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Frequency perception as a measure of room acoustic quality
Applied Acoustics 60 (2000) 63±79
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Frequency perception as a measure of room acoustic quality Daeup Jeong*, Fergus R. Fricke Department of Architectural and Design Science, University of Sydney, NSW 2006, Australia Received 5 October 1998; received in revised form 18 May 1999; accepted 18 June 1999
Abstract For the development of auditorium acoustics as a science, a reliable procedure for rating halls needs to be developed which is independent of the music, performers and other confounding non-acoustical variables. The present work is aimed at producing a test which can be used to rate the acoustic quality of rooms for music. Listeners' perceptions of frequency change in sounds were investigated in the search for a useful subjective indicator of the acoustics of a room. The in¯uence of the acoustics of the listening environment on the listeners' perceptions of frequency change at mid- and high-frequencies was observed and analysed based on the 2AFC (two alternative forced choice) and 2-down, 1-up adaptive test paradigm. Also, listeners' preferred positions for listening to music were investigated in a reverberant listening condition using music excerpts recorded under anechoic conditions. It was found that dierent listening positions in a room have a signi®cant eect on the listeners' perceptions of frequency change. A negative relationship was found between a listener's preference (ie. the acoustics) for the ¯ute recordings and the JNDs of frequency change while the perception of frequency change was negatively correlated with the listening preference for the guitar and percussion recordings. # 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction It has been suggested [1] that one of the major factors impeding the development of design methodologies for concert halls is the absence of an acoustical assessment method which is free from in¯uences such as the quality of the musical group playing in the auditorium, visual biases and the music played. If a particular hall is to be * Corresponding author. 0003-682X/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0003-682X(99)00032-8
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improved, it would be useful to have a method based on a repeatable and quanti®able subjective assessment, independent of the music and performers, to determine how much of an improvement has been achieved. A concert hall is a place which provides the acoustic link between the music performer and the listener. Performers produce subtle changes in tonal quality as a component of musical expression. The listeners' impressions of the acoustics of the hall are believed to be in¯uenced by how well those small changes can be perceived. A successful room for music is the one in which the listener is able to hear such subtleties of musical expressions [2]. In a room, the sound from a source tends to be modi®ed by the acoustic characteristics of the room, which implies that a listener, placed at a certain position in a room, will perceive the original sound as modi®ed or coloured [3]. By varying the acoustic conditions of a listening environment or simply changing the listening position in a room, small but signi®cant dierences can be made in the frequency spectrum and time history of the sound. The study of aural responses to transient tones is necessary for understanding the perception of speech and musical sounds which involve continuous changes in amplitude and frequency. As to frequency change, in particular, it is well known that a tone with a large and gradual change in frequency conveys a clear impression of pitch change. Even if the pitch change is not caused by the frequency change, this change may have an in¯uence on the quality of a tone. The transition in formant frequency of speech is said to be an important cue for the identi®cation of a phoneme [4]. In musical sounds, especially in wind instrument tones, the frequency change near the onset of a note may be one of the characteristic features of this kind of instrument [5,6]. A number of researches [7±11] have explored the in¯uence of room acoustics on FM (frequency modulated) and AM (amplitude-modulated) sounds propagating in listening environments. These studies were mainly concerned with how room transmission characteristics physically aect AM and FM sounds. The present study investigated the psychoacoustic eect of room acoustics on the perception of frequency change in short duration tones. The present work was also aimed at exploring the relationship between the perception of frequency change in short duration sounds and the perceived acoustic quality of listening environments. Musicians' preferences for re¯ections with dierent delay times and intensities have been studied by Marshall [12], Barron [13], Yamaguchi [14] and others. In this work, more complex sequences of re¯ections are included and perceived changes in frequency are studied to determine whether perception of change could be used as a measure of room acoustic quality. The hypothesis under investigation in this work is that the acoustics of the listening environment aects the listeners' perception of frequency change in short duration tones and the amount of the perceivable frequency change may be related to listeners' preferences. To verify this, two experiments were carried out at dierent listening positions in a reverberation chamber using short duration tones of 110 ms in duration (including rise/decay time) and four dierent anechoic music recordings. It would normally be assumed that the perception of frequency change is due to the acoustic properties of the transmission path between the source and the receiver
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positions where the dummy head is located. Three questions have been investigated in this work: 1. How much does the acoustics of dierent listening positions in a room aect the listener's perception of frequency change? 2. Are there some frequencies which are more readily perceived to change than others by room re¯ections (preliminary tests suggested that frequency perception at the mid and high frequencies were more in¯uenced than low frequencies by the acoustics of the listening environment)? 3. Is there any relationship between a listener's perception of frequency change and the preferred listening environment? In the ®rst experiment, short duration tones at mid (1 kHz) and high frequency (4 kHz), with linear frequency changes from the onset to the oset, were produced, and recorded in a reverberant room using an omni-directional loudspeaker and dummy head setup. The recorded sounds were reproduced for the listeners through headphones and the listeners' JNDs of frequency change were measured at dierent listening positions through the experimental paradigm based on the adaptive staircase method that would produce 70.7% correct responses [15]. The listener's task was simply to determine which of a pair of sounds had a frequency change. In the second experiment, anechoic music recordings featuring four dierent musical instruments were reproduced in a reverberation chamber and recorded at three dierent listening positions, using exactly the same setup as in the ®rst experiment. The recorded music samples were paired and presented to listeners through headphones. A two alternative forced choice (2AFC) paradigm was used for determining preferences. Listeners were asked to choose the preferred recording and hence the listening position they preferred for listening to dierent music recordings. 2. Experiment 1. The dependence of frequency change perception on the listening position in a reverberant room 2.1. Experimental arrangement Five dierent listening positions, in the reverberation chamber (whd: 6.354.105.15 m) and one listening position in the anechoic chamber (whd: 4.503.603.0 m), were selected to investigate the eect of the acoustics of dierent listening environments on the perception of frequency change (see Fig. 1). The reverberation time (RT500±2000) of the reverberation chamber, where the dummy head recordings were made, was 3.7 s (see Fig. 2). The perception of dierences at dierent listening positions in a room may be a function of dierent acoustics at each position. An omni-directional speaker (Soundsphere model 110) was used to eliminate the eect of the directional characteristics of the source. In the reverberation chamber, the loudspeaker was placed slightly o the centerline of the room, 3.5 m from the right hand side wall and 1 m from the front wall and 1.2 m from the
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Fig. 1. Experimental setup for recording and reproducing sounds. The sounds were recorded in a reverberation chamber and anechoic chamber.
¯oor. Position P0 was also located slightly o the centerline of the room, 3.5 m from the right hand side wall, and 3.5 m from the front wall. The other positions (P1, P2, P3, and P4) were chosen at the distance of 2.5 m from the source by simply varying the angle from the axis between the source and P0 (22.5 ). This arrangement allowed the distance from the front and rear walls to the dummy head to be varied from 1 to 3.5 m. Also, the distance from the left wall to the dummy head was varied from 2.85 to 5.35 m and the distance from the right wall to the dummy head was varied from 1 to 3.5 m. Thus, dierent re¯ection levels from the each surface with dierent delay times were introduced to each listening position, whilst the reverberant conditions remained the same. The source and the dummy head were placed at a height of 1.2 m from the ¯oor and the dummy head at each listening position always pointed toward the source. To minimise any directionality eects, the direction of the omni-directional speaker
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Fig. 2. Octave band reverberation times in the room where the dummy head recordings were made.
was changed by the same angle as the dummy head position, so the dummy head always faced the same part of the speaker. In the anechoic chamber only one position was chosen at the distance of 2.5 m from the source. The omni-directional speaker and the dummy head were installed at 1.2 m from the ¯oor and 1 m from the wall. The tones for the tests were generated by the audio processing system and supporting program, CSound for Macintosh. The generated test tones were saved on a digital audio tape (DAT) and reproduced in the reverberation chamber and anechoic chamber through an omni-directional speaker. The produced tones were binaurally recorded as 16 bit sounds (at a sampling rate of 44.1 kHz) using a digital audio tape recorder (Sony, PCM-2700A) through a dummy head (Kemar Head DB062) using 1/2 inch microphones (B&K type 4165) installed at the pinnae. The sounds recorded on a DAT tape were fed into the computer hard-disk (PMac 7200/120) and digitally converted to System 7 sound ®les for the presentation. All stimuli were digitally stored on the computer hard-disk and reproduced to listeners using a program written with a psychological experimental software, psychlab, through an open type headphone [16] (AKG, K1000). The listening level was kept constant at approximately 70dB(A). The processed stimuli were low-pass ®ltered at 10 kHz by the bandpass ®lter (Kronhite 3550), to remove the noise from the output of the computer, and ampli®ed (Onkyo Integrated Stereo Ampli®er A-9210) before being presented to a listener through headphones. 2.2. Procedure A 2AFC and 2-down, 1-up adaptive experimental procedure were employed. Two consecutive correct responses lead to the smaller frequency change, while the magnitude of frequency change was increased at every wrong response. The experiment was designed to terminate after achieving 12 reversals for each listener. The ®rst three reversals were discarded and the arithmetic mean of the last nine trials was taken as a JND.
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During the assessment listeners were seated in a quiet chamber and a computer keyboard and monitor were provided for the presentation and collection of responses. A 30 min practice session was provided to the all listeners, just before the main test session, to make them familiar with the stimuli and operation of the program. The listener's task was simply to determine which of a pair of signals presented to him/her through headphones had a frequency change. No feedback on the accuracy of the response was provided to the listeners in the main test while feedback was used in the practice session. 2.3. Stimuli In the present work, 1 and 4 kHz short duration tones with linear ascending frequency changes were used (refer to Fig. 3). Preliminary tests using two dierent frequency change patterns (ascending and descending linear frequency changes) at three frequencies (0.25, 1 and 4 kHz) in an anechoic and reverberant sound ®eld (a reverberation chamber and lecture room) suggested that the ascending frequency change is more useful for this purpose: no signi®cant eect of the acoustics of a listening environment was found at 0.25 kHz and remarkable individual dierences were observed for the descending frequency change. This result con®rms the ®ndings from the other researchers. Gardner and Wilson [17] suggested the existence, in the auditory system, of channels speci®c to upward FM and downward FM in a frequency change detection experiment using upward and downward sweeping signals. Dooley and Moore [18,19] found discrepancies between listeners, and argued such discrepancies were caused by the dierences in stimuli. Also, signi®cant dierences
Fig. 3. Time±frequency plot of the stimuli used in the experiment.
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in the perceived frequency change between the anechoic and reverberant sound ®elds were observed for the mid and high frequency short duration tones. The frequency of the comparison stimulus was linearly swept from 1 kHz to the speci®ed target frequency throughout its duration. The standard stimuli had a constant frequency of 1 kHz from the onset to the oset (see Fig. 3). The duration of the stimulus used in this test was 110 ms. To minimise switching transients at the onset and oset of the stimuli, rise/decay times of 5 s were used. The amount of frequency change of the comparison stimulus was increased by 0.25 Hz from 1 k to 1.001 kHz and then by 1Hz from 1.001 to 1.028 kHz for the 1 kHz short duration tones at every wrong response from listeners. For the 4 kHz short duration tones, a 0.2 Hz step was applied to the region between 4 and 4.002 kHz and then increased by 2 Hz from 4.002 to 4.030 kHz. 2.4. Listeners Two male postgraduate listeners including one of the authors (listener DU) and two female undergraduate students voluntarily participated in this experiment. Three listeners were known to have musical backgrounds. One male listener, DC, and one female listener, JH, were amateur musicians (BMus) and the other female listener, MK, also had a background as an amateur music performer (AMEB Grade 7). DU was 34 years old and the other listeners were between 20 and 30. They had previously taken part in other auditory perception experiments. They had normal audiometric threshold (ISO 389, 1991) at standard octave frequencies from 0.125 to 8 kHz. 2.5. Results and discussion The purpose of this work was to investigate the eect of the acoustics of the listening environment on the detection threshold of frequency change and to ®nd out which frequency range is most in¯uenced by the room re¯ections. The thresholds of detection for frequency change at dierent listening positions are shown in Figs. 4 and 5. Listener JH and DU showed lower JNDs at every listening position in a reverberation chamber than in an anechoic chamber at 1 kHz. Also, better performance was observed in the JNDs of listener DU and DC, at P0 in the reverberation chamber than in the anechoic condition, for the 4 kHz signal. This result suggests that the re¯ections from room surfaces may improve the listeners' ability to perceive frequency change. However, it appears that there are signi®cant individual dierences in perception and a dependency on the frequency of the sounds used. To verify if the room re¯ections can contribute to the improvement of the frequency change perception, more extensive experiments using arti®cial sound ®elds, featuring a number of re¯ections, are required. In the reverberation chamber, three listeners showed lowest JNDs at the listening position of P0, while the listener, JH showed slightly better performance at P1 for the mid-frequency (1 kHz). Position P2 turned out to be the worst listening position for all listeners except DC. Listener DC showed his worst JNDs at P1 and P4 at 1 kHz,
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Fig. 4. Measured JNDs for 1 kHz frequency changed pure tones in an anechoic chamber and at ®ve different positions in a reverberation chamber (A: anechoic chamber, P0 to P4: dierent positions in a reverberation chamber) (the errors bars showstandard errors).
Fig 5. Measured JNDs for 4 kHz frequency changed pure tones in an anechoic chamber and at ®ve different positions in a reverberation chamber) (A: anechoic chamber, P0 to P4: dierent positions in a reverberation chamber) (the error bars showstandard errors).
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and P1, P2, P3, P4 at 4 kHz. Two questions, as follows, were examined using a t-test. ``Do the dierent listening positions in a room have any signi®cant eect on the listeners perceptions of frequency change?'' and ``Is any listener response signi®cantly dierent from any other at both 1 and 4 kHz?'' A t-test (paired means comparison) showed that a signi®cant dierence (p<0.05) exists between the perceptions of frequency change at dierent listening positions (Table 1) for both 1 and 4 kHz. In particular, it was found that the measured JNDs at P0, at which listeners showed the lowest JNDs, were signi®cantly dierent from the JNDs at the other positions. More signi®cant and consistent data across the listeners were obtained at the high frequency (4kHz), where all listeners showed the lowest JNDs at P0 and the highest at P1. Although the listener MK showed slightly better performance at P2 and P3 than at P0, the dierences between these three listening positions were negligible (less than 0.2 Hz). The position P4 was the next most dicult position to detect frequency change at 4 kHz. Also, there exist some individual dierences in frequency perception between listeners. A t-test showed that the JNDs of listener MK were signi®cantly dierent from the others at 1 kHz (p<0.05), but no signi®cant dierence was found between the other listeners. At 4 kHz, listener JH showed signi®cantly dierent sensitivity to the other listeners (p<0.05) (Table 2). The in¯uence of the listening position on the JNDs of these two listeners at each position was found to be more remarkable than the other listeners. This result suggests that listeners in a room may have dierent sensitivities to the room re¯ections, especially for the perception of frequency change. Normalised JNDs from all listeners with mean and standard errors for both 1 and 4 kHz, at dierent listening positions, are shown in Figs. 6 and 7. The normalised JNDs were obtained by accumulating the JNDs at dierent positions in the reverberation chamber for each listener, and then dividing by the JND obtained in the anechoic room for the same listener. At both frequencies P0 turned out to be the best position for detecting frequency changes. For 1 kHz, listeners showed the highest JNDs at P1 and P2, while P1 was the worst listening position for detecting frequency Table 1 The signi®cance of listening positions in a room on the perception of frequency change using a paired means comparison test Frequency
Listening positions
t-value
p-value
1 kHz
P0 P0 P0 P2
vs P1 vs P2 vs P3 vs P3
ÿ2.817 ÿ4.395 ÿ2.487 2.142
0.0119 0.0004 0.0236 0.0470
(<0.05) (<0.001) (<0.05) (<0.05)
4 kHz
P0 P0 P0 P0 P1 P2
vs P1 vs P2 vs P3 vs P4 vs P3 vs P4
ÿ4.547 ÿ2.846 ÿ2.156 ÿ5.620 2.684 ÿ2.204
0.0003 0.0112 0.0457 <0.0001 0.0157 0.0416
(<0.001) (<0.05) (<0.05) (<0.05) (<0.05)
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Table 2 The signi®cance of listeners' perceptions of frequency change on frequency using a paired means comparison test Frequency
Listeners
t-value
p-value
1 kHz
MK vs DC MK vs DU MK vs JH
2.86910 ÿ2.34940 ÿ2.86910
0.005937 (<0.01) 0.022637 (<0.05) 0.005937 (<0.01)
4 kHz
JH vs DC JH vs DU JH vs MK
2.1680 ÿ2.5130 ÿ1.4480
0.04070 (<0.05) 0.01940 (<0.05) 0.15990 (>0.05)
Fig. 6. JNDs for change in frequency of 1 kHz pure tone measured at dierent positions in a reverberation chamber and normalised with the JNDs obtained in an anechoic chamber (P0 to P4: dierent positions in a reverberation chamber) (the error bars showstandard errors).
change at 4 kHz. The dierences between the other four positions (P1 to P4) at 1 kHz seem to be negligible (normalised JND values exist within the maximum and minimum standard error range of the other listening positions). It is known that the sound level produced by a source of constant power output produces a greater level in a room with larger reverberation time [20]. Also, the directivity of the source introduces loudness dierences to the dierent listening positions in a room. Generally, an increase in loudness improves the detectability of sounds. This was not a factor in the present work because of the use of an omni-directional source. It is also known that early room re¯ections, which arrive within 80 ms after the direct sound, help in the perception of speech. However any signi®cant dierence in early sound ®elds was not observed between the impulse responses recorded at the listening positions. In the present experiment, re¯ections with dierent delay times and spectral structures were introduced to each listening position by varying the listening positions whilst maintaining the same distance from the source. Therefore, the
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Fig. 7. JNDs for change in frequency of 4 kHz pure tone measured at dierent positions in a reverberation chamber and normalised with the JNDs obtained in an anechoic chamber (P0 to P4: dierent positions in a reverberation chamber) (the error bars showstandard errors).
observed dierences between JNDs at listening positions seem to be due to the central masking (or reinforcing) caused by the delayed re¯ections at the two ears with dierent temporal and spectral structures. 3. Experiment 2. Preferred positions for listening to music Listeners' preferred positions for listening to music were investigated using anechoic music recordings featuring four dierent solo instruments. Three listening positions were chosen based on the results of Experiment 1. 3.1. Experimental arrangement and procedure In this experiment, three listening positions (P0, P1, and P2), which bracket the worst and the best position for discriminating frequency change, were visited again for the preference test, considering the results of the Experiment 1. In the previous experiment, P0 was found to be the best position for discriminating the frequency change at 1 and 4 kHz, while P2 and P1 were the worst positions for discriminating 1 and 4 kHz tones, respectively. Three anechoic music recordings (cello, guitar, and xylophone) were chosen from ``Music for Archimedes'' [21] and one anechoic music recording (Flute) ``DENON Professional CDs'' [22] as follows: 1. 2. 3. 4.
Theme, Cello, composed by Weber Bourre, Guitar, composed by J.S. Bach Sabre Dance, Xylophone, composed by Khachaturian Syrinx, Flute, composed by Debussy
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The length of the music sample used for preference testing is important. It has been found earlier that sounds or pieces of orchestral music between 8 and 35 seconds in duration gave rise to reproducible emotional responses in listeners [23]. However, the use of long music samples should be avoided because of the short-term auditory memory. Therefore, the duration of music samples used in this work was chosen to be approximately 10 s. These music excerpts were reproduced in the reverberation chamber through an omni-directional speaker (Soundspere Model 110) and recorded on a DAT tape using a KEMAR dummy head. Exactly the same recording setup as in the Experiment 1 was used for the recording. The recording levels at each listening position were controlled to be equal when measured with a single 1/2 inch free-®eld microphone (B&K, Type 4190). The music samples, recorded on a DAT tape, were fed into the computer hard-disk (PMac 7200/120) and digitally converted to System 7 sound ®les for the presentation. A two-alternative forced choice method (2AFC) was used. Music samples recorded at each position were paired with each other and stored on a computer hard-disk. They were presented to the listeners in a random order through headphones (AKG, K1000). Each listener in the present experiment undertook four dierent sessions, which took approximately 1.5 h to complete. Each session featured a dierent musical instrument (cello, ¯ute, guitar, and xylophone). The session order was randomised for each listener. Also, the order of presenting music pairs was randomised within each session. Each pair of music recordings was presented four times in a session. Listeners were seated in a quiet room and a computer keyboard and monitor were provided to control the experiment. The listener's task was simply to determine which of two recordings in a given pair was preferred. Listeners were allowed to hear the sounds as many times as they wished before making a choice. 3.2. Listeners Two male postgraduate listeners including one of the authors (listener DU) and four female undergraduate students voluntarily participated in this experiment. Three listeners (DC, DU, and MK) who participated in the Experiment 1, came back for this test. All the listeners, except DU, were known to have musical backgrounds. A male listener, DC is an amateur musician (BMus) and the female listener, CHJ, is a music student. The other female listeners were known to have backgrounds as amateur music performers (AMEB Grade 7). Four listeners had previously taken part in other auditory perception experiments. 3.3. Results and discussion In the present experiment, a paired comparison test (2AFC) was carried out for obtaining listeners' subjective preferences. The paired comparison method is extremely useful, since listeners can simply judge which of two stimuli they prefer. Preference scores can be obtained for each pair of stimuli by giving scores of +1 and ÿ1, corresponding to positive and negative judgements, respectively, for each
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presentation of the pair. The normalised preference score can be produced by accumulating the scores for all stimuli (S) tested and all listeners (L), and then dividing by the factor L(Sÿ1) for further statistical analysis. However, the preference score itself cannot be represented as a linear scale. To obtain the comparable preference value which can be regarded as a linear psychological distance between stimuli, the law of comparative judgment [24,25] provides a basis [26]. In the present study, the preference scores were linearly scaled based on the law of comparative judgment. Eq. (1) shows the complete form of the law of comparative judgment. p 2 j k2 2rjk j k 1 Sk ÿ Sj where, Sj, Sk: xjk: j , k : r:
the psychological scale values of the two compared stimuli. the normal deviate corresponding to the theoretical proportion of times stimulus k is judged greater than stimulus j discriminal dispersion of stimulus j and k. the correlation between the pairs of discriminal processes dj and dk.
The measured preferences are presented in Fig. 8. It was found that listening position, P1, was the preferred position for listening to the guitar and xylophone. However, for the ¯ute, P0 was the preferred listening position. Any major eect of listening position on the preference was not found for the cello recording. This result suggests that there exist preferred positions for listening to music in a room, depending on the instruments being played. Since the recordings were made in a highly controlled listening environment, the arrival time and relative level of re¯ected
Fig 8. Preferred positions for listening to dierent instrumental music (P0 to P2: the listening positions in a reverberaiton chamber).
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sounds (introduced by the varied distance from the room surfaces to the dierent listening positions) seem to be a major factor in¯uencing the listeners' preferences. Another possibility is that listeners might have dierent criteria for each instrument type. Discussions with the listeners after the listening tests support this possibility. Most listeners reported that they made their choice based mainly on the clarity of sounds for guitar and percussion (xylophone) while each listener seemed to apply dierent criteria for the cello and ¯ute, such as binaural dissimilarity and the degree of resonance. In terms of temporal structure, plucked string or struck instruments are characterised by a sharp attack and long decay while wind instruments and bowed strings have completely opposite characteristics. Meyer [27] suggested, after his analysis of individual instruments, that strings and, particularly, woodwinds need the reverberation of the room as their own decay times are shorter. However this is not the case in this experiment, since the variation of reverberation time should be limited within a room. The pitch of the plucked string and percussion instrument, during the attack, is very dicult to detect [5,6]. Therefore, clear audibility of the attack seems to form an important part in preference for guitar and percussion sounds, while the masking of decaying procedure by the clouds of re¯ections may in¯uence the preference for cello and ¯ute. As discussed earlier, P0 was found to be the best position for the listener to discriminate the frequency change at both mid and high frequencies (see Figs. 6 and 7). P1 was the worst listening position for the frequency change perception at 4 kHz. The frequency change JNDs at 1 and 4 kHz did not show any signi®cant correlation with listeners' overall preferences. Therefore normalised JNDs and the magnitude of JND dierences were calculated and their relationships with scale values of preference were tested using a correlation matrix. The magnitude of JND dierences (JND(|PnÿA|)s) were obtained by taking the magnitude of the dierence between the JNDs at each listening position in the reverberant room and in the anechoic chamber, for each listener. Fig. 9 shows the accumulated magnitudes of JND dierences calculated at three listening positions in a reverberation chamber [The data points in Fig. 9 include the accumulated JND(|PnÿA|) values from four listeners involved in the previous experiment]. The question of whether the preferred listening positions are signi®cantly related to the magnitude of the JND dierences (JND(|PnÿA|)s) was examined using a correlation matrix (Table 3). To determine if coecients are statistically dierent from zero, a Fisher's r to z transformation was carried out. It was found that correlation coecients exceeding r=0.335 were signi®cant at the level of p<0.005. No signi®cant correlation was found between the JND(|PnÿA|)s at 1 kHz and the preferred listening positions. A signi®cant correlation was found between JND(|PnÿA|)s at 4 kHz and preferred listening position. The JND(|Pn ÿA|)s at 4 kHz are negatively correlated with the scale values of preference for cello and ¯ute (with the correlation coecients of ÿ0.364 and ÿ0.496, respectively). Positive correlations between the scale values of preference for xylophone and guitar and the JND(|PnÿA|)s at 4 kHz were found to be signi®cant (with the correlation coecients of 0.335 and 0.444, respectively).
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Fig. 9. The JND (|PnÿA|) at three listening positions in a reverberation chamber at 1 and 4 kHz. JND (|PnÿA|) was obtained by taking the magnitute of the dierence between JNDs at each position in a reverberation chamber and in an anechoic chamber for each listener. P0P2 present listenin positions in a reverberation chamber. (Error bars showstandard errors.)
Table 3 A correlation matrix that explains the correlation between the scale values of preference for dierent instrumental music and JND (jPn ÿ Aj) at 1 and 4 kHz Cello
Flute
Guitar
Xylophone
JND jPn ÿ Aj at 1 kHz
ÿ 0.046 (p=0.6946)
ÿ0.131 (p=0.2641)
0.156 (p=0.1820)
0.146 (p=0.2122)
JND jPn ÿ ÿAj at 4 kHz
ÿ0.364 (p<0.005)
ÿ0.509 (p<0.0001)
0.444 (p<0.0001)
0.335 (p<0.005)
4. Conclusions The experimental results show that, in nominally the same acoustics, a listener's ability to perceive frequency change appears to vary depending on the listening position in the room and the dierence limen of frequency change detection has a relationship with the preferred acoustics for a given instrument. This has important implications for developing a method of evaluating the acoustic quality at dierent seats in a concert hall or in dierent concert halls. It also has important implications for the design of a concert hall. A consistent eect of listening position on the perception of frequency change was found at the high frequency (4 kHz). The type of musical instrument being played appears to be one of the important factors for listeners in deciding their preferred listening position in a room. The high frequency JND may be a good indicator of the acoustics of spaces for instruments which are plucked or struck, i.e. which have many overtones of high intensities. Listeners may prefer listening to the guitar and percussion playing in the listening environment which does not provide good
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frequency change discrimination (it is likely that the good discriminability of frequency change may not be a prerequisite of a listening environment for listening to plucked strings and percussions). This result suggests that a listening environment, in which listeners can perceive frequency changes well at high frequency, may be good for listening to cello and ¯ute. However, clear audibility of frequency change at high frequency appears not to be suitable for listening to guitar and xylophone. The correlation results suggest that an anechoic listening environment would be preferable for listening to ¯ute and cello. This is unlikely to be the case. It would seem necessary to use this method of assessment in conjunction with reverberation time. Alternatively there may be an optimum JND(|PnÿA|) for a given instrument. The interaural relationships such as ITD (interaural time delay) and ILD (interaural loudness dierence) caused by re¯ections from the surface of the room seem to have a signi®cant eect on the perception of frequency change, but this was not directly veri®ed in the present work. To verify this, JNDs will be measured with dierent arrival times of re¯ections and dierent relative levels of re¯ections using a more controlled experimental setup. References [1] Beranek LL. Concert hall acoustics Ð 1992. J Acoust Soc Am 1992;92(1):1±39. [2] Goad PJ, Keefe DH. Timbre discrimination of musical instruments in a concert hall. Music Perception 1992;10(1):43±62. [3] Bilsen FA. Thresholds of perception of repetition pitch. Conclusions concerning coloration in room acoustics and correlation in the hearing organ. Acustica 1968;19:7±32. [4] Lieberman AM, Delattre PC, Gerstman LJ, Cooper FS. Tempo of frequency change as a cue for distinguishing classes of speech sounds. J Exp Psychol 1984;52:127±37. [5] Freedman MD. Analysis of musical instrument tones. J Acoust Soc Am 1967;41:793±806. [6] Luce D, Clark M. Physical correlates of brass instrument tones. J Acoust Soc Am 1967;42:1232±43. [7] Ozimek E. Physical and psychoacoustic evaluation of FM signals propagating in a room. Proceedings of 13th ICA, 1989. p. 177±180. [8] Ozimek E, Sek A. AM and FM dierence limens and their reference to amplitude-frequency changes of a sound in a room. Acta Acustica 1996;82:114±22. [9] Ozimek E, Rutkowski L. Deformation of frequency modulated (FM) signals propagating in a room. Applied Acoustics 1989;26:217±30. [10] Rutkowski L, Ozimek E. Linear and jump frequency changes of a signal in a room. Archives of Acoustics 1995;20(2):115±38. [11] Rutkowski L, Ozimek E. Linear and sinusoidal frequency changes of a signal in a room. Acta Acustica 1997;83:881±90. [12] Marshall AH. A note on the importance of room cross-section in concert halls. J Sound Vib 1967;5(1):100±12. [13] Barron M. Subjective study of British Symphony Concert Hall. Acustica 1988;66(1):1±14. [14] Yamaguchi K. Multivariate analysis of subjective and physical measures of hall acoustics. J Acoust Soc Am 1972;52(5):1271±9. [15] Levitt H. Transformed up-down procedures in psychoacoustics. J Acoust Soc Am 1970;49:467±77. [16] Mùller H. Fundamentals of binaural technology. Applied Acoustics 1992;36:171±218. [17] Gardner RB, Wilson JP. Evidence for direction-speci®c channels in the processing of frequency modulation. J Acoust Soc Am 1979;66(3):704±9. [18] Dooley GJ, Moore BCJ. Detection of linear frequency glides as a function of frequency and duration. J Acoust Soc Am 1988;84(6):2045±57.
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[19] Dooley GJ, Moore BCJ. Duration discrimination of steady and gliding tones: A new method for estimating sensitivity to rate of change. J Acoust Soc Am 1988;84(4):1332±7. [20] Zwicker E, Fastl H. Psychoacoustics Ð Facts and Models. Berlin: Springer Verlag, 1990. p. 314±6. [21] Hansen V, Munch G. Making recordings for simulation tests in the Archimedes Project. J Audio Eng Soc 1991;39(10):768±74. [22] Hidaka T, Kageyama K. Recording of anechoic orchestral music and measurement of its physical characteristics based on the auto-correlation function. Acustica 1988;67:68±70. [23] NielzeÂn S, Cesazrec Z. On the perception of emotional meaning in music. Psychology of Music 1981;9(2):17±31. [24] Thurston LL. A law of comparative judgment. Psychological Review 1927;34:273±86. [25] Torgerson WS. Theory and methods of scaling. New York: John Wiley & Sons Inc., 1962. [26] Ando Y. Concert hall acoustics. Berlin: Springer-Verlag, 1985. [27] Meyer J. The sound of the orchestra. J Audio Eng Soc 1993;41(4):203±13.
www.elsevier.com/locate/apacoust
Frequency perception as a measure of room acoustic quality Daeup Jeong*, Fergus R. Fricke Department of Architectural and Design Science, University of Sydney, NSW 2006, Australia Received 5 October 1998; received in revised form 18 May 1999; accepted 18 June 1999
Abstract For the development of auditorium acoustics as a science, a reliable procedure for rating halls needs to be developed which is independent of the music, performers and other confounding non-acoustical variables. The present work is aimed at producing a test which can be used to rate the acoustic quality of rooms for music. Listeners' perceptions of frequency change in sounds were investigated in the search for a useful subjective indicator of the acoustics of a room. The in¯uence of the acoustics of the listening environment on the listeners' perceptions of frequency change at mid- and high-frequencies was observed and analysed based on the 2AFC (two alternative forced choice) and 2-down, 1-up adaptive test paradigm. Also, listeners' preferred positions for listening to music were investigated in a reverberant listening condition using music excerpts recorded under anechoic conditions. It was found that dierent listening positions in a room have a signi®cant eect on the listeners' perceptions of frequency change. A negative relationship was found between a listener's preference (ie. the acoustics) for the ¯ute recordings and the JNDs of frequency change while the perception of frequency change was negatively correlated with the listening preference for the guitar and percussion recordings. # 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction It has been suggested [1] that one of the major factors impeding the development of design methodologies for concert halls is the absence of an acoustical assessment method which is free from in¯uences such as the quality of the musical group playing in the auditorium, visual biases and the music played. If a particular hall is to be * Corresponding author. 0003-682X/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0003-682X(99)00032-8
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improved, it would be useful to have a method based on a repeatable and quanti®able subjective assessment, independent of the music and performers, to determine how much of an improvement has been achieved. A concert hall is a place which provides the acoustic link between the music performer and the listener. Performers produce subtle changes in tonal quality as a component of musical expression. The listeners' impressions of the acoustics of the hall are believed to be in¯uenced by how well those small changes can be perceived. A successful room for music is the one in which the listener is able to hear such subtleties of musical expressions [2]. In a room, the sound from a source tends to be modi®ed by the acoustic characteristics of the room, which implies that a listener, placed at a certain position in a room, will perceive the original sound as modi®ed or coloured [3]. By varying the acoustic conditions of a listening environment or simply changing the listening position in a room, small but signi®cant dierences can be made in the frequency spectrum and time history of the sound. The study of aural responses to transient tones is necessary for understanding the perception of speech and musical sounds which involve continuous changes in amplitude and frequency. As to frequency change, in particular, it is well known that a tone with a large and gradual change in frequency conveys a clear impression of pitch change. Even if the pitch change is not caused by the frequency change, this change may have an in¯uence on the quality of a tone. The transition in formant frequency of speech is said to be an important cue for the identi®cation of a phoneme [4]. In musical sounds, especially in wind instrument tones, the frequency change near the onset of a note may be one of the characteristic features of this kind of instrument [5,6]. A number of researches [7±11] have explored the in¯uence of room acoustics on FM (frequency modulated) and AM (amplitude-modulated) sounds propagating in listening environments. These studies were mainly concerned with how room transmission characteristics physically aect AM and FM sounds. The present study investigated the psychoacoustic eect of room acoustics on the perception of frequency change in short duration tones. The present work was also aimed at exploring the relationship between the perception of frequency change in short duration sounds and the perceived acoustic quality of listening environments. Musicians' preferences for re¯ections with dierent delay times and intensities have been studied by Marshall [12], Barron [13], Yamaguchi [14] and others. In this work, more complex sequences of re¯ections are included and perceived changes in frequency are studied to determine whether perception of change could be used as a measure of room acoustic quality. The hypothesis under investigation in this work is that the acoustics of the listening environment aects the listeners' perception of frequency change in short duration tones and the amount of the perceivable frequency change may be related to listeners' preferences. To verify this, two experiments were carried out at dierent listening positions in a reverberation chamber using short duration tones of 110 ms in duration (including rise/decay time) and four dierent anechoic music recordings. It would normally be assumed that the perception of frequency change is due to the acoustic properties of the transmission path between the source and the receiver
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positions where the dummy head is located. Three questions have been investigated in this work: 1. How much does the acoustics of dierent listening positions in a room aect the listener's perception of frequency change? 2. Are there some frequencies which are more readily perceived to change than others by room re¯ections (preliminary tests suggested that frequency perception at the mid and high frequencies were more in¯uenced than low frequencies by the acoustics of the listening environment)? 3. Is there any relationship between a listener's perception of frequency change and the preferred listening environment? In the ®rst experiment, short duration tones at mid (1 kHz) and high frequency (4 kHz), with linear frequency changes from the onset to the oset, were produced, and recorded in a reverberant room using an omni-directional loudspeaker and dummy head setup. The recorded sounds were reproduced for the listeners through headphones and the listeners' JNDs of frequency change were measured at dierent listening positions through the experimental paradigm based on the adaptive staircase method that would produce 70.7% correct responses [15]. The listener's task was simply to determine which of a pair of sounds had a frequency change. In the second experiment, anechoic music recordings featuring four dierent musical instruments were reproduced in a reverberation chamber and recorded at three dierent listening positions, using exactly the same setup as in the ®rst experiment. The recorded music samples were paired and presented to listeners through headphones. A two alternative forced choice (2AFC) paradigm was used for determining preferences. Listeners were asked to choose the preferred recording and hence the listening position they preferred for listening to dierent music recordings. 2. Experiment 1. The dependence of frequency change perception on the listening position in a reverberant room 2.1. Experimental arrangement Five dierent listening positions, in the reverberation chamber (whd: 6.354.105.15 m) and one listening position in the anechoic chamber (whd: 4.503.603.0 m), were selected to investigate the eect of the acoustics of dierent listening environments on the perception of frequency change (see Fig. 1). The reverberation time (RT500±2000) of the reverberation chamber, where the dummy head recordings were made, was 3.7 s (see Fig. 2). The perception of dierences at dierent listening positions in a room may be a function of dierent acoustics at each position. An omni-directional speaker (Soundsphere model 110) was used to eliminate the eect of the directional characteristics of the source. In the reverberation chamber, the loudspeaker was placed slightly o the centerline of the room, 3.5 m from the right hand side wall and 1 m from the front wall and 1.2 m from the
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Fig. 1. Experimental setup for recording and reproducing sounds. The sounds were recorded in a reverberation chamber and anechoic chamber.
¯oor. Position P0 was also located slightly o the centerline of the room, 3.5 m from the right hand side wall, and 3.5 m from the front wall. The other positions (P1, P2, P3, and P4) were chosen at the distance of 2.5 m from the source by simply varying the angle from the axis between the source and P0 (22.5 ). This arrangement allowed the distance from the front and rear walls to the dummy head to be varied from 1 to 3.5 m. Also, the distance from the left wall to the dummy head was varied from 2.85 to 5.35 m and the distance from the right wall to the dummy head was varied from 1 to 3.5 m. Thus, dierent re¯ection levels from the each surface with dierent delay times were introduced to each listening position, whilst the reverberant conditions remained the same. The source and the dummy head were placed at a height of 1.2 m from the ¯oor and the dummy head at each listening position always pointed toward the source. To minimise any directionality eects, the direction of the omni-directional speaker
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Fig. 2. Octave band reverberation times in the room where the dummy head recordings were made.
was changed by the same angle as the dummy head position, so the dummy head always faced the same part of the speaker. In the anechoic chamber only one position was chosen at the distance of 2.5 m from the source. The omni-directional speaker and the dummy head were installed at 1.2 m from the ¯oor and 1 m from the wall. The tones for the tests were generated by the audio processing system and supporting program, CSound for Macintosh. The generated test tones were saved on a digital audio tape (DAT) and reproduced in the reverberation chamber and anechoic chamber through an omni-directional speaker. The produced tones were binaurally recorded as 16 bit sounds (at a sampling rate of 44.1 kHz) using a digital audio tape recorder (Sony, PCM-2700A) through a dummy head (Kemar Head DB062) using 1/2 inch microphones (B&K type 4165) installed at the pinnae. The sounds recorded on a DAT tape were fed into the computer hard-disk (PMac 7200/120) and digitally converted to System 7 sound ®les for the presentation. All stimuli were digitally stored on the computer hard-disk and reproduced to listeners using a program written with a psychological experimental software, psychlab, through an open type headphone [16] (AKG, K1000). The listening level was kept constant at approximately 70dB(A). The processed stimuli were low-pass ®ltered at 10 kHz by the bandpass ®lter (Kronhite 3550), to remove the noise from the output of the computer, and ampli®ed (Onkyo Integrated Stereo Ampli®er A-9210) before being presented to a listener through headphones. 2.2. Procedure A 2AFC and 2-down, 1-up adaptive experimental procedure were employed. Two consecutive correct responses lead to the smaller frequency change, while the magnitude of frequency change was increased at every wrong response. The experiment was designed to terminate after achieving 12 reversals for each listener. The ®rst three reversals were discarded and the arithmetic mean of the last nine trials was taken as a JND.
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During the assessment listeners were seated in a quiet chamber and a computer keyboard and monitor were provided for the presentation and collection of responses. A 30 min practice session was provided to the all listeners, just before the main test session, to make them familiar with the stimuli and operation of the program. The listener's task was simply to determine which of a pair of signals presented to him/her through headphones had a frequency change. No feedback on the accuracy of the response was provided to the listeners in the main test while feedback was used in the practice session. 2.3. Stimuli In the present work, 1 and 4 kHz short duration tones with linear ascending frequency changes were used (refer to Fig. 3). Preliminary tests using two dierent frequency change patterns (ascending and descending linear frequency changes) at three frequencies (0.25, 1 and 4 kHz) in an anechoic and reverberant sound ®eld (a reverberation chamber and lecture room) suggested that the ascending frequency change is more useful for this purpose: no signi®cant eect of the acoustics of a listening environment was found at 0.25 kHz and remarkable individual dierences were observed for the descending frequency change. This result con®rms the ®ndings from the other researchers. Gardner and Wilson [17] suggested the existence, in the auditory system, of channels speci®c to upward FM and downward FM in a frequency change detection experiment using upward and downward sweeping signals. Dooley and Moore [18,19] found discrepancies between listeners, and argued such discrepancies were caused by the dierences in stimuli. Also, signi®cant dierences
Fig. 3. Time±frequency plot of the stimuli used in the experiment.
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in the perceived frequency change between the anechoic and reverberant sound ®elds were observed for the mid and high frequency short duration tones. The frequency of the comparison stimulus was linearly swept from 1 kHz to the speci®ed target frequency throughout its duration. The standard stimuli had a constant frequency of 1 kHz from the onset to the oset (see Fig. 3). The duration of the stimulus used in this test was 110 ms. To minimise switching transients at the onset and oset of the stimuli, rise/decay times of 5 s were used. The amount of frequency change of the comparison stimulus was increased by 0.25 Hz from 1 k to 1.001 kHz and then by 1Hz from 1.001 to 1.028 kHz for the 1 kHz short duration tones at every wrong response from listeners. For the 4 kHz short duration tones, a 0.2 Hz step was applied to the region between 4 and 4.002 kHz and then increased by 2 Hz from 4.002 to 4.030 kHz. 2.4. Listeners Two male postgraduate listeners including one of the authors (listener DU) and two female undergraduate students voluntarily participated in this experiment. Three listeners were known to have musical backgrounds. One male listener, DC, and one female listener, JH, were amateur musicians (BMus) and the other female listener, MK, also had a background as an amateur music performer (AMEB Grade 7). DU was 34 years old and the other listeners were between 20 and 30. They had previously taken part in other auditory perception experiments. They had normal audiometric threshold (ISO 389, 1991) at standard octave frequencies from 0.125 to 8 kHz. 2.5. Results and discussion The purpose of this work was to investigate the eect of the acoustics of the listening environment on the detection threshold of frequency change and to ®nd out which frequency range is most in¯uenced by the room re¯ections. The thresholds of detection for frequency change at dierent listening positions are shown in Figs. 4 and 5. Listener JH and DU showed lower JNDs at every listening position in a reverberation chamber than in an anechoic chamber at 1 kHz. Also, better performance was observed in the JNDs of listener DU and DC, at P0 in the reverberation chamber than in the anechoic condition, for the 4 kHz signal. This result suggests that the re¯ections from room surfaces may improve the listeners' ability to perceive frequency change. However, it appears that there are signi®cant individual dierences in perception and a dependency on the frequency of the sounds used. To verify if the room re¯ections can contribute to the improvement of the frequency change perception, more extensive experiments using arti®cial sound ®elds, featuring a number of re¯ections, are required. In the reverberation chamber, three listeners showed lowest JNDs at the listening position of P0, while the listener, JH showed slightly better performance at P1 for the mid-frequency (1 kHz). Position P2 turned out to be the worst listening position for all listeners except DC. Listener DC showed his worst JNDs at P1 and P4 at 1 kHz,
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Fig. 4. Measured JNDs for 1 kHz frequency changed pure tones in an anechoic chamber and at ®ve different positions in a reverberation chamber (A: anechoic chamber, P0 to P4: dierent positions in a reverberation chamber) (the errors bars showstandard errors).
Fig 5. Measured JNDs for 4 kHz frequency changed pure tones in an anechoic chamber and at ®ve different positions in a reverberation chamber) (A: anechoic chamber, P0 to P4: dierent positions in a reverberation chamber) (the error bars showstandard errors).
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and P1, P2, P3, P4 at 4 kHz. Two questions, as follows, were examined using a t-test. ``Do the dierent listening positions in a room have any signi®cant eect on the listeners perceptions of frequency change?'' and ``Is any listener response signi®cantly dierent from any other at both 1 and 4 kHz?'' A t-test (paired means comparison) showed that a signi®cant dierence (p<0.05) exists between the perceptions of frequency change at dierent listening positions (Table 1) for both 1 and 4 kHz. In particular, it was found that the measured JNDs at P0, at which listeners showed the lowest JNDs, were signi®cantly dierent from the JNDs at the other positions. More signi®cant and consistent data across the listeners were obtained at the high frequency (4kHz), where all listeners showed the lowest JNDs at P0 and the highest at P1. Although the listener MK showed slightly better performance at P2 and P3 than at P0, the dierences between these three listening positions were negligible (less than 0.2 Hz). The position P4 was the next most dicult position to detect frequency change at 4 kHz. Also, there exist some individual dierences in frequency perception between listeners. A t-test showed that the JNDs of listener MK were signi®cantly dierent from the others at 1 kHz (p<0.05), but no signi®cant dierence was found between the other listeners. At 4 kHz, listener JH showed signi®cantly dierent sensitivity to the other listeners (p<0.05) (Table 2). The in¯uence of the listening position on the JNDs of these two listeners at each position was found to be more remarkable than the other listeners. This result suggests that listeners in a room may have dierent sensitivities to the room re¯ections, especially for the perception of frequency change. Normalised JNDs from all listeners with mean and standard errors for both 1 and 4 kHz, at dierent listening positions, are shown in Figs. 6 and 7. The normalised JNDs were obtained by accumulating the JNDs at dierent positions in the reverberation chamber for each listener, and then dividing by the JND obtained in the anechoic room for the same listener. At both frequencies P0 turned out to be the best position for detecting frequency changes. For 1 kHz, listeners showed the highest JNDs at P1 and P2, while P1 was the worst listening position for detecting frequency Table 1 The signi®cance of listening positions in a room on the perception of frequency change using a paired means comparison test Frequency
Listening positions
t-value
p-value
1 kHz
P0 P0 P0 P2
vs P1 vs P2 vs P3 vs P3
ÿ2.817 ÿ4.395 ÿ2.487 2.142
0.0119 0.0004 0.0236 0.0470
(<0.05) (<0.001) (<0.05) (<0.05)
4 kHz
P0 P0 P0 P0 P1 P2
vs P1 vs P2 vs P3 vs P4 vs P3 vs P4
ÿ4.547 ÿ2.846 ÿ2.156 ÿ5.620 2.684 ÿ2.204
0.0003 0.0112 0.0457 <0.0001 0.0157 0.0416
(<0.001) (<0.05) (<0.05) (<0.05) (<0.05)
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Table 2 The signi®cance of listeners' perceptions of frequency change on frequency using a paired means comparison test Frequency
Listeners
t-value
p-value
1 kHz
MK vs DC MK vs DU MK vs JH
2.86910 ÿ2.34940 ÿ2.86910
0.005937 (<0.01) 0.022637 (<0.05) 0.005937 (<0.01)
4 kHz
JH vs DC JH vs DU JH vs MK
2.1680 ÿ2.5130 ÿ1.4480
0.04070 (<0.05) 0.01940 (<0.05) 0.15990 (>0.05)
Fig. 6. JNDs for change in frequency of 1 kHz pure tone measured at dierent positions in a reverberation chamber and normalised with the JNDs obtained in an anechoic chamber (P0 to P4: dierent positions in a reverberation chamber) (the error bars showstandard errors).
change at 4 kHz. The dierences between the other four positions (P1 to P4) at 1 kHz seem to be negligible (normalised JND values exist within the maximum and minimum standard error range of the other listening positions). It is known that the sound level produced by a source of constant power output produces a greater level in a room with larger reverberation time [20]. Also, the directivity of the source introduces loudness dierences to the dierent listening positions in a room. Generally, an increase in loudness improves the detectability of sounds. This was not a factor in the present work because of the use of an omni-directional source. It is also known that early room re¯ections, which arrive within 80 ms after the direct sound, help in the perception of speech. However any signi®cant dierence in early sound ®elds was not observed between the impulse responses recorded at the listening positions. In the present experiment, re¯ections with dierent delay times and spectral structures were introduced to each listening position by varying the listening positions whilst maintaining the same distance from the source. Therefore, the
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Fig. 7. JNDs for change in frequency of 4 kHz pure tone measured at dierent positions in a reverberation chamber and normalised with the JNDs obtained in an anechoic chamber (P0 to P4: dierent positions in a reverberation chamber) (the error bars showstandard errors).
observed dierences between JNDs at listening positions seem to be due to the central masking (or reinforcing) caused by the delayed re¯ections at the two ears with dierent temporal and spectral structures. 3. Experiment 2. Preferred positions for listening to music Listeners' preferred positions for listening to music were investigated using anechoic music recordings featuring four dierent solo instruments. Three listening positions were chosen based on the results of Experiment 1. 3.1. Experimental arrangement and procedure In this experiment, three listening positions (P0, P1, and P2), which bracket the worst and the best position for discriminating frequency change, were visited again for the preference test, considering the results of the Experiment 1. In the previous experiment, P0 was found to be the best position for discriminating the frequency change at 1 and 4 kHz, while P2 and P1 were the worst positions for discriminating 1 and 4 kHz tones, respectively. Three anechoic music recordings (cello, guitar, and xylophone) were chosen from ``Music for Archimedes'' [21] and one anechoic music recording (Flute) ``DENON Professional CDs'' [22] as follows: 1. 2. 3. 4.
Theme, Cello, composed by Weber Bourre, Guitar, composed by J.S. Bach Sabre Dance, Xylophone, composed by Khachaturian Syrinx, Flute, composed by Debussy
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The length of the music sample used for preference testing is important. It has been found earlier that sounds or pieces of orchestral music between 8 and 35 seconds in duration gave rise to reproducible emotional responses in listeners [23]. However, the use of long music samples should be avoided because of the short-term auditory memory. Therefore, the duration of music samples used in this work was chosen to be approximately 10 s. These music excerpts were reproduced in the reverberation chamber through an omni-directional speaker (Soundspere Model 110) and recorded on a DAT tape using a KEMAR dummy head. Exactly the same recording setup as in the Experiment 1 was used for the recording. The recording levels at each listening position were controlled to be equal when measured with a single 1/2 inch free-®eld microphone (B&K, Type 4190). The music samples, recorded on a DAT tape, were fed into the computer hard-disk (PMac 7200/120) and digitally converted to System 7 sound ®les for the presentation. A two-alternative forced choice method (2AFC) was used. Music samples recorded at each position were paired with each other and stored on a computer hard-disk. They were presented to the listeners in a random order through headphones (AKG, K1000). Each listener in the present experiment undertook four dierent sessions, which took approximately 1.5 h to complete. Each session featured a dierent musical instrument (cello, ¯ute, guitar, and xylophone). The session order was randomised for each listener. Also, the order of presenting music pairs was randomised within each session. Each pair of music recordings was presented four times in a session. Listeners were seated in a quiet room and a computer keyboard and monitor were provided to control the experiment. The listener's task was simply to determine which of two recordings in a given pair was preferred. Listeners were allowed to hear the sounds as many times as they wished before making a choice. 3.2. Listeners Two male postgraduate listeners including one of the authors (listener DU) and four female undergraduate students voluntarily participated in this experiment. Three listeners (DC, DU, and MK) who participated in the Experiment 1, came back for this test. All the listeners, except DU, were known to have musical backgrounds. A male listener, DC is an amateur musician (BMus) and the female listener, CHJ, is a music student. The other female listeners were known to have backgrounds as amateur music performers (AMEB Grade 7). Four listeners had previously taken part in other auditory perception experiments. 3.3. Results and discussion In the present experiment, a paired comparison test (2AFC) was carried out for obtaining listeners' subjective preferences. The paired comparison method is extremely useful, since listeners can simply judge which of two stimuli they prefer. Preference scores can be obtained for each pair of stimuli by giving scores of +1 and ÿ1, corresponding to positive and negative judgements, respectively, for each
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presentation of the pair. The normalised preference score can be produced by accumulating the scores for all stimuli (S) tested and all listeners (L), and then dividing by the factor L(Sÿ1) for further statistical analysis. However, the preference score itself cannot be represented as a linear scale. To obtain the comparable preference value which can be regarded as a linear psychological distance between stimuli, the law of comparative judgment [24,25] provides a basis [26]. In the present study, the preference scores were linearly scaled based on the law of comparative judgment. Eq. (1) shows the complete form of the law of comparative judgment. p 2 j k2 2rjk j k 1 Sk ÿ Sj where, Sj, Sk: xjk: j , k : r:
the psychological scale values of the two compared stimuli. the normal deviate corresponding to the theoretical proportion of times stimulus k is judged greater than stimulus j discriminal dispersion of stimulus j and k. the correlation between the pairs of discriminal processes dj and dk.
The measured preferences are presented in Fig. 8. It was found that listening position, P1, was the preferred position for listening to the guitar and xylophone. However, for the ¯ute, P0 was the preferred listening position. Any major eect of listening position on the preference was not found for the cello recording. This result suggests that there exist preferred positions for listening to music in a room, depending on the instruments being played. Since the recordings were made in a highly controlled listening environment, the arrival time and relative level of re¯ected
Fig 8. Preferred positions for listening to dierent instrumental music (P0 to P2: the listening positions in a reverberaiton chamber).
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sounds (introduced by the varied distance from the room surfaces to the dierent listening positions) seem to be a major factor in¯uencing the listeners' preferences. Another possibility is that listeners might have dierent criteria for each instrument type. Discussions with the listeners after the listening tests support this possibility. Most listeners reported that they made their choice based mainly on the clarity of sounds for guitar and percussion (xylophone) while each listener seemed to apply dierent criteria for the cello and ¯ute, such as binaural dissimilarity and the degree of resonance. In terms of temporal structure, plucked string or struck instruments are characterised by a sharp attack and long decay while wind instruments and bowed strings have completely opposite characteristics. Meyer [27] suggested, after his analysis of individual instruments, that strings and, particularly, woodwinds need the reverberation of the room as their own decay times are shorter. However this is not the case in this experiment, since the variation of reverberation time should be limited within a room. The pitch of the plucked string and percussion instrument, during the attack, is very dicult to detect [5,6]. Therefore, clear audibility of the attack seems to form an important part in preference for guitar and percussion sounds, while the masking of decaying procedure by the clouds of re¯ections may in¯uence the preference for cello and ¯ute. As discussed earlier, P0 was found to be the best position for the listener to discriminate the frequency change at both mid and high frequencies (see Figs. 6 and 7). P1 was the worst listening position for the frequency change perception at 4 kHz. The frequency change JNDs at 1 and 4 kHz did not show any signi®cant correlation with listeners' overall preferences. Therefore normalised JNDs and the magnitude of JND dierences were calculated and their relationships with scale values of preference were tested using a correlation matrix. The magnitude of JND dierences (JND(|PnÿA|)s) were obtained by taking the magnitude of the dierence between the JNDs at each listening position in the reverberant room and in the anechoic chamber, for each listener. Fig. 9 shows the accumulated magnitudes of JND dierences calculated at three listening positions in a reverberation chamber [The data points in Fig. 9 include the accumulated JND(|PnÿA|) values from four listeners involved in the previous experiment]. The question of whether the preferred listening positions are signi®cantly related to the magnitude of the JND dierences (JND(|PnÿA|)s) was examined using a correlation matrix (Table 3). To determine if coecients are statistically dierent from zero, a Fisher's r to z transformation was carried out. It was found that correlation coecients exceeding r=0.335 were signi®cant at the level of p<0.005. No signi®cant correlation was found between the JND(|PnÿA|)s at 1 kHz and the preferred listening positions. A signi®cant correlation was found between JND(|PnÿA|)s at 4 kHz and preferred listening position. The JND(|Pn ÿA|)s at 4 kHz are negatively correlated with the scale values of preference for cello and ¯ute (with the correlation coecients of ÿ0.364 and ÿ0.496, respectively). Positive correlations between the scale values of preference for xylophone and guitar and the JND(|PnÿA|)s at 4 kHz were found to be signi®cant (with the correlation coecients of 0.335 and 0.444, respectively).
D. Jeong, F.R. Fricke / Applied Acoustics 60 (2000) 63±79
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Fig. 9. The JND (|PnÿA|) at three listening positions in a reverberation chamber at 1 and 4 kHz. JND (|PnÿA|) was obtained by taking the magnitute of the dierence between JNDs at each position in a reverberation chamber and in an anechoic chamber for each listener. P0P2 present listenin positions in a reverberation chamber. (Error bars showstandard errors.)
Table 3 A correlation matrix that explains the correlation between the scale values of preference for dierent instrumental music and JND (jPn ÿ Aj) at 1 and 4 kHz Cello
Flute
Guitar
Xylophone
JND jPn ÿ Aj at 1 kHz
ÿ 0.046 (p=0.6946)
ÿ0.131 (p=0.2641)
0.156 (p=0.1820)
0.146 (p=0.2122)
JND jPn ÿ ÿAj at 4 kHz
ÿ0.364 (p<0.005)
ÿ0.509 (p<0.0001)
0.444 (p<0.0001)
0.335 (p<0.005)
4. Conclusions The experimental results show that, in nominally the same acoustics, a listener's ability to perceive frequency change appears to vary depending on the listening position in the room and the dierence limen of frequency change detection has a relationship with the preferred acoustics for a given instrument. This has important implications for developing a method of evaluating the acoustic quality at dierent seats in a concert hall or in dierent concert halls. It also has important implications for the design of a concert hall. A consistent eect of listening position on the perception of frequency change was found at the high frequency (4 kHz). The type of musical instrument being played appears to be one of the important factors for listeners in deciding their preferred listening position in a room. The high frequency JND may be a good indicator of the acoustics of spaces for instruments which are plucked or struck, i.e. which have many overtones of high intensities. Listeners may prefer listening to the guitar and percussion playing in the listening environment which does not provide good
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frequency change discrimination (it is likely that the good discriminability of frequency change may not be a prerequisite of a listening environment for listening to plucked strings and percussions). This result suggests that a listening environment, in which listeners can perceive frequency changes well at high frequency, may be good for listening to cello and ¯ute. However, clear audibility of frequency change at high frequency appears not to be suitable for listening to guitar and xylophone. The correlation results suggest that an anechoic listening environment would be preferable for listening to ¯ute and cello. This is unlikely to be the case. It would seem necessary to use this method of assessment in conjunction with reverberation time. Alternatively there may be an optimum JND(|PnÿA|) for a given instrument. The interaural relationships such as ITD (interaural time delay) and ILD (interaural loudness dierence) caused by re¯ections from the surface of the room seem to have a signi®cant eect on the perception of frequency change, but this was not directly veri®ed in the present work. To verify this, JNDs will be measured with dierent arrival times of re¯ections and dierent relative levels of re¯ections using a more controlled experimental setup. References [1] Beranek LL. Concert hall acoustics Ð 1992. J Acoust Soc Am 1992;92(1):1±39. [2] Goad PJ, Keefe DH. Timbre discrimination of musical instruments in a concert hall. Music Perception 1992;10(1):43±62. [3] Bilsen FA. Thresholds of perception of repetition pitch. Conclusions concerning coloration in room acoustics and correlation in the hearing organ. Acustica 1968;19:7±32. [4] Lieberman AM, Delattre PC, Gerstman LJ, Cooper FS. Tempo of frequency change as a cue for distinguishing classes of speech sounds. J Exp Psychol 1984;52:127±37. [5] Freedman MD. Analysis of musical instrument tones. J Acoust Soc Am 1967;41:793±806. [6] Luce D, Clark M. Physical correlates of brass instrument tones. J Acoust Soc Am 1967;42:1232±43. [7] Ozimek E. Physical and psychoacoustic evaluation of FM signals propagating in a room. Proceedings of 13th ICA, 1989. p. 177±180. [8] Ozimek E, Sek A. AM and FM dierence limens and their reference to amplitude-frequency changes of a sound in a room. Acta Acustica 1996;82:114±22. [9] Ozimek E, Rutkowski L. Deformation of frequency modulated (FM) signals propagating in a room. Applied Acoustics 1989;26:217±30. [10] Rutkowski L, Ozimek E. Linear and jump frequency changes of a signal in a room. Archives of Acoustics 1995;20(2):115±38. [11] Rutkowski L, Ozimek E. Linear and sinusoidal frequency changes of a signal in a room. Acta Acustica 1997;83:881±90. [12] Marshall AH. A note on the importance of room cross-section in concert halls. J Sound Vib 1967;5(1):100±12. [13] Barron M. Subjective study of British Symphony Concert Hall. Acustica 1988;66(1):1±14. [14] Yamaguchi K. Multivariate analysis of subjective and physical measures of hall acoustics. J Acoust Soc Am 1972;52(5):1271±9. [15] Levitt H. Transformed up-down procedures in psychoacoustics. J Acoust Soc Am 1970;49:467±77. [16] Mùller H. Fundamentals of binaural technology. Applied Acoustics 1992;36:171±218. [17] Gardner RB, Wilson JP. Evidence for direction-speci®c channels in the processing of frequency modulation. J Acoust Soc Am 1979;66(3):704±9. [18] Dooley GJ, Moore BCJ. Detection of linear frequency glides as a function of frequency and duration. J Acoust Soc Am 1988;84(6):2045±57.
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