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The influence of first reflection distribution on the quality of concert halls
Applied Acoustics 35 (1992) 233-245
The Influence of First Reflection Distribution on the Quality of Concert Halls
A. Fischetti & J. J o u h a n e a u Laboratoire d'Acoustique, Conservatoire National des Arts et M6tiers, 292 rue St Martin, 75141 Paris Cedex 03, France (Received 8 July 1991; accepted 19 September 1991)
A BSTRA CT Experiments were carried out concerning relationships between subjective clarity and criterion C80 in a variable acoustical room. Some discrepancies appeared and the effect of early reverberant energy was then investigated with artificial reverberation. It wasfound that subjective clarity could be increased by a great interspike gap (GIG).
1 INTRODUCTION Early reverberant energy in a room is usually assumed to be perceived in conjunction with direct sound and to improve clarity. Subjective clarity of music is often estimated by criterion C80, defined as the ratio of the energy arising within the first 80 ms to the remaining energy. In the first part of this study we have investigated relationships between subjective clarity, criterion C80 and geometrical parameters of ESPRO, the variable geometrical room of IRCAM (Paris). Since discrepancies between C80 and subjective clarity were observed, the role of early reverberant energy has been revisited by recreating artificial reverberation. Energetic distribution included in the first 80 ms was changed, criterion C80 being kept constant. The results led to an interpretative analysis of subjective clarity conception. Some hypotheses taking into account time delays of early reflections were proposed. 233 Applied Acoustics 0003-682X/92/$05.00 © 1992 Elsevier Science Publishers Ltd, England. Printed in Great Britain
A. Fischetti, J. Jouhaneau
234
2 RELATIONS BETWEEN SUBJECTIVE CLARITY GEOMETRICAL PARAMETERS OF ESPRO
AND
2.1 Method
2.1.1 Physical measures E S P R O is a p a r a l l e l e p i p e d i c r o o m (24 m l o n g a n d 15 m wide) w h o s e walls are c o m p o s e d o f o r i e n t a b l e t r i h e d r a l s w i t h a b s o r p t i v e - d i f f u s i n g - r e f l e c t i n g sides. T h e h e i g h t o f t h e ceiling is v a r i a b l e f r o m 2.5 to 12.5 m. T h e f o l l o w i n g e x p e r i m e n t w a s realized b y J. J o u h a n e a u , J.-P. Jullien a n d J. D. P o l a c k . A n TABLE 1 Definitions
Geometrical parameters of ESPRO H, height of ceiling: high (10m), median (7.5 m), low (5 m) A/D, ratio of absorptive to diffusing panel numbers: strong (oo), median (200%), weak (30%) P, receiver position in the room: P1 and P3 in the axis of the room, respectively at 7 and 16m from the loudspeaker line, P2 close to a lateral wall at 12m from the loudspeaker line G, absorptive panel distribution gradient, which indicates that absorptive panels are either predominantly grouped or uniformed distributed in the room (three values) R, percentage of reflecting panels (three values)
Acoustic criteria C80 (dB) = 10 log ff°'°a p2(t) dt/S~.oap2(t) dt) C50 (dB) = 10 log (S°'°5 p2(t) dt/~p2(t) dt) S/B (dB) = 10 log (~o.o95a(t)p2(t) dt/~.o9 s p2(t )dt) Ca(t) is a weighting function, a(0) = 1, a(0.095) = O)
Perceptive parameter Subjective clarity was evaluated by subjects according to the following definition: 'Subjective clarity is high if sound details and attacks are well perceived. This characteristic is often designed by definition or precision. You have only to evaluate subjective clarity independently of other parameters, like tone coloration for example'
Parameters of artificial reverberation Distribution: simulated impulse response, or this impulse response convolved with an anechoic musical sample Direct impulse: burst of white noise of 5 ms duration which simulated direct sound Spike: burst of white noise which simulated a reflection Temporal energetic barycentre computed in the first 80 ms (ms): S°'°a tp2(t)dt/S°o'°ap2(t)dt Spike time delay (ms): difference of time between spike and direct sound Spike width (ms): duration of white noise burst (narrow, 5 ms; wide, greater or equal to 10ms) Interspike gap (ms): difference of time delay between two successive spikes GIG: great interspike gap (ms): interspike gap greater than other interspike gaps
First reflection distribution in concert hails
235
anechoic musical signal was diffused in ESPRO through two loudspeakers and recorded at three positions with two stereophonic microphones placed on each side of a dummy head. Impulse responses were also measured and 15 acoustical criteria were computed. This procedure was repeated for 81 configurations of ESPRO, chosen by different combinations of three values of each geometrical parameter (see Table 1). Preliminary tests have shown that variations of absorptive panel distribution in the room were poorly discriminated through listening tests, while reflecting and diffusing panels produced nearly identical effects. So we only selected 31 configurations based upon variations of position in the room (P), ratio of absorptive to diffusing panels (A/D) and ceiling height (H). Only one criterion (C80 for mean octave band value) was investigated for the present study.
2.1.2 Perceptive tests The message under test was a 15-s duration piece of Schubert's 14th string quartet. Sound samples were presented by pairs and three kinds of pairs were arranged, according to the geometrical parameter A/D, P or H. All pairs were mixed in the sequence and the place of samples in each pair was randomized. Since musical samples were recorded in ESPRO with a dummy head, they were diffused through headphones to preserve the two channel characteristics. Ten sound engineering students, poorly experienced in listening tests, were required to discriminate subjective clarity between each sample on a scale ranging from - 1 0 to +10 with reference to the first sample of each pair. Subjective clarity was defined as described in Table 1. Subjects could listen to each pair as long as they wished. 2.2 Results and discussion
2.2.1 Results Figure 1 illustrates for each geometrical parameter the average variation of subjective clarity (responses of all subjects were normalized) as a function of C80 variation (dB). Absorption AID. AID varied from weak to median values while the ceiling was kept high. Increases of C80 were identical for all receiver positions. However, subjective clarity increased markedly for distant position P3 and decreased for excentred position P2. Ceiling height H. H varied from median (7.5 m) to low (5 m) values in a weakly absorptive configuration. C80 was emphasized on position P1 or P3, but subjective clarity was unchanged in position P2 in spite of high variation of C80.
236
A. Fischetti, J. Jouhaneau
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+ 10 in reference to the first sample of the pair). HP1-HP2-HP3: decrease of ceiling height (respectively in position P1, P2, P3); A/D P1-A/D P2-A/D P3: increase of room absorption (respectively in position P1, P2, P3); P2/31-P2/3 h: variation of receiver position from P2 to P3 (respectively in low and high ceiling configuration).
Receiver position P. Receiver position varied from P2 to P3 in a weakly absorptive high ceiling P2 position would seem greater than predicted by in excentred position P2. However, subjective clarity decreased in high ceiling configuration. 2.2.2 Analysis and discussion These experiments would seem to reveal significant discrepancies between subjective clarity and C80. In particular, subjective clarity in the weakly absorptive high ceiling P2 position would seem greater than predicted by C80. In effect, inversely to C80, subjective clarity was (i) greater in P2 than P3. This could suggest that subjective clarity in position P1 weak absorption high ceiling is greater than predicted by C80. Other criteria, like reduces less the subjective clarity in near position P1 than in distant position P3. This could suggest that subjective clarity in position P1 weak absorption high ceiling is greater than predicted by C80. Other criteria, like C50 and S/B (see Table 1), were also measured in ESPRO, but both of them were strongly correlated to C801 and consequently not to subjective clarity. So this could give rise to some remarks about criterion C80. The latter compare auditory processes to energy measurement systems in the first 80ms. But (i) 80ms is an arguable limit for there are few reports about auditive time constants, (ii) early reflections might induce different perceptions (because of masking for example) and perceptive processes could not be reduced to a mere sum of energies and (iii) temporal patterns of early reflections could also be relevant in perception. Consequently it would
237
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Fig. 2. Early part of some impulse responses measured in ESPRO (median absorption). (a) Pl-high ceiling; (b) P2-high ceiling; (c) Pl-low ceiling; (d) P3-high ceiling.
seem that the conception of criterion C80 had to be revisited. That led to a study of impulse responses of ESPRO (Fig. 2): two very early reflections followed by a 25 ms interspike gap can be seen in position P2, while strong reflections are separated by about 50 ms interspike gaps in P1, and they are rather grouped in P3. This could suggest a relevance of early reflection temporal patterns in perception which is not taken into account by C80.'* More accurate examination of this question was the purpose of the following experiment.
3 STUDY OF FIRST REFLECTIONS 3.1 M e t h o d
3.1.1 Impulse response simulation A computer was used to construct different kinds of impulse responses. At first each reflection was simulated by a 'Dirac' function. But time delay variations were poorly discriminated and that led to building o f impulse
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A. Fischetti, J. Jouhaneau
responses by 'shaping' a white noise. Direct sound and early spikes were then simulated by white noise bursts. Diffuse sound started 80 ms after direct sound with a 30 dB lower level. It was obtained by 'cutting' white noise with an exponential curve (RT was 1 s). Impulse response was then convolved with both channels o f a stereophonic anechoic musical sample and the result was listened to through headphones. Early reflections simulated by such a m e t h o d are more realistic, for actual reflections in a concert hall are not 'Dirac' functions. The stochastic character o f a diffuse field is also respected, and very fair sound quality is obtained. Parameters o f impulse response are defined in Table 1. 3.1.2 Perceptive tests The anechoic signal was a 3-s duration sound o f drums. Samples were presented by pairs to subjects, and each pair was diffused three times. Five tests between the two samples were performed; modified parameters were: Test 1: Impulse response shape. 17 impulse responses were built with different early time-energy characteristics but the same energetic a m o u n t in the first 80 ms (C80 remained constant). Samples were c o m p a r e d to the same reference. Twelve subjects were asked to report which o f the two samples seemed clearest. They were also required to rate the overall difference between the two samples on a scale ranging from 0 to + 10. Test 2: Time delay or spike crest level. Two kinds o f spikes, either n a r r o w (5ms duration) or wide (10ms), were used. The same energy variation p r o d u c e d either by width or crest level change was also tested. Sixteen subjects had to rate the difference o f subjective clarity on a scale ranging from - 10 to + 10 in reference to the first sample o f each pair. They were also required to rate the overall difference between the two samples on a scale ranging from 0 to + 10. TABLE 2
Time Delays and Interspike Gaps of Distributions Used in Test 3 (last spike time delay was increased by 5 ms steps) Case
Time delay (ms) of spike no.
Interspike gap (ms) no.
no.
1 2 3 4
1
2
3
1
2
3
25 10 10 5
30-60 25 15--45 15
-30-60 -20-50
25 10 10 5
5-35 15 15-35 10
-5-35 -5-35
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Fig. 3. Early part of some distributions used in Test 1. Results of discrimination and subjective clarity are listed in Table 4. Direct sound is simulated by the first white noise burst of 5 ms duration.
Test 3: Latest spike time delay. In a distribution composed of several narrow spikes (time delays are listed in Table 2) the latest spike time delay was increased by 5 ms steps, and three highly experienced subjects had to rate the difference in subjective clarity on a scale ranging from - 10 to + 10 in reference to the first sample. Test 4: Spike width. Two kinds of situations were tested, spike arising either with or without a time delay after direct sound. The spike level was equal or --15 dB lower than direct sound. The width was increased by 5 ms steps, and three highly experienced subjects had to rate the difference in subjective clarity on a scale ranging from - 1 0 to + 10 in reference to the first sample. Test 5: Respective time delays of several narrow spikes. Three impulse responses based upon variations of time delays of four narrow spikes (Table TABLE 3 Time Delays and Interspike Gaps of Distributions Used in Test 5 Distribution
Time delay (ms) of spike no.
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1
2
3
4
1
2
3
4
5 5 5
25 25 25
55 45 45
65 75 65
5 5 5
15 15 15
25 15 15
5 25 15
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240
3) were built. Seven subjects were asked to report which sample sounded clearest. Spike crest levels were equal to direct sound, except in Test 2, where it was changed from - 1 0 to 0 dB below direct sound, and in Test 4, where it was - 15 dB lower. 3.2 Results 3.2.1 Discrimination Test 1. Average discrimination (between 0 and I0) appeared to be closely related to differences of temporal energetic barycentre computed in the first 80 ms (see Table 1). However, some exceptions appeared. Figure 3 shows, for example, distribution Tl(c), which was better discriminated from reference Tl(a) than from Tl(b) and Tl(d) in spite of the same barycentre values (see Table 4). Test 2. Average discrimination is plotted in Fig. 4 against spike time delay. When the spike was narrow, increases of time delay (nos 1, 2, 3) were better discriminated than increases of crest level (nos 7, 8, 9). Time delay variations of a wide spike (nos 4, 5, 6) were poorly discriminated when it arose less than 8-
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Fig. 4. Test 2: results of discrimination (evaluated on a scale ranging from 0 to +10). Nos I - 2 - 3 (respectively nos 4-5-6): increase of time delay for a narrow spike o f 5 ms duration (respectively wide spike o f ! 5 ms duration). In nos 1-6 the spike crest level was equal to direct sound. Time delay variation was 15ms for nos 1 and 4, 10ms for nos 2 and 5, and 25ms for nos 3 and 6. Nos 7 - 8 - 9 (respectively no. 10): increase o f narrow spike (respectively wide spike of 25ms) crest level from - 1 0 to 0 d B below direct sound. No. 11: increase of width of a narrow spike (with the same energetic variation as no. 10).
241
First reflection distribution in concert halls TABLE 4 Distributions Used in Test 1 (Fig. 3)
Temporal energetic Average discrimination Subjective clarity: with reference Tl(a) % of subjects which barycentre (ranging from 0 to 10) evaluated distribution (ms) clearer than reference Tl(a) Tl(a) Tl(b) Tl(c) Tl(d)
32-97 24-02 22.14 25.77
-2.5 6.0 3-0
-25 16.7 66.7
20 ms after direct sound. The width increase of a narrow spike (no. 11) would seem better discriminated than a crest level increase of a wide spike (no. 10) in spite of the same energy variation. 3.2.2 Subjective clarity Test 1. Subjective clarity was roughly negatively correlated to temporal energetic barycentre. But some distributions provided a non-explained greater subjective clarity; for example, Tl(d) was evaluated clearer than Tl(b) and Tl(c) in spite of some barycentre values (Fig. 3 and Table 4). Test 2. Figure 5(a) illustrates variations of subjective clarity as a function of spike time delay. Increase of time delay (nos 1-6) reduced subjective clarity, except when the spike arose about 25-30 ms after direct sound (nos 2 and 6). Increase of crest level emphasized subjective clarity when the spike was narrow (nos 7, 8, 9) and reduced it when it was wide (no. 10). Subjective clarity was also emphasized by an increase of narrow spike width (no. 11). Test 3. Figure 5(b) shows variations of subjective clarity induced by the last spike time delay increase. Subjective clarity decreased in cases nos 1 and 2, and increased in cases nos 3 and 4 (see Table 2). Test 4. Figure 5(c) depicts variations of subjective clarity as a function of spike width. The greatest subjective clarity was related to an optimum width. The latter seems to depend on spike level (but not on spike time delay): lO15ms for the same level as direct sound (nos 1 and 3) and 35ms for the - 1 5 dB level (no. 2). Test 5. 71% of subjects evaluated distribution T5(a) clearer than T5(b) and 100% evaluated T5(b) clearer than T5(c) (see Table 3).
A. Fischetti, J. Jouhaneau
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Fig. 5. (a) Test 2: subjective clarity variation. Percentage ofsubjects which evaluated second sample clearer than first sample. Pairs representation is the same as in Fig. 4. In (b) and (c) a subjective scale was constructed in the following way: last spike time delay (b) or spike width (c) was increased by 5 ms steps and subjects evaluated subjective clarity on a scale ranged from - 1 0 to +10 in reference to the first sample of each pair, (b) Test 3: subjective clarity variation as a function of the last spike time delay (remaining time delays are listed in Table 2). (c) Test 4: subjective clarity variation as a function of spike width. 1, Spike crest level equal to direct sound with a 65 ms time delay; 2, spike crest level 15 dB lower than direct sound without a time delay; 3, spike crest level equal to direct sound without a time delay.
3.3 D i s c u s s i o n
3.3.1 Analysis Discrimination. In Test 1 the best d i s c r i m i n a t e d d i s t r i b u t i o n , Tl(c), differed f r o m the r e f e r e n c e T l ( a ) b y a n a r r o w e r e a r l y spike w i d t h (20 ms) t h a n T l ( b ) (40 ms) (Fig. 3). Test 2 w o u l d suggest t h a t n a r r o w spike t i m e delays are the best d i s c r i m i n a t e d p a r a m e t e r s . T h e s e results t e n d to indicate a relative i m p o r t a n c e o f early e n e r g y t e m p o r a l p a t t e r n . Subjective clarity. T h e results o f Test 1 seem to s h o w t h a t a g r e a t e r e a r l y gap, T l ( d ) , m i g h t increase subjective clarity. T h a t also a p p e a r e d in Test 2 w h e n a n a r r o w spike a r o s e 20 ms a f t e r d i r e c t s o u n d (Fig. 5(a)). Subjective
First reflection distribution in concert hails
243
clarity also seemed to increase in cases nos 3 and 4 of Test 3 (Fig. 5(b)). It can be seen (Table 2) that for the same number of spikes (nos 1-3 and nos 2-4) subjective clarity was increased when the remaining spikes had the weakest time delays (nos 3 and 4). This led to calculation of interspike gaps (see definition, Table 1). Therefore subjective clarity would seem to be increased by a relatively great interspike gap. The latter can be named 'great interspike gap' (GIG) (see Table 1). Interspike gaps of the distributions used in Test 4 are also listed in Table 3. It can be seen that only the less clear distribution (no. 3) did not have a GIG. Both distributions (1 and 2) had a 25 ms GIG; moreover, the GIG occurred earlier in the clearest distribution 1 (third gap) than in no.2 (fourth gap).
3.3.2 Interpretation These results would seem in contradiction with usual assertions that (i) intelligibility is negatively correlated with single echo delay, according to Haas's results, 2 and (ii) early reflection distribution is not perceived, and subjective clarity is positively related to reverberant energy in the first 80 ms, according to criterion C80. However, the tested signal was the sound of drums and consequently consisted of successive direct impulses (Table 1). According to the definition of C80, subjective clarity could be interpreted as discrimination of direct impulses and early reverberant over late energy on a level axis. But subjective clarity could also be assumed to be related to temporal discrimination of successive direct impulses. The latter could be perceptively linked by early reverberant energy. A GIG could act as a relaxative system improving temporal discrimination. It can also be argued that auditive relaxation due to a GIG might reduce the duration of early energy perceived within each direct impulse. A high frequency level would be consequently increased, improving perception of transients and discrimination of simultaneous sounds. However, such an interpretation deals with impulsive sounds, which could explain discrepancies with Haas's experiments focused on speech. 2 Moreover, Haas used single reflections, and even without GIG temporal discrimination could be less decreased than with 5 ms duration spikes.
4 CONCLUSION The first experiment conducted in ESPRO had shown some discrepancies between subjective clarity and criterion C80. In order to study the effect of early reverberant energy distribution, tests of subjective clarity were performed with artificial reverberation. The results provided strong evidence that variations ofearly reflection patterns could be perceived. Spike
244
A. Fischetti, J. Jouhaneau
temporal distribution appeared to be particularly relevant in discrimination. In some cases subjective clarity appeared to be emphasized with increasing spike time delay. That would seem to be due to a relatively great interspike gap (GIG). Subjective clarity was then assumed to be related to temporal discrimination of successive direct impulsions. Some previous results could also be analysed in terms of temporal integration; for example, subjective clarity was reduced when the wide spike crest level increased, even if associated with a C80 increase (Section 3.2.2, Test 2). That could be induced by a dramatic deterioration of successive direct spike temporal discrimination. The same reason could also explain that above an optimum value subjective clarity was reduced by an increase in spike width (Section 3.2.2, Test 4). Discrepancies between C80 and subjective clarity results in ESPRO (Section 2.2.2) could also be explained through early interspike gaps. In particular, it has been seen that excentred position P2 seemed to have a greater subjective clarity than predicted by C80; Fig. 2 shows an actual relative GIG of 25 ms. Values of approximately 20 ms appeared to be relevant in several cases: (i) time delay variation of a wide spike was discriminated when arising more than 20 ms after direct sound (Section 3.2.1, Test 2, Fig. 4); (ii) subjective clarity improved when the narrow spike time delay reached 20ms after direct sound (Section 3.2.2, Test 2, Fig. 5(a)); (iii) the spike width optimum for the greatest clarity (Section 3.2.2, Test 4, Fig. 5(c)) was about 10-15 ms (when the spike crest level was equal to direct sound). These values of 20 ms are consistent with previous experiments about time constant, a However, they are smaller than the 80 ms limit of the C80 criterion. A useful way to detect GIG could be provided by histograms. Figure 6 shows interspike gap histograms of the distributions used in Test 5 (Table 3). The temporal position of the GIG is represented by a number. A great relative gap only appears for TS(a) and TS(b); it is earlier for the clearest TS(a) (third gap) than for TS(b) (fourth gap). Further investigations should give more details on the effect of gap position and histogram relevance. These experiments could give rise to practical applications in a concert
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Fig. 6. Test 5: histograms of interspike gaps. Spike time delays are listed in Table 3. Positions of interspike gaps are numerated. A GIG appears in TS(a) and T5(b).
First reflection distribution in concert halls
245
hall. In effect, proscenium surfaces are usually used to provide strong early reflections. But they could also be m a d e o f relatively small and spaced reflecting parts, distributed in order to produce a GIG.
ACKNOWLEDGEMENT The authors gratefully thank I R C A M , who have supported the main part o f this research. REFERENCES I. Jullien, J. P., Correlations among criteria of room acoustics. Proceedings of the 12th International Congress on Acoustics, E4-9, Toronto, Canada, 1986. 2. Haas, H., The influence of a single echo on the auditibility of speech. JASA, 20 (1972) 146-55. 3. Reichardt, W., Subjectiveand objective measurement of the loudness level of single and repeated impulses. JASA, 47 (1970) 1557-62. 4. Fischetti, A., Jouhaneau, J. & Hemin, Y., La caract6risation perceptive des salles de concert: insuflisance des crit6res traditionnels et proposition d'un mod61e bas6 sur la distribution des premi6res r6flexions. Proceedings from the 13th International Congress on Acoustics, Vol. 2, Belgrade, 1989, pp. 165-8.
The Influence of First Reflection Distribution on the Quality of Concert Halls
A. Fischetti & J. J o u h a n e a u Laboratoire d'Acoustique, Conservatoire National des Arts et M6tiers, 292 rue St Martin, 75141 Paris Cedex 03, France (Received 8 July 1991; accepted 19 September 1991)
A BSTRA CT Experiments were carried out concerning relationships between subjective clarity and criterion C80 in a variable acoustical room. Some discrepancies appeared and the effect of early reverberant energy was then investigated with artificial reverberation. It wasfound that subjective clarity could be increased by a great interspike gap (GIG).
1 INTRODUCTION Early reverberant energy in a room is usually assumed to be perceived in conjunction with direct sound and to improve clarity. Subjective clarity of music is often estimated by criterion C80, defined as the ratio of the energy arising within the first 80 ms to the remaining energy. In the first part of this study we have investigated relationships between subjective clarity, criterion C80 and geometrical parameters of ESPRO, the variable geometrical room of IRCAM (Paris). Since discrepancies between C80 and subjective clarity were observed, the role of early reverberant energy has been revisited by recreating artificial reverberation. Energetic distribution included in the first 80 ms was changed, criterion C80 being kept constant. The results led to an interpretative analysis of subjective clarity conception. Some hypotheses taking into account time delays of early reflections were proposed. 233 Applied Acoustics 0003-682X/92/$05.00 © 1992 Elsevier Science Publishers Ltd, England. Printed in Great Britain
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2 RELATIONS BETWEEN SUBJECTIVE CLARITY GEOMETRICAL PARAMETERS OF ESPRO
AND
2.1 Method
2.1.1 Physical measures E S P R O is a p a r a l l e l e p i p e d i c r o o m (24 m l o n g a n d 15 m wide) w h o s e walls are c o m p o s e d o f o r i e n t a b l e t r i h e d r a l s w i t h a b s o r p t i v e - d i f f u s i n g - r e f l e c t i n g sides. T h e h e i g h t o f t h e ceiling is v a r i a b l e f r o m 2.5 to 12.5 m. T h e f o l l o w i n g e x p e r i m e n t w a s realized b y J. J o u h a n e a u , J.-P. Jullien a n d J. D. P o l a c k . A n TABLE 1 Definitions
Geometrical parameters of ESPRO H, height of ceiling: high (10m), median (7.5 m), low (5 m) A/D, ratio of absorptive to diffusing panel numbers: strong (oo), median (200%), weak (30%) P, receiver position in the room: P1 and P3 in the axis of the room, respectively at 7 and 16m from the loudspeaker line, P2 close to a lateral wall at 12m from the loudspeaker line G, absorptive panel distribution gradient, which indicates that absorptive panels are either predominantly grouped or uniformed distributed in the room (three values) R, percentage of reflecting panels (three values)
Acoustic criteria C80 (dB) = 10 log ff°'°a p2(t) dt/S~.oap2(t) dt) C50 (dB) = 10 log (S°'°5 p2(t) dt/~p2(t) dt) S/B (dB) = 10 log (~o.o95a(t)p2(t) dt/~.o9 s p2(t )dt) Ca(t) is a weighting function, a(0) = 1, a(0.095) = O)
Perceptive parameter Subjective clarity was evaluated by subjects according to the following definition: 'Subjective clarity is high if sound details and attacks are well perceived. This characteristic is often designed by definition or precision. You have only to evaluate subjective clarity independently of other parameters, like tone coloration for example'
Parameters of artificial reverberation Distribution: simulated impulse response, or this impulse response convolved with an anechoic musical sample Direct impulse: burst of white noise of 5 ms duration which simulated direct sound Spike: burst of white noise which simulated a reflection Temporal energetic barycentre computed in the first 80 ms (ms): S°'°a tp2(t)dt/S°o'°ap2(t)dt Spike time delay (ms): difference of time between spike and direct sound Spike width (ms): duration of white noise burst (narrow, 5 ms; wide, greater or equal to 10ms) Interspike gap (ms): difference of time delay between two successive spikes GIG: great interspike gap (ms): interspike gap greater than other interspike gaps
First reflection distribution in concert hails
235
anechoic musical signal was diffused in ESPRO through two loudspeakers and recorded at three positions with two stereophonic microphones placed on each side of a dummy head. Impulse responses were also measured and 15 acoustical criteria were computed. This procedure was repeated for 81 configurations of ESPRO, chosen by different combinations of three values of each geometrical parameter (see Table 1). Preliminary tests have shown that variations of absorptive panel distribution in the room were poorly discriminated through listening tests, while reflecting and diffusing panels produced nearly identical effects. So we only selected 31 configurations based upon variations of position in the room (P), ratio of absorptive to diffusing panels (A/D) and ceiling height (H). Only one criterion (C80 for mean octave band value) was investigated for the present study.
2.1.2 Perceptive tests The message under test was a 15-s duration piece of Schubert's 14th string quartet. Sound samples were presented by pairs and three kinds of pairs were arranged, according to the geometrical parameter A/D, P or H. All pairs were mixed in the sequence and the place of samples in each pair was randomized. Since musical samples were recorded in ESPRO with a dummy head, they were diffused through headphones to preserve the two channel characteristics. Ten sound engineering students, poorly experienced in listening tests, were required to discriminate subjective clarity between each sample on a scale ranging from - 1 0 to +10 with reference to the first sample of each pair. Subjective clarity was defined as described in Table 1. Subjects could listen to each pair as long as they wished. 2.2 Results and discussion
2.2.1 Results Figure 1 illustrates for each geometrical parameter the average variation of subjective clarity (responses of all subjects were normalized) as a function of C80 variation (dB). Absorption AID. AID varied from weak to median values while the ceiling was kept high. Increases of C80 were identical for all receiver positions. However, subjective clarity increased markedly for distant position P3 and decreased for excentred position P2. Ceiling height H. H varied from median (7.5 m) to low (5 m) values in a weakly absorptive configuration. C80 was emphasized on position P1 or P3, but subjective clarity was unchanged in position P2 in spite of high variation of C80.
236
A. Fischetti, J. Jouhaneau
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+ 10 in reference to the first sample of the pair). HP1-HP2-HP3: decrease of ceiling height (respectively in position P1, P2, P3); A/D P1-A/D P2-A/D P3: increase of room absorption (respectively in position P1, P2, P3); P2/31-P2/3 h: variation of receiver position from P2 to P3 (respectively in low and high ceiling configuration).
Receiver position P. Receiver position varied from P2 to P3 in a weakly absorptive high ceiling P2 position would seem greater than predicted by in excentred position P2. However, subjective clarity decreased in high ceiling configuration. 2.2.2 Analysis and discussion These experiments would seem to reveal significant discrepancies between subjective clarity and C80. In particular, subjective clarity in the weakly absorptive high ceiling P2 position would seem greater than predicted by C80. In effect, inversely to C80, subjective clarity was (i) greater in P2 than P3. This could suggest that subjective clarity in position P1 weak absorption high ceiling is greater than predicted by C80. Other criteria, like reduces less the subjective clarity in near position P1 than in distant position P3. This could suggest that subjective clarity in position P1 weak absorption high ceiling is greater than predicted by C80. Other criteria, like C50 and S/B (see Table 1), were also measured in ESPRO, but both of them were strongly correlated to C801 and consequently not to subjective clarity. So this could give rise to some remarks about criterion C80. The latter compare auditory processes to energy measurement systems in the first 80ms. But (i) 80ms is an arguable limit for there are few reports about auditive time constants, (ii) early reflections might induce different perceptions (because of masking for example) and perceptive processes could not be reduced to a mere sum of energies and (iii) temporal patterns of early reflections could also be relevant in perception. Consequently it would
237
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Fig. 2. Early part of some impulse responses measured in ESPRO (median absorption). (a) Pl-high ceiling; (b) P2-high ceiling; (c) Pl-low ceiling; (d) P3-high ceiling.
seem that the conception of criterion C80 had to be revisited. That led to a study of impulse responses of ESPRO (Fig. 2): two very early reflections followed by a 25 ms interspike gap can be seen in position P2, while strong reflections are separated by about 50 ms interspike gaps in P1, and they are rather grouped in P3. This could suggest a relevance of early reflection temporal patterns in perception which is not taken into account by C80.'* More accurate examination of this question was the purpose of the following experiment.
3 STUDY OF FIRST REFLECTIONS 3.1 M e t h o d
3.1.1 Impulse response simulation A computer was used to construct different kinds of impulse responses. At first each reflection was simulated by a 'Dirac' function. But time delay variations were poorly discriminated and that led to building o f impulse
238
A. Fischetti, J. Jouhaneau
responses by 'shaping' a white noise. Direct sound and early spikes were then simulated by white noise bursts. Diffuse sound started 80 ms after direct sound with a 30 dB lower level. It was obtained by 'cutting' white noise with an exponential curve (RT was 1 s). Impulse response was then convolved with both channels o f a stereophonic anechoic musical sample and the result was listened to through headphones. Early reflections simulated by such a m e t h o d are more realistic, for actual reflections in a concert hall are not 'Dirac' functions. The stochastic character o f a diffuse field is also respected, and very fair sound quality is obtained. Parameters o f impulse response are defined in Table 1. 3.1.2 Perceptive tests The anechoic signal was a 3-s duration sound o f drums. Samples were presented by pairs to subjects, and each pair was diffused three times. Five tests between the two samples were performed; modified parameters were: Test 1: Impulse response shape. 17 impulse responses were built with different early time-energy characteristics but the same energetic a m o u n t in the first 80 ms (C80 remained constant). Samples were c o m p a r e d to the same reference. Twelve subjects were asked to report which o f the two samples seemed clearest. They were also required to rate the overall difference between the two samples on a scale ranging from 0 to + 10. Test 2: Time delay or spike crest level. Two kinds o f spikes, either n a r r o w (5ms duration) or wide (10ms), were used. The same energy variation p r o d u c e d either by width or crest level change was also tested. Sixteen subjects had to rate the difference o f subjective clarity on a scale ranging from - 10 to + 10 in reference to the first sample o f each pair. They were also required to rate the overall difference between the two samples on a scale ranging from 0 to + 10. TABLE 2
Time Delays and Interspike Gaps of Distributions Used in Test 3 (last spike time delay was increased by 5 ms steps) Case
Time delay (ms) of spike no.
Interspike gap (ms) no.
no.
1 2 3 4
1
2
3
1
2
3
25 10 10 5
30-60 25 15--45 15
-30-60 -20-50
25 10 10 5
5-35 15 15-35 10
-5-35 -5-35
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Fig. 3. Early part of some distributions used in Test 1. Results of discrimination and subjective clarity are listed in Table 4. Direct sound is simulated by the first white noise burst of 5 ms duration.
Test 3: Latest spike time delay. In a distribution composed of several narrow spikes (time delays are listed in Table 2) the latest spike time delay was increased by 5 ms steps, and three highly experienced subjects had to rate the difference in subjective clarity on a scale ranging from - 10 to + 10 in reference to the first sample. Test 4: Spike width. Two kinds of situations were tested, spike arising either with or without a time delay after direct sound. The spike level was equal or --15 dB lower than direct sound. The width was increased by 5 ms steps, and three highly experienced subjects had to rate the difference in subjective clarity on a scale ranging from - 1 0 to + 10 in reference to the first sample. Test 5: Respective time delays of several narrow spikes. Three impulse responses based upon variations of time delays of four narrow spikes (Table TABLE 3 Time Delays and Interspike Gaps of Distributions Used in Test 5 Distribution
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no.
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1
2
3
4
1
2
3
4
5 5 5
25 25 25
55 45 45
65 75 65
5 5 5
15 15 15
25 15 15
5 25 15
A. Fischetti, J. Jouhaneau
240
3) were built. Seven subjects were asked to report which sample sounded clearest. Spike crest levels were equal to direct sound, except in Test 2, where it was changed from - 1 0 to 0 dB below direct sound, and in Test 4, where it was - 15 dB lower. 3.2 Results 3.2.1 Discrimination Test 1. Average discrimination (between 0 and I0) appeared to be closely related to differences of temporal energetic barycentre computed in the first 80 ms (see Table 1). However, some exceptions appeared. Figure 3 shows, for example, distribution Tl(c), which was better discriminated from reference Tl(a) than from Tl(b) and Tl(d) in spite of the same barycentre values (see Table 4). Test 2. Average discrimination is plotted in Fig. 4 against spike time delay. When the spike was narrow, increases of time delay (nos 1, 2, 3) were better discriminated than increases of crest level (nos 7, 8, 9). Time delay variations of a wide spike (nos 4, 5, 6) were poorly discriminated when it arose less than 8-
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241
First reflection distribution in concert halls TABLE 4 Distributions Used in Test 1 (Fig. 3)
Temporal energetic Average discrimination Subjective clarity: with reference Tl(a) % of subjects which barycentre (ranging from 0 to 10) evaluated distribution (ms) clearer than reference Tl(a) Tl(a) Tl(b) Tl(c) Tl(d)
32-97 24-02 22.14 25.77
-2.5 6.0 3-0
-25 16.7 66.7
20 ms after direct sound. The width increase of a narrow spike (no. 11) would seem better discriminated than a crest level increase of a wide spike (no. 10) in spite of the same energy variation. 3.2.2 Subjective clarity Test 1. Subjective clarity was roughly negatively correlated to temporal energetic barycentre. But some distributions provided a non-explained greater subjective clarity; for example, Tl(d) was evaluated clearer than Tl(b) and Tl(c) in spite of some barycentre values (Fig. 3 and Table 4). Test 2. Figure 5(a) illustrates variations of subjective clarity as a function of spike time delay. Increase of time delay (nos 1-6) reduced subjective clarity, except when the spike arose about 25-30 ms after direct sound (nos 2 and 6). Increase of crest level emphasized subjective clarity when the spike was narrow (nos 7, 8, 9) and reduced it when it was wide (no. 10). Subjective clarity was also emphasized by an increase of narrow spike width (no. 11). Test 3. Figure 5(b) shows variations of subjective clarity induced by the last spike time delay increase. Subjective clarity decreased in cases nos 1 and 2, and increased in cases nos 3 and 4 (see Table 2). Test 4. Figure 5(c) depicts variations of subjective clarity as a function of spike width. The greatest subjective clarity was related to an optimum width. The latter seems to depend on spike level (but not on spike time delay): lO15ms for the same level as direct sound (nos 1 and 3) and 35ms for the - 1 5 dB level (no. 2). Test 5. 71% of subjects evaluated distribution T5(a) clearer than T5(b) and 100% evaluated T5(b) clearer than T5(c) (see Table 3).
A. Fischetti, J. Jouhaneau
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3.3 D i s c u s s i o n
3.3.1 Analysis Discrimination. In Test 1 the best d i s c r i m i n a t e d d i s t r i b u t i o n , Tl(c), differed f r o m the r e f e r e n c e T l ( a ) b y a n a r r o w e r e a r l y spike w i d t h (20 ms) t h a n T l ( b ) (40 ms) (Fig. 3). Test 2 w o u l d suggest t h a t n a r r o w spike t i m e delays are the best d i s c r i m i n a t e d p a r a m e t e r s . T h e s e results t e n d to indicate a relative i m p o r t a n c e o f early e n e r g y t e m p o r a l p a t t e r n . Subjective clarity. T h e results o f Test 1 seem to s h o w t h a t a g r e a t e r e a r l y gap, T l ( d ) , m i g h t increase subjective clarity. T h a t also a p p e a r e d in Test 2 w h e n a n a r r o w spike a r o s e 20 ms a f t e r d i r e c t s o u n d (Fig. 5(a)). Subjective
First reflection distribution in concert hails
243
clarity also seemed to increase in cases nos 3 and 4 of Test 3 (Fig. 5(b)). It can be seen (Table 2) that for the same number of spikes (nos 1-3 and nos 2-4) subjective clarity was increased when the remaining spikes had the weakest time delays (nos 3 and 4). This led to calculation of interspike gaps (see definition, Table 1). Therefore subjective clarity would seem to be increased by a relatively great interspike gap. The latter can be named 'great interspike gap' (GIG) (see Table 1). Interspike gaps of the distributions used in Test 4 are also listed in Table 3. It can be seen that only the less clear distribution (no. 3) did not have a GIG. Both distributions (1 and 2) had a 25 ms GIG; moreover, the GIG occurred earlier in the clearest distribution 1 (third gap) than in no.2 (fourth gap).
3.3.2 Interpretation These results would seem in contradiction with usual assertions that (i) intelligibility is negatively correlated with single echo delay, according to Haas's results, 2 and (ii) early reflection distribution is not perceived, and subjective clarity is positively related to reverberant energy in the first 80 ms, according to criterion C80. However, the tested signal was the sound of drums and consequently consisted of successive direct impulses (Table 1). According to the definition of C80, subjective clarity could be interpreted as discrimination of direct impulses and early reverberant over late energy on a level axis. But subjective clarity could also be assumed to be related to temporal discrimination of successive direct impulses. The latter could be perceptively linked by early reverberant energy. A GIG could act as a relaxative system improving temporal discrimination. It can also be argued that auditive relaxation due to a GIG might reduce the duration of early energy perceived within each direct impulse. A high frequency level would be consequently increased, improving perception of transients and discrimination of simultaneous sounds. However, such an interpretation deals with impulsive sounds, which could explain discrepancies with Haas's experiments focused on speech. 2 Moreover, Haas used single reflections, and even without GIG temporal discrimination could be less decreased than with 5 ms duration spikes.
4 CONCLUSION The first experiment conducted in ESPRO had shown some discrepancies between subjective clarity and criterion C80. In order to study the effect of early reverberant energy distribution, tests of subjective clarity were performed with artificial reverberation. The results provided strong evidence that variations ofearly reflection patterns could be perceived. Spike
244
A. Fischetti, J. Jouhaneau
temporal distribution appeared to be particularly relevant in discrimination. In some cases subjective clarity appeared to be emphasized with increasing spike time delay. That would seem to be due to a relatively great interspike gap (GIG). Subjective clarity was then assumed to be related to temporal discrimination of successive direct impulsions. Some previous results could also be analysed in terms of temporal integration; for example, subjective clarity was reduced when the wide spike crest level increased, even if associated with a C80 increase (Section 3.2.2, Test 2). That could be induced by a dramatic deterioration of successive direct spike temporal discrimination. The same reason could also explain that above an optimum value subjective clarity was reduced by an increase in spike width (Section 3.2.2, Test 4). Discrepancies between C80 and subjective clarity results in ESPRO (Section 2.2.2) could also be explained through early interspike gaps. In particular, it has been seen that excentred position P2 seemed to have a greater subjective clarity than predicted by C80; Fig. 2 shows an actual relative GIG of 25 ms. Values of approximately 20 ms appeared to be relevant in several cases: (i) time delay variation of a wide spike was discriminated when arising more than 20 ms after direct sound (Section 3.2.1, Test 2, Fig. 4); (ii) subjective clarity improved when the narrow spike time delay reached 20ms after direct sound (Section 3.2.2, Test 2, Fig. 5(a)); (iii) the spike width optimum for the greatest clarity (Section 3.2.2, Test 4, Fig. 5(c)) was about 10-15 ms (when the spike crest level was equal to direct sound). These values of 20 ms are consistent with previous experiments about time constant, a However, they are smaller than the 80 ms limit of the C80 criterion. A useful way to detect GIG could be provided by histograms. Figure 6 shows interspike gap histograms of the distributions used in Test 5 (Table 3). The temporal position of the GIG is represented by a number. A great relative gap only appears for TS(a) and TS(b); it is earlier for the clearest TS(a) (third gap) than for TS(b) (fourth gap). Further investigations should give more details on the effect of gap position and histogram relevance. These experiments could give rise to practical applications in a concert
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First reflection distribution in concert halls
245
hall. In effect, proscenium surfaces are usually used to provide strong early reflections. But they could also be m a d e o f relatively small and spaced reflecting parts, distributed in order to produce a GIG.
ACKNOWLEDGEMENT The authors gratefully thank I R C A M , who have supported the main part o f this research. REFERENCES I. Jullien, J. P., Correlations among criteria of room acoustics. Proceedings of the 12th International Congress on Acoustics, E4-9, Toronto, Canada, 1986. 2. Haas, H., The influence of a single echo on the auditibility of speech. JASA, 20 (1972) 146-55. 3. Reichardt, W., Subjectiveand objective measurement of the loudness level of single and repeated impulses. JASA, 47 (1970) 1557-62. 4. Fischetti, A., Jouhaneau, J. & Hemin, Y., La caract6risation perceptive des salles de concert: insuflisance des crit6res traditionnels et proposition d'un mod61e bas6 sur la distribution des premi6res r6flexions. Proceedings from the 13th International Congress on Acoustics, Vol. 2, Belgrade, 1989, pp. 165-8.