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Modeling chairs and occupants to closely approximate the sound absorption of occupied full scale theatre chairs
Applied Acoustics 75 (2014) 52–58
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Modeling chairs and occupants to closely approximate the sound absorption of occupied full scale theatre chairs Young-Ji Choi ⇑ Department of Architectural Engineering, Chonbuk National University, 664-14 1Ga, Duckjin-Dong, Duckjin-Gu, Jeonju, Jeonbuk 561-156, Republic of Korea
a r t i c l e
i n f o
Article history: Received 15 May 2013 Received in revised form 3 July 2013 Accepted 3 July 2013 Available online 2 August 2013 Keywords: Sound absorption Model chairs Model listeners
a b s t r a c t The present work reports on the process of modeling chairs and occupants to closely approximate the sound absorption of occupied full scale theatre chairs and explains how the best form of model listener was determined. Modifying the form of the model listeners to have shorter upper legs and narrower lower legs, led to improved agreement between model and full scale occupied chairs at all frequencies including at 125 Hz. The measured absorption coefficients of single blocks of model chairs with or without model listeners agreed well with the measured values for both full scale types E and G chairs. However, the estimated values for larger sample blocks of model chairs with P/A = 0.5 m1 showed better agreement with the measured values for full scale type G chairs than type E chairs due to the different slopes of the regression lines versus P/A. The present results demonstrate that the model chair and listener accurately simulate the sound absorption characteristics of a particular type of quite absorptive full scale occupied chairs for all sample sizes of the full scale chairs. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction The successful modeling of chairs and occupants in scale models is particularly important because the audience is usually the largest single component of absorption in an auditorium. Some previous studies [1–6] have demonstrated the development of model listeners for adding the sound absorption of an audience, seated on chairs, in model lecture rooms or auditoria. The model listeners have varied in form from egg cartons [1] to the simplified human forms [2,3,5]. Table 1 summarizes the details of model listeners developed in some earlier studies. Day [2] developed 1/10 scale model listeners with a simplified human form to investigate the effect of the degree of occupancy on the audience absorption in a model lecture room. The model listener consisted of smoothed softwood body and hardwood head. A single layer of surgical gauze was used for simulating human clothing. Hegvold [3] investigated the effects of the amount of clothing worn by a person on the absorption of a group of people and developed 1/8 scale model listeners to simulate the sound absorption of a typically dressed Sydney audience. The model listeners were constructed with a rigid polyurethane foam body and a sanded pine head. Hegvold adopted Day’s simplified human form to model the sound absorption of model Sydney listeners. Cremer et al. [4] used cardboard egg cartons to simulate the sound absorption of the audience in a 1/16 scale model of a ⇑ Tel.: +82 1029270563. E-mail addresses: [email protected], [email protected] 0003-682X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apacoust.2013.07.003
multi-purpose hall and found that the cardboard egg cartons were capable of simulating the partially diffuse reflections from the audience. In a recent study, Tahara et al. [5,6] developed 1/16 scale model listeners to simulate the audience absorption in a model auditorium. The model listeners consisted of a wooden head with a single layer of 1 mm felt simulating human hair and a wooden torso. Their model listeners were more similar to a head and torso simulator rather than the simplified more complete human form of model listeners developed by Day and Hegvold. Similar types of head and torso forms of model listeners were used in other recently reported scale model predictions [7]. The absorption coefficients of the simple forms of model listener were obtained from the measurements of single blocks of model chairs, and therefore the results were never justified to closely approximate the absorption characteristics of occupied full scale chairs for all sample sizes of the chairs. Some earlier studies demonstrated the value of using model tests to better understand the sound absorption of theatre chairs [3,8,9]. Reverberation chamber tests of both model and full scale chairs showed that the sound absorption characteristics of smaller blocks of chairs are not representative of those found for the larger blocks of chairs in an auditorium because the measured absorption coefficients vary with the dimensions of the sample and larger samples tend to have lower absorption coefficients due to edge effects [3,8–10]. The edge absorption may be reduced by using screens around the seating blocks, but diffraction effects are not eliminated by this method and the absorption coefficients of the seating blocks would still vary with the sample perimeter-to-area
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This process could enhance the credibility of future studies using these model chairs and occupants such as for investigation of the incremental effects of occupants in chairs or carpet under chairs on the absorption coefficients of each combination. It is important to better understand such interactive effects rather than to depend on measurements of every new situation. The goal of this work is to develop model chairs and occupants as closely representative as possible of common types of full scale theatre chairs with audiences. The present work reports on the process of modeling chairs and occupants to closely approximate the sound absorption of occupied full scale theatre chairs and describes how the best form of model listeners was determined. This paper was also intended to give some useful guidelines on constructing model occupants by investigating how the form and dimensions of model listeners affect to the measured sound absorption characteristics of occupied theatre chairs. 2. Measurement procedures Fig. 1. A photo of high absorption model chairs occupied with model listeners in the model reverberation chamber.
(P/A) ratio both in full scale [9–11] and in model tests [3,9]. This would also be true for the approach of putting absorbing samples in a well in the floor of the reverberation chamber [12]. This latter method is also not very practical for objects as large as chairs. The sound absorption coefficients of blocks of theatre chars have been shown to be linearly related to the sample P/A ratio [10]. One can predict the expected absorption coefficients of the larger blocks of chairs found in auditoria using linear regression fits to the measured values obtained from 5 or more sample blocks of chairs with varied P/A ratio in a reverberation chamber. This method is referred to as the P/A method. This method can more completely characterize particular types of chairs than one sound absorption test of a single sample block of chairs in the reverberation chamber. The method has been shown to accurately predict the absorption coefficients of chairs measured in an auditorium both in full scale [10,11] and in model tests [9,13]. Many have previously used scale model chairs and listeners, but previous efforts to create scale model chairs and occupants have not always been very thorough. There are a range of absorption characteristics for occupied and unoccupied theatre chairs and the absorption characteristics vary with P/A and the form of this variation varies with chair type [13]. It is not enough to compare single samples in reverberation chamber tests. One should validate model chairs and occupants to confirm that the variation of absorption coefficients with P/A for the model chairs and listeners are similar to those for common types of full scale chairs with occupants. This would ensure that absorption coefficients of the model chairs would be similar to the corresponding full scale chairs for all sample sizes of the chairs.
2.1. Model chair construction The main purpose of developing various types of model theatre chairs and listeners was to accurately simulate their sound absorption characteristics and to better understand how they affect to the acoustical conditions in real auditoria. The model chairs and occupants were developed to have similar absorption characteristics, as a function of P/A, to typical more highly absorbing full scale chairs and occupants. The previously reported results for full scale types E and G chairs [10,11] were used as design goals for the model chairs and occupants. The type E chairs were considered very highly absorptive chairs and the type G chairs more typically highly absorptive. These high absorption full scale theatre chairs had thicker absorbing material and included thick absorbing pads on the chair backs. The type E chairs also had absorptive padding on the rear of the seat backs as well as on the arm rests and sides of the chairs. They also had quite thick and absorptive seat cushions and perforated seat pans over glass fiber material. Chair type G had cloth covered metal of the rears of the seat backs. The slopes and intercept values of the regression lines for both unoccupied and occupied types E and G chairs were included in Ref. [13]. Beranek and Hidaka’s [14] estimates of the average absorption coefficients of the most absorptive group of chairs (their Group 1) were less absorptive than the values calculated from the measured types E and G full scale occupied chairs with P/A = 0.5 m1. A variety of 1/10 scale model chairs were systematically tested to develop a model chair with absorption characteristics similar to those for full scale high absorption chairs. Model chairs having a width of 0.6 m (full scale) were constructed as connected seats with arm rests and underpasses as shown in Fig. 1. The height of
Table 1 Summary of the model listeners developed in previous studies. The symbol ‘‘Ø’’ indicates the diameter of the model listener’s head measured in mm. Researcher (year)
Materials
Scale
Size (width length depth), mm
Brebek et al. [1] Cremer et al. [4] Day [2]
Egg cartons
1/10 1/16 1/10
–
Hegvold [3]
Head: sanded pine Body: polyurethane foam Cloth: no cloth
1/8
Tahara et al. [5]
Head: wood Torso: wood Cloth: felt 1 mm adding on the torso and head
1/16
Head: softwood Body: hardwood Cloth: surgical gauze adding on the body
Head: Ø25 Torso: 35 75 18 Hips: 35 45 18 Legs: 35 47 18 Head: 22.2 (depth) 31.7 (length) Torso: 54.6 73.6 22.2 Hips: 54.6 57.2 22.2 Legs: 54.6 58.4 22.2 Head: Ø10 Torso: 24 38 12
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Y.-J. Choi / Applied Acoustics 75 (2014) 52–58 1.0
Absorption coefficient
0.8
0.6
0.4
0.2 Felt 1 mm Bubble wrap 3mm Bubble wrap+Felt
0.0 125
250
500
1000
2000
4000
Frequency, Hz Fig. 2. Absorption coefficients of the materials added to model chairs (full scale).
the gap under the chairs was 280 mm (full scale). A 1.0 mm single layer of felt combined with a 3.0 mm single layer of bubble wrap was added on the seat back, rear of back, sides and undersides of the model chairs and these chairs are referred to as high absorption model chairs. The measured absorption coefficients of the materials when added to the model chairs are plotted in Fig. 2. The samples tested had an area of 10.2 m2 (full scale), which corresponds to a P/A ratio of 1.25 m1. The absorption characteristics of the combined bubble wrap and felt materials showed very similar trends over frequency to those found for unoccupied full scale chairs (see Fig. 5). The sound absorption coefficients of blocks of theatre chairs vary with sample P/A ratio and the sample blocks of model chairs should have the same P/A ratio as the full scale blocks of chairs used as design goals. The tests used blocks of 3 rows of 5 model chairs (R3C5) having a P/A ratio of 1.40 m1 (full scale) which was very similar to P/A of 1.39 m1 and 1.37 m1 for the blocks of 3 rows of 6 chairs (R3C6) of both unoccupied full scale types E and G chairs. 2.2. Model listener construction One-tenth scale model listeners were constructed using 10 mm thick expanded PVC (poly vinyl chloride) pieces and were scaled to be representative of average Koreans. The selection of materials for constructing model listeners similar to the absorption coefficients 1.0
Felt 0.5 mm Fabric 0.5 mm Pineapple sheet 0.5 mm Expanded PVC 10 mm
0.9
Absorption coefficient
0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 125
250
500
1000
2000
4000
Frequency, Hz Fig. 3. Absorption coefficients of the materials added to model listeners (full scale).
of the human body was carefully considered according to some earlier studies on the measurements of the absorption coefficients of the human body and hair [15,16]. Fig. 3 plots the measured absorption coefficients of the materials used for model listener construction. The samples tested had an area of 10.2 m2 (full scale), which corresponds to a P/A ratio of 1.25 m1. The absorption coefficients of the PVC pieces are very similar to those measured for real human body surfaces found in Ref. [15]. The absorption coefficients of human hair increased with increasing frequency but decreased above 2 kHz. The absorption coefficients of the 1 mm felt showed similar trends as measured for human hair found in Ref. [16]. It was found that there was considerable variation in the absorption coefficients resulting from different dress types worn by audiences [3]. The audiences wore light to medium clothes, such as a short sleeved shirts or suits, during the measurements in the full scale reverberation chamber tests and the materials added to the model listeners were selected to approximate the absorption characteristics of the dress types worn by full scale audiences. Prior to adding absorbing materials to the model listeners, the best form of model listeners was determined by testing 7 types of model listeners with various forms and dimensions. The absorption coefficients of each set of model listeners were measured while seated on the high absorption model chairs. A total of 7 types of model listeners with various forms and dimensions are illustrated in Fig. 4 and they are described in Table 2. ‘Upper legs’ are the portion of the legs above the knees and ‘lower legs, are the portion below the knees. Model listener type A had the widest upper and lower legs and was similar to the model occupant developed by Day [2] and Hegvold [3]. Model listener type B consisted of torso and upper legs, but had no lower legs. Type C had shorter upper legs than types A and B model listeners and no lower legs. Model listener type D had no upper and lower legs and was similar to the model listeners developed by Tahara et al. [5,6]. Although model listener type D had no upper and lower legs, and hence was not representative of real human body forms, it was intended to investigate the effects of the presence of the upper and lower legs on the occupied chair absorption characteristics. Model listener types E, F and G were modified versions of the type A listeners. Type E listeners had narrower lower legs than those of the type A model listeners. Type F had shorter upper legs and narrower lower legs than the other models. Type G had the two thin separate lower legs making them more similar to real humans. The best form of model listener was selected and tested with varied added absorbing material to develop a model listener with absorption characteristics most similar to those found for the full scale occupied theatre chairs. Table 3 describes various absorbing materials added to the best form of model. Four different types of absorbing materials were added to the head, torso and upper legs of the model listeners. No absorbing materials were added to the lower legs of model listeners. A total of 6 model listeners with varied added absorbing materials were constructed. The absorbing material named ‘pineapple sheet’ is a 0.5 mm thick paper having a pineapple pattern on it. Three absorbing materials, felt 0.5 mm, fabric 0.5 mm and pineapple sheet 0.5 mm were added to the torso and upper legs of the model listeners. Felt, 1.0 mm thick, was added to the head of model listeners to simulate the sound absorption by human hair (see Table 3). All tests were of sample blocks of 5 rows of 3 chairs in each row with a row-to-row spacing of 0.91 m (full scale). This particular sample block of model chairs had a P/A ratio of 1.55 m1 that is similar to P/A of 1.53 m1 for both unoccupied full scale types E and G chairs. After selecting the most successful model listener, the absorption coefficients of larger sample blocks of occupied model chairs with P/A = 0.5 m1 were estimated from the reverberation chamber measurements of seven different sample blocks of
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Fig. 4. Sketch of front and side views of 7 types of model listeners (unit: mm, full scale).
1.8
Table 2 7 Types of model listeners with various forms and dimensions. The symbol ‘‘Ø’’ indicates the diameter of the model listener’s head, measured in mm.
1.6
Name
Absorption coefficient
1.4 1.2
Type Type Type Type Type Type Type
1.0 0.8 0.6 0.4
A B C D E F G
Size (width length depth), mm Head
Torso
Upper legs
Lower legs
Ø25 Ø25 Ø25 Ø25 Ø25 Ø25 Ø25
36 60 10 36 60 10 36 60 10 36 60 10 36 60 10 36 60 10 36 60 10
36 40 10 36 40 10 36 25 10 – 36 40 10 36 30 10 36 30 10
36 30 10 – – – 20 30 10 15 30 5 5 30 5
Full scale type E chairs (unoccupied, P/A=1.39m-1) Full scale type G chairs (unoccupied, P/A=1.37m-1) Average, full scale types E and G chairs Model chairs (unoccupied, P/A=1.40 m-1)
0.2 0.0 125
250
500
1000
2000
4000
Frequency, Hz Fig. 5. Measured absorption coefficients of an R3C6 block of unoccupied model chairs compared with those measured for R3C6 blocks of both unoccupied full scale types E and G chairs as well as the average values of all two types of full scale chairs [10,13].
chairs having a range of P/A values between 1.29 and 2.40 m1 and compared to those estimated for full scale occupied chairs. The measured absorption coefficients of sample blocks of chairs vary with sample P/A and comparing the estimated absorption coefficients of larger sample blocks of chairs typically found in auditoria for both model and full scale chairs would be more appropriate. 2.3. Reverberation chamber measurements The volume of the 1/10 scale model reverberation chamber was 300 m3 (full scale) and it was built using 20 mm thick acrylic panels. In the model measurements, a 1.37-s logarithmic sine sweep from 1 kHz to 100 kHz was used, which corresponds to full-scale
frequencies from 100 Hz to 10 kHz. To eliminate the unwanted effects of air absorption, the model chamber was filled with nitrogen during each test. The reverberation chamber was kept at a constant temperature of 22 °C and a relative humidity of 4%. Six combinations of two source positions and three receiver positions were selected for measuring the absorption coefficients of the unoccupied chairs. A 20 dB range of each decay, from 5 dB to 25 dB, was used to calculate reverberation times according to the procedures described in ISO 354 [12]. Prior to the measurements, the diffusivity of the sound field in the reverberation room was examined according to ISO 354 [12]. There was no evidence of non-linear decays over this range when the absorption of the chairs was measured. The measurements were made in 1/3 octave bands, but the absorption coefficients were presented as octave band values derived by averaging the three individual 1/3 octave sound absorption coefficients in each octave band. A repeatability test of the measurements for the absorption coefficients of the chairs was carried out to check whether the results were consistent for each measurement. The measurements were repeated three times, and the results were presented as the mean absorption coefficients.
Table 3 Absorbing materials added to the best form of model listener. The meaning of letter symbols is explained in the table below the main table. Name
Best Best Best Best Best Best
type-Fe type-F type-PF type-PP type-33FePP type-27FePP
Materials, thickness mm Head
Torso
Upper legs
Lower legs
– – – – Ø33Felt, 1 Ø27Felt, 1
Felt, 0.5 Fabric, 0.5 Pineapple sheet, Pineapple sheet, Pineapple sheet, Pineapple sheet,
Felt, 0.5 Fabric, 0.5 Fabric, 0.5 Pineapple sheet, 0.5 Pineapple sheet, 0.5 Pineapple sheet, 0.5
– – – – – –
0.5 0.5 0.5 0.5
Symbol
Meaning
F P Fe
Fabric Pineapple sheet Felt
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Y.-J. Choi / Applied Acoustics 75 (2014) 52–58
By comparison the full scale measurements were performed in the 254 m3 reverberation chamber, at the National Research Council in Ottawa, having fixed diffuser panels as well as a large rotating vane. More details are included in Refs. [10] and [13].
3. Results and discussion 3.1. Developing model chairs and listeners representative of the sound absorption of occupied theatre chairs
1.6
1.6
1.4
1.4
1.2
1.2
1.0 0.8 0.6
Unoccupied model chairs Type A+chairs Type B+chairs Type C+chairs Type D+chairs
0.4 0.2 0.0
Absorption coefficient
Absorption coefficient
The measured absorption coefficients of the unoccupied high absorption model chairs were compared in Fig. 5 with those values measured for unoccupied full scale types E and G chairs as well as the average of both types of full scale chairs. The measured absorption coefficients of unoccupied model chairs in Fig. 5 show more similar characteristics in absorption coefficient over the frequency to those measured for type G chairs than with the type E chairs results. Chair type G had slightly higher absorption coefficients at 250 Hz than those measured for model chairs and the type E chairs. This is most likely due to the resonant sound absorption of the thin metal seat pans of type G chairs. The average absorption coefficients of the more absorptive two types of theatre chairs showed good agreement with the measured results for the model chairs. The materials of highly absorbing chairs can influence the absorption characteristics of the occupied chairs, but the occupants are the most important factor determining their absorption characteristics [13]. Various types of model audiences were developed in earlier studies, but guidelines on how to construct more accurate model listeners were generally not available. Therefore, it was very useful to provide measurement results showing how the form and dimensions of model listeners affect to the measured absorption characteristics of occupied theatre chairs. Fig. 6 compares the measured absorption coefficients of the types A to D model listeners seated on the high absorption model chairs to illustrate the effects of the form of the model listener. Adding model listeners to the chairs generally led to increases in the absorption coefficients which were largest at 125 Hz. The increases in 125 Hz absorption coefficients varied from 0.06 to 0.27. The largest increases in 125 Hz absorption coefficients occurred for the types A and B listeners. The type D model listener having no upper and lower legs had the smallest increase in absorption coefficient at 125 Hz. Since the absorption of the expanded PVC pieces at 125 Hz (Fig. 3) was quite small, the increases in the absorption coefficients at this frequency with model listeners added to the chairs, were likely due to
changes to the form of the model listener. The inclusion of upper and lower legs in the model listeners led to larger increases in absorption coefficients at 125 Hz. At 125 Hz the cavities between and under the chairs may act as resonant cavities and putting legs into them might change the cavities enough to change the resonant absorption of the cavities and to increase the low frequency absorption of the model theatre chairs. Adding model listener type D to the chairs caused a larger increase in absorption coefficients at 2000 and 4000 Hz than when adding the other model listener types to the chairs. This was because the model type D listeners, consisting of a torso only, did not cover some absorptive areas of the seat of the chairs that other model listener types, with upper and lower legs, did cover. Except for model listener type D, adding model listeners to the chairs caused a decrease in absorption coefficients at 4000 Hz. The greatest decrease was 0.14 which occurred when the type A model listeners were in the chairs. The decreased absorption at 4000 Hz was due to the model listeners covering some absorptive areas of the seat and back of the chairs and reducing the total absorption of the occupied chairs at high frequencies. Fig. 7 shows the measured absorption coefficients of high absorption model chairs occupied with model listener types A, E, F and G. Model listener types A, E, F and G have the same form of torso, but the dimensions of upper and lower legs varied (see Table 1). Types F and G listeners have the shortest upper legs and narrowest lower legs among the four model listener types. Adding model listener type E, having narrower lower legs, to the chairs slightly decreased the absorption coefficients at 1000 and 2000 Hz compared to chairs occupied with model listener type A. The differences were 0.03 and 0.05 at 1000 and 2000 Hz. Adding model listener type F having the shortest upper legs and the narrowest lower legs to the chairs led to a smaller increase in the absorption coefficients at 125 Hz than occurred for chairs occupied with types A and E model listeners. Type G model listeners, having two thin lower legs, had a slightly increased absorption coefficient at 125 Hz compared to chairs occupied with types A and E model listeners. However, the difference in absorption coefficients at 125 Hz between chairs occupied with types A and G listeners was only 0.04. Chairs with type G listeners were less absorptive at 1000 and 2000 Hz than chairs occupied with type A listeners. Adding type G model listeners to the chairs decreased the absorption coefficients at 1000 and 2000 Hz more than those found for chairs occupied with type A model listeners. The differences were 0.09 and 0.12 at 1000 and 2000 Hz respectively. Type F listeners, a
1.0 0.8 0.6
Unoccupied model chairs Type A+chairs Type E+chairs Type F+chairs Type G+chairs
0.4 0.2 0.0
125
250
500
1000
2000
4000
Frequency, Hz Fig. 6. Measured absorption coefficients of occupied chairs with varied forms of model listeners compared with the absorption coefficients of the unoccupied chairs.
125
250
500
1000
2000
4000
Frequency, Hz Fig. 7. Absorption coefficients of model listeners with varied dimensions of the legs of the model listeners.
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Y.-J. Choi / Applied Acoustics 75 (2014) 52–58 0.6
1.8
(a)
1.4
0.4
1.2
Slope
Absorption coefficient
1.6
1.0
Type F-Fe+chairs Type F-F+chairs Type F-PF+chairs Type F-PP+chairs Type F-33FePP+chairs Type F-27FePP+chairs Full scale type E chairs, occupied Full scale type G chairs, occupied Average, full scale types E and G chairs
0.8
0.6
0.2
Type E chairs Type G chairs Model chairs
0.0
0.4 125
250
500
1000
2000
4000
125
250
Frequency, Hz
500
1000
2000
4000
Frequency, Hz
Fig. 8. Measured absorption coefficients of type F form model listeners with varied added absorbing materials.
1.8 1.6
(b)
1.4 1.2
Intercept
modified human form of model listener having shorter upper legs and narrower lower legs, reduced the increase in the absorption coefficients at 125 Hz to 0.07. Therefore, model listener type F was selected as the best form of model listeners. Fig. 8 shows the measured absorption coefficients of type F form model listeners with varied added absorbing material. The measured absorption coefficients of full scale occupied theatre chairs are also plotted for comparison in Fig. 8. Adding 0.5 mm felt absorbing material to the model listeners reduced the absorption coefficients of the occupied chairs at most frequencies. That is, the felt did not increase the absorption coefficients of the occupied chairs as desired. Because the expanded PVC pieces and the felt had similar absorption coefficients (see Fig. 4), adding the 0.5 mm felt absorbing material to the model listeners covered the expanded PVC pieces and reduced the total absorption of the occupied chairs at most frequencies. Adding 0.5 mm fabric absorbing material to the model listeners led to increases in absorption coefficients at most frequencies and the increase in absorption coefficients at 125 Hz was the largest among the six model listeners with varied added absorbing materials. Adding 0.5 mm pineapple sheet absorbing material to the model listeners slightly reduced the absorption coefficients at 125 Hz and increased the absorption coefficients at other frequencies. Adding 1 mm felt absorbing material to the Ø33 mm head of the model listeners increased the absorption coefficients at most frequencies and was found to be the most successful model listener with similar absorption coefficients to the full scale occupied theatre chairs. At 125 Hz, model listener type F-33FePP had a slightly higher absorption coefficient than the full scale occupied type G chairs with a difference of 0.06. The occupied model chairs with model listeners type F-33Fe-PP showed good agreement of their absorption coefficients with the occupied full scale type G chairs. The difference between the absorption coefficients, averaged over
1.0 0.8 0.6 0.4 Type E chairs Type G chairs Model chairs
0.2 0.0 125
250
500
1000
2000
4000
Frequency, Hz Fig. 9. (a) Slopes and (b) intercepts of regression lines of absorption coefficients versus P/A for occupied full scale theatre chairs and model chairs.
frequency, for model chairs occupied with the most successful model listeners, and those measured for the full scale occupied type G chairs, were no more than 0.1.
3.2. Comparison of estimated absorption coefficients for samples of occupied model chairs with P/A = 0.5 m1 with those for full scale occupied chairs Linear regression lines were fitted to plots of absorption coefficient versus P/A for various blocks of chairs measured in the model reverberation chamber. Theses analyses were performed using the Origin plotting and analysis software [17] to perform standard least squares linear regression fits to the data. The slopes and intercept values of the regression lines were included in Table 4 as well as the statistical significance of the results. Fig. 9(a) plots the slopes
Table 4 Summary of: the standard errors of the estimate of the octave-band data about the regression lines (r), the statistical significance of the octave-band regression results (p-value), The values of slopes (b), and intercepts (a1) for high absorption model chairs (ns = not significant, p < 0.2, p < 0.05, p < 0.01). Chair type
N
Model chairs, occupied
7
Frequency, Hz
r p b
a¥
125
250
500
1000
2000
4000
0.052 0.106 0.664
0.040 0.307 0.686
0.049 0.264 0.953
0.061 0.317 0.949
0.047 0.394 0.868
0.043 0.470 0.786
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Y.-J. Choi / Applied Acoustics 75 (2014) 52–58 1.8 1.6
Absorption coefficient
1.4 1.2 1.0 0.8 0.6 0.4
Full scale type E chairs ,occupied Full scale type G chairs ,occupied Average, full scale types E and G chairs Model chairs ,occupied
0.2 0.0 125
250
500
1000
2000
4000
Frequency, Hz Fig. 10. Comparison of absorption coefficients versus frequency for samples of occupied model chairs with P/A = 0.5 m1 with Bradley et al.’s [13] estimates of the absorption coefficients of two types of full scale occupied chairs as well as the average of all two types of full scale chairs.
of the regression lines versus frequency for occupied theatre chairs and model chairs. The intercepts of the regression lines are plotted versus octave band frequency in Fig. 9(b). The slopes and intercepts of the regression lines for the type G and model chairs in Fig. 9(a) and (b) are very similar at most frequencies. The type E chairs have higher absorption coefficient values at most frequencies. The model chairs are more representative of the more intermediate characteristics of the type G chairs than the more highly absorptive type E chairs. The absorption coefficients of model chairs for larger sample blocks of chairs with P/A = 0.5 m1 were calculated and compared with those estimated for full scale types E and G occupied chairs shown in Fig. 10. This gives more reliable comparisons by using the mean trend over several measurements of blocks of chairs. The estimated absorption coefficients of full scale type G occupied chairs were good agreement with those calculated for model chairs over frequency. Although the measured absorption coefficients from one single sample blocks of chairs for full scale chair E type were very similar to those measured for model chairs (see Fig. 5), the estimated values for larger sample blocks of chairs were quite different at most frequencies. This is likely due to the different slope values over frequency for both full scale type E and model chairs shown in Fig. 9(a). It is needed to understand the differences in characteristics of different types of chairs. 4. Conclusions The process of modeling chairs and occupants resulted in absorption characteristics that closely approximated the sound absorption of full scale occupied theatre chairs and the details of the successful model listeners were reported. A modified form of model listener (type F), having shorter upper legs and narrower lower legs than a typical human form of model listener (type A), reduced the increase in the absorption coefficient at 125 Hz and achieved better agreement with the absorption coefficients of occupied full scale chairs. The differences between the absorption coefficients, averaged over frequency for model chairs, occupied
with the most successful model listeners (type F-33FePP), and those measured for the full scale occupied theatre chairs were no more than 0.1. The measured absorption coefficients from one single sample block of chairs for model chairs with or without model listeners showed good agreement with the measured values for both full scale types E and G chairs. However, the estimated values for larger sample blocks of model chairs representative of chairs in an auditorium (P/A = 0.5 m1) were closer to the measured values for full scale type G chairs than to the type E chair results. That is, the model chairs and occupants were a better model of the P/ A effect for the type G chairs. The present results demonstrate that the model chair and listener that were developed accurately simulated the sound absorption characteristics of the type G full scale chairs. As the type G chairs are representative of many more absorptive full scale chairs, the model chairs and listeners should be more widely useful. The effects of P/A on the incremental effects of adding occupants have not been fully explored and the incremental effects of carpet under chairs have also not been considered. These are issues that also need to be better understood so that we can better estimate their effects on the total sound absorption in an auditorium. They will be discussed in a forthcoming companion paper. Acknowledgements I would like to thank Prof. Dae-Up Jeong at Chonbuk National University for his help with the development of the model listeners. I also would like to acknowledge Dr. John S. Bradley at National Research Council in Canada for his advice and discussions with the preparation of the manuscript. This work was partly supported by National Research Foundation of Korea Grant funded by the Korean Government (2012R1A1B5000491). References [1] Brebeck D, Bucklein R, Krauth E, Spandock R. Acoustically similar models as an aid in space acoustics. Acustica 1967;18:213–26. [2] Day BF. A tenth-scale model audience. Appl Acoust 1968;1:121–35. [3] Hegvold LW. A 1:8 scale model auditor. Appl Acoust 1971;4:237–56. [4] Cremer L, Muller HA, Schultz TJ. Principles and applications of room acoustics, vol. 1. Applied Science Publishers LTD.; 1982; p. 179. [5] Tahara Y, Shimoda H. 1/16 Scale model experiment for room acoustics. J Environ Eng AIJ 2006;608:1–7. [6] Tahara Y, Shimoda H. 1/16 Scale model experiment for room acoustics physical properties and auralized sound quality. Proceedings of 19th ICA; 2007. [7] Nagata Acoustics. News 12-04 (No.292); 2012. [8] Barron M, Coleman S. Measurements of the absorption by auditorium seating – a model study. J Sound Vib 2001;239(4):573–87. [9] Choi YJ, Bradley JS, Jeong DU. Effects of edge screens on the absorption of blocks of theatre chairs. Appl Acoust 2012;73:470–7. [10] Bradley JS. Predicting theatre chair absorption from reverberation chamber measurements. J Acoust Soc Am 1992;91:1514–24. [11] Martellota F, Cirillo E. Experimental studies of sound absorption by church pews. Appl Acoust 2009;70(3):441–9. [12] ISO 354-Acoustics. Measurement of sound absorption in a reverberation room; 2003. [13] Bradley JS, Choi YJ, Jeong DU. Understanding chair absorption characteristics using the perimeter-to-area method. Appl Acoust 2013;74:1060–8. [14] Beranek LL, Hidaka T. Sound absorption in concert halls by seats, occupied and unoccupied, and by the hall’s interior surfaces. J Acoust Soc Am 1998;104(6):3169–77. [15] von Gierke HE. Sound absorption at the surface of the body of man and animals. J Acoust Soc Am 1949;21(1):55. [16] Treeby Bradley E, Pan Jie, Paurobally Roshun M. An experimental study of acoustics impedance characteristics of human hair. J Acoust Soc Am 2007;122(4):2107–17. [17] OriginLab data analysis and graphing software.
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Applied Acoustics journal homepage: www.elsevier.com/locate/apacoust
Modeling chairs and occupants to closely approximate the sound absorption of occupied full scale theatre chairs Young-Ji Choi ⇑ Department of Architectural Engineering, Chonbuk National University, 664-14 1Ga, Duckjin-Dong, Duckjin-Gu, Jeonju, Jeonbuk 561-156, Republic of Korea
a r t i c l e
i n f o
Article history: Received 15 May 2013 Received in revised form 3 July 2013 Accepted 3 July 2013 Available online 2 August 2013 Keywords: Sound absorption Model chairs Model listeners
a b s t r a c t The present work reports on the process of modeling chairs and occupants to closely approximate the sound absorption of occupied full scale theatre chairs and explains how the best form of model listener was determined. Modifying the form of the model listeners to have shorter upper legs and narrower lower legs, led to improved agreement between model and full scale occupied chairs at all frequencies including at 125 Hz. The measured absorption coefficients of single blocks of model chairs with or without model listeners agreed well with the measured values for both full scale types E and G chairs. However, the estimated values for larger sample blocks of model chairs with P/A = 0.5 m1 showed better agreement with the measured values for full scale type G chairs than type E chairs due to the different slopes of the regression lines versus P/A. The present results demonstrate that the model chair and listener accurately simulate the sound absorption characteristics of a particular type of quite absorptive full scale occupied chairs for all sample sizes of the full scale chairs. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction The successful modeling of chairs and occupants in scale models is particularly important because the audience is usually the largest single component of absorption in an auditorium. Some previous studies [1–6] have demonstrated the development of model listeners for adding the sound absorption of an audience, seated on chairs, in model lecture rooms or auditoria. The model listeners have varied in form from egg cartons [1] to the simplified human forms [2,3,5]. Table 1 summarizes the details of model listeners developed in some earlier studies. Day [2] developed 1/10 scale model listeners with a simplified human form to investigate the effect of the degree of occupancy on the audience absorption in a model lecture room. The model listener consisted of smoothed softwood body and hardwood head. A single layer of surgical gauze was used for simulating human clothing. Hegvold [3] investigated the effects of the amount of clothing worn by a person on the absorption of a group of people and developed 1/8 scale model listeners to simulate the sound absorption of a typically dressed Sydney audience. The model listeners were constructed with a rigid polyurethane foam body and a sanded pine head. Hegvold adopted Day’s simplified human form to model the sound absorption of model Sydney listeners. Cremer et al. [4] used cardboard egg cartons to simulate the sound absorption of the audience in a 1/16 scale model of a ⇑ Tel.: +82 1029270563. E-mail addresses: [email protected], [email protected] 0003-682X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apacoust.2013.07.003
multi-purpose hall and found that the cardboard egg cartons were capable of simulating the partially diffuse reflections from the audience. In a recent study, Tahara et al. [5,6] developed 1/16 scale model listeners to simulate the audience absorption in a model auditorium. The model listeners consisted of a wooden head with a single layer of 1 mm felt simulating human hair and a wooden torso. Their model listeners were more similar to a head and torso simulator rather than the simplified more complete human form of model listeners developed by Day and Hegvold. Similar types of head and torso forms of model listeners were used in other recently reported scale model predictions [7]. The absorption coefficients of the simple forms of model listener were obtained from the measurements of single blocks of model chairs, and therefore the results were never justified to closely approximate the absorption characteristics of occupied full scale chairs for all sample sizes of the chairs. Some earlier studies demonstrated the value of using model tests to better understand the sound absorption of theatre chairs [3,8,9]. Reverberation chamber tests of both model and full scale chairs showed that the sound absorption characteristics of smaller blocks of chairs are not representative of those found for the larger blocks of chairs in an auditorium because the measured absorption coefficients vary with the dimensions of the sample and larger samples tend to have lower absorption coefficients due to edge effects [3,8–10]. The edge absorption may be reduced by using screens around the seating blocks, but diffraction effects are not eliminated by this method and the absorption coefficients of the seating blocks would still vary with the sample perimeter-to-area
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This process could enhance the credibility of future studies using these model chairs and occupants such as for investigation of the incremental effects of occupants in chairs or carpet under chairs on the absorption coefficients of each combination. It is important to better understand such interactive effects rather than to depend on measurements of every new situation. The goal of this work is to develop model chairs and occupants as closely representative as possible of common types of full scale theatre chairs with audiences. The present work reports on the process of modeling chairs and occupants to closely approximate the sound absorption of occupied full scale theatre chairs and describes how the best form of model listeners was determined. This paper was also intended to give some useful guidelines on constructing model occupants by investigating how the form and dimensions of model listeners affect to the measured sound absorption characteristics of occupied theatre chairs. 2. Measurement procedures Fig. 1. A photo of high absorption model chairs occupied with model listeners in the model reverberation chamber.
(P/A) ratio both in full scale [9–11] and in model tests [3,9]. This would also be true for the approach of putting absorbing samples in a well in the floor of the reverberation chamber [12]. This latter method is also not very practical for objects as large as chairs. The sound absorption coefficients of blocks of theatre chars have been shown to be linearly related to the sample P/A ratio [10]. One can predict the expected absorption coefficients of the larger blocks of chairs found in auditoria using linear regression fits to the measured values obtained from 5 or more sample blocks of chairs with varied P/A ratio in a reverberation chamber. This method is referred to as the P/A method. This method can more completely characterize particular types of chairs than one sound absorption test of a single sample block of chairs in the reverberation chamber. The method has been shown to accurately predict the absorption coefficients of chairs measured in an auditorium both in full scale [10,11] and in model tests [9,13]. Many have previously used scale model chairs and listeners, but previous efforts to create scale model chairs and occupants have not always been very thorough. There are a range of absorption characteristics for occupied and unoccupied theatre chairs and the absorption characteristics vary with P/A and the form of this variation varies with chair type [13]. It is not enough to compare single samples in reverberation chamber tests. One should validate model chairs and occupants to confirm that the variation of absorption coefficients with P/A for the model chairs and listeners are similar to those for common types of full scale chairs with occupants. This would ensure that absorption coefficients of the model chairs would be similar to the corresponding full scale chairs for all sample sizes of the chairs.
2.1. Model chair construction The main purpose of developing various types of model theatre chairs and listeners was to accurately simulate their sound absorption characteristics and to better understand how they affect to the acoustical conditions in real auditoria. The model chairs and occupants were developed to have similar absorption characteristics, as a function of P/A, to typical more highly absorbing full scale chairs and occupants. The previously reported results for full scale types E and G chairs [10,11] were used as design goals for the model chairs and occupants. The type E chairs were considered very highly absorptive chairs and the type G chairs more typically highly absorptive. These high absorption full scale theatre chairs had thicker absorbing material and included thick absorbing pads on the chair backs. The type E chairs also had absorptive padding on the rear of the seat backs as well as on the arm rests and sides of the chairs. They also had quite thick and absorptive seat cushions and perforated seat pans over glass fiber material. Chair type G had cloth covered metal of the rears of the seat backs. The slopes and intercept values of the regression lines for both unoccupied and occupied types E and G chairs were included in Ref. [13]. Beranek and Hidaka’s [14] estimates of the average absorption coefficients of the most absorptive group of chairs (their Group 1) were less absorptive than the values calculated from the measured types E and G full scale occupied chairs with P/A = 0.5 m1. A variety of 1/10 scale model chairs were systematically tested to develop a model chair with absorption characteristics similar to those for full scale high absorption chairs. Model chairs having a width of 0.6 m (full scale) were constructed as connected seats with arm rests and underpasses as shown in Fig. 1. The height of
Table 1 Summary of the model listeners developed in previous studies. The symbol ‘‘Ø’’ indicates the diameter of the model listener’s head measured in mm. Researcher (year)
Materials
Scale
Size (width length depth), mm
Brebek et al. [1] Cremer et al. [4] Day [2]
Egg cartons
1/10 1/16 1/10
–
Hegvold [3]
Head: sanded pine Body: polyurethane foam Cloth: no cloth
1/8
Tahara et al. [5]
Head: wood Torso: wood Cloth: felt 1 mm adding on the torso and head
1/16
Head: softwood Body: hardwood Cloth: surgical gauze adding on the body
Head: Ø25 Torso: 35 75 18 Hips: 35 45 18 Legs: 35 47 18 Head: 22.2 (depth) 31.7 (length) Torso: 54.6 73.6 22.2 Hips: 54.6 57.2 22.2 Legs: 54.6 58.4 22.2 Head: Ø10 Torso: 24 38 12
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Y.-J. Choi / Applied Acoustics 75 (2014) 52–58 1.0
Absorption coefficient
0.8
0.6
0.4
0.2 Felt 1 mm Bubble wrap 3mm Bubble wrap+Felt
0.0 125
250
500
1000
2000
4000
Frequency, Hz Fig. 2. Absorption coefficients of the materials added to model chairs (full scale).
the gap under the chairs was 280 mm (full scale). A 1.0 mm single layer of felt combined with a 3.0 mm single layer of bubble wrap was added on the seat back, rear of back, sides and undersides of the model chairs and these chairs are referred to as high absorption model chairs. The measured absorption coefficients of the materials when added to the model chairs are plotted in Fig. 2. The samples tested had an area of 10.2 m2 (full scale), which corresponds to a P/A ratio of 1.25 m1. The absorption characteristics of the combined bubble wrap and felt materials showed very similar trends over frequency to those found for unoccupied full scale chairs (see Fig. 5). The sound absorption coefficients of blocks of theatre chairs vary with sample P/A ratio and the sample blocks of model chairs should have the same P/A ratio as the full scale blocks of chairs used as design goals. The tests used blocks of 3 rows of 5 model chairs (R3C5) having a P/A ratio of 1.40 m1 (full scale) which was very similar to P/A of 1.39 m1 and 1.37 m1 for the blocks of 3 rows of 6 chairs (R3C6) of both unoccupied full scale types E and G chairs. 2.2. Model listener construction One-tenth scale model listeners were constructed using 10 mm thick expanded PVC (poly vinyl chloride) pieces and were scaled to be representative of average Koreans. The selection of materials for constructing model listeners similar to the absorption coefficients 1.0
Felt 0.5 mm Fabric 0.5 mm Pineapple sheet 0.5 mm Expanded PVC 10 mm
0.9
Absorption coefficient
0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 125
250
500
1000
2000
4000
Frequency, Hz Fig. 3. Absorption coefficients of the materials added to model listeners (full scale).
of the human body was carefully considered according to some earlier studies on the measurements of the absorption coefficients of the human body and hair [15,16]. Fig. 3 plots the measured absorption coefficients of the materials used for model listener construction. The samples tested had an area of 10.2 m2 (full scale), which corresponds to a P/A ratio of 1.25 m1. The absorption coefficients of the PVC pieces are very similar to those measured for real human body surfaces found in Ref. [15]. The absorption coefficients of human hair increased with increasing frequency but decreased above 2 kHz. The absorption coefficients of the 1 mm felt showed similar trends as measured for human hair found in Ref. [16]. It was found that there was considerable variation in the absorption coefficients resulting from different dress types worn by audiences [3]. The audiences wore light to medium clothes, such as a short sleeved shirts or suits, during the measurements in the full scale reverberation chamber tests and the materials added to the model listeners were selected to approximate the absorption characteristics of the dress types worn by full scale audiences. Prior to adding absorbing materials to the model listeners, the best form of model listeners was determined by testing 7 types of model listeners with various forms and dimensions. The absorption coefficients of each set of model listeners were measured while seated on the high absorption model chairs. A total of 7 types of model listeners with various forms and dimensions are illustrated in Fig. 4 and they are described in Table 2. ‘Upper legs’ are the portion of the legs above the knees and ‘lower legs, are the portion below the knees. Model listener type A had the widest upper and lower legs and was similar to the model occupant developed by Day [2] and Hegvold [3]. Model listener type B consisted of torso and upper legs, but had no lower legs. Type C had shorter upper legs than types A and B model listeners and no lower legs. Model listener type D had no upper and lower legs and was similar to the model listeners developed by Tahara et al. [5,6]. Although model listener type D had no upper and lower legs, and hence was not representative of real human body forms, it was intended to investigate the effects of the presence of the upper and lower legs on the occupied chair absorption characteristics. Model listener types E, F and G were modified versions of the type A listeners. Type E listeners had narrower lower legs than those of the type A model listeners. Type F had shorter upper legs and narrower lower legs than the other models. Type G had the two thin separate lower legs making them more similar to real humans. The best form of model listener was selected and tested with varied added absorbing material to develop a model listener with absorption characteristics most similar to those found for the full scale occupied theatre chairs. Table 3 describes various absorbing materials added to the best form of model. Four different types of absorbing materials were added to the head, torso and upper legs of the model listeners. No absorbing materials were added to the lower legs of model listeners. A total of 6 model listeners with varied added absorbing materials were constructed. The absorbing material named ‘pineapple sheet’ is a 0.5 mm thick paper having a pineapple pattern on it. Three absorbing materials, felt 0.5 mm, fabric 0.5 mm and pineapple sheet 0.5 mm were added to the torso and upper legs of the model listeners. Felt, 1.0 mm thick, was added to the head of model listeners to simulate the sound absorption by human hair (see Table 3). All tests were of sample blocks of 5 rows of 3 chairs in each row with a row-to-row spacing of 0.91 m (full scale). This particular sample block of model chairs had a P/A ratio of 1.55 m1 that is similar to P/A of 1.53 m1 for both unoccupied full scale types E and G chairs. After selecting the most successful model listener, the absorption coefficients of larger sample blocks of occupied model chairs with P/A = 0.5 m1 were estimated from the reverberation chamber measurements of seven different sample blocks of
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Fig. 4. Sketch of front and side views of 7 types of model listeners (unit: mm, full scale).
1.8
Table 2 7 Types of model listeners with various forms and dimensions. The symbol ‘‘Ø’’ indicates the diameter of the model listener’s head, measured in mm.
1.6
Name
Absorption coefficient
1.4 1.2
Type Type Type Type Type Type Type
1.0 0.8 0.6 0.4
A B C D E F G
Size (width length depth), mm Head
Torso
Upper legs
Lower legs
Ø25 Ø25 Ø25 Ø25 Ø25 Ø25 Ø25
36 60 10 36 60 10 36 60 10 36 60 10 36 60 10 36 60 10 36 60 10
36 40 10 36 40 10 36 25 10 – 36 40 10 36 30 10 36 30 10
36 30 10 – – – 20 30 10 15 30 5 5 30 5
Full scale type E chairs (unoccupied, P/A=1.39m-1) Full scale type G chairs (unoccupied, P/A=1.37m-1) Average, full scale types E and G chairs Model chairs (unoccupied, P/A=1.40 m-1)
0.2 0.0 125
250
500
1000
2000
4000
Frequency, Hz Fig. 5. Measured absorption coefficients of an R3C6 block of unoccupied model chairs compared with those measured for R3C6 blocks of both unoccupied full scale types E and G chairs as well as the average values of all two types of full scale chairs [10,13].
chairs having a range of P/A values between 1.29 and 2.40 m1 and compared to those estimated for full scale occupied chairs. The measured absorption coefficients of sample blocks of chairs vary with sample P/A and comparing the estimated absorption coefficients of larger sample blocks of chairs typically found in auditoria for both model and full scale chairs would be more appropriate. 2.3. Reverberation chamber measurements The volume of the 1/10 scale model reverberation chamber was 300 m3 (full scale) and it was built using 20 mm thick acrylic panels. In the model measurements, a 1.37-s logarithmic sine sweep from 1 kHz to 100 kHz was used, which corresponds to full-scale
frequencies from 100 Hz to 10 kHz. To eliminate the unwanted effects of air absorption, the model chamber was filled with nitrogen during each test. The reverberation chamber was kept at a constant temperature of 22 °C and a relative humidity of 4%. Six combinations of two source positions and three receiver positions were selected for measuring the absorption coefficients of the unoccupied chairs. A 20 dB range of each decay, from 5 dB to 25 dB, was used to calculate reverberation times according to the procedures described in ISO 354 [12]. Prior to the measurements, the diffusivity of the sound field in the reverberation room was examined according to ISO 354 [12]. There was no evidence of non-linear decays over this range when the absorption of the chairs was measured. The measurements were made in 1/3 octave bands, but the absorption coefficients were presented as octave band values derived by averaging the three individual 1/3 octave sound absorption coefficients in each octave band. A repeatability test of the measurements for the absorption coefficients of the chairs was carried out to check whether the results were consistent for each measurement. The measurements were repeated three times, and the results were presented as the mean absorption coefficients.
Table 3 Absorbing materials added to the best form of model listener. The meaning of letter symbols is explained in the table below the main table. Name
Best Best Best Best Best Best
type-Fe type-F type-PF type-PP type-33FePP type-27FePP
Materials, thickness mm Head
Torso
Upper legs
Lower legs
– – – – Ø33Felt, 1 Ø27Felt, 1
Felt, 0.5 Fabric, 0.5 Pineapple sheet, Pineapple sheet, Pineapple sheet, Pineapple sheet,
Felt, 0.5 Fabric, 0.5 Fabric, 0.5 Pineapple sheet, 0.5 Pineapple sheet, 0.5 Pineapple sheet, 0.5
– – – – – –
0.5 0.5 0.5 0.5
Symbol
Meaning
F P Fe
Fabric Pineapple sheet Felt
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Y.-J. Choi / Applied Acoustics 75 (2014) 52–58
By comparison the full scale measurements were performed in the 254 m3 reverberation chamber, at the National Research Council in Ottawa, having fixed diffuser panels as well as a large rotating vane. More details are included in Refs. [10] and [13].
3. Results and discussion 3.1. Developing model chairs and listeners representative of the sound absorption of occupied theatre chairs
1.6
1.6
1.4
1.4
1.2
1.2
1.0 0.8 0.6
Unoccupied model chairs Type A+chairs Type B+chairs Type C+chairs Type D+chairs
0.4 0.2 0.0
Absorption coefficient
Absorption coefficient
The measured absorption coefficients of the unoccupied high absorption model chairs were compared in Fig. 5 with those values measured for unoccupied full scale types E and G chairs as well as the average of both types of full scale chairs. The measured absorption coefficients of unoccupied model chairs in Fig. 5 show more similar characteristics in absorption coefficient over the frequency to those measured for type G chairs than with the type E chairs results. Chair type G had slightly higher absorption coefficients at 250 Hz than those measured for model chairs and the type E chairs. This is most likely due to the resonant sound absorption of the thin metal seat pans of type G chairs. The average absorption coefficients of the more absorptive two types of theatre chairs showed good agreement with the measured results for the model chairs. The materials of highly absorbing chairs can influence the absorption characteristics of the occupied chairs, but the occupants are the most important factor determining their absorption characteristics [13]. Various types of model audiences were developed in earlier studies, but guidelines on how to construct more accurate model listeners were generally not available. Therefore, it was very useful to provide measurement results showing how the form and dimensions of model listeners affect to the measured absorption characteristics of occupied theatre chairs. Fig. 6 compares the measured absorption coefficients of the types A to D model listeners seated on the high absorption model chairs to illustrate the effects of the form of the model listener. Adding model listeners to the chairs generally led to increases in the absorption coefficients which were largest at 125 Hz. The increases in 125 Hz absorption coefficients varied from 0.06 to 0.27. The largest increases in 125 Hz absorption coefficients occurred for the types A and B listeners. The type D model listener having no upper and lower legs had the smallest increase in absorption coefficient at 125 Hz. Since the absorption of the expanded PVC pieces at 125 Hz (Fig. 3) was quite small, the increases in the absorption coefficients at this frequency with model listeners added to the chairs, were likely due to
changes to the form of the model listener. The inclusion of upper and lower legs in the model listeners led to larger increases in absorption coefficients at 125 Hz. At 125 Hz the cavities between and under the chairs may act as resonant cavities and putting legs into them might change the cavities enough to change the resonant absorption of the cavities and to increase the low frequency absorption of the model theatre chairs. Adding model listener type D to the chairs caused a larger increase in absorption coefficients at 2000 and 4000 Hz than when adding the other model listener types to the chairs. This was because the model type D listeners, consisting of a torso only, did not cover some absorptive areas of the seat of the chairs that other model listener types, with upper and lower legs, did cover. Except for model listener type D, adding model listeners to the chairs caused a decrease in absorption coefficients at 4000 Hz. The greatest decrease was 0.14 which occurred when the type A model listeners were in the chairs. The decreased absorption at 4000 Hz was due to the model listeners covering some absorptive areas of the seat and back of the chairs and reducing the total absorption of the occupied chairs at high frequencies. Fig. 7 shows the measured absorption coefficients of high absorption model chairs occupied with model listener types A, E, F and G. Model listener types A, E, F and G have the same form of torso, but the dimensions of upper and lower legs varied (see Table 1). Types F and G listeners have the shortest upper legs and narrowest lower legs among the four model listener types. Adding model listener type E, having narrower lower legs, to the chairs slightly decreased the absorption coefficients at 1000 and 2000 Hz compared to chairs occupied with model listener type A. The differences were 0.03 and 0.05 at 1000 and 2000 Hz. Adding model listener type F having the shortest upper legs and the narrowest lower legs to the chairs led to a smaller increase in the absorption coefficients at 125 Hz than occurred for chairs occupied with types A and E model listeners. Type G model listeners, having two thin lower legs, had a slightly increased absorption coefficient at 125 Hz compared to chairs occupied with types A and E model listeners. However, the difference in absorption coefficients at 125 Hz between chairs occupied with types A and G listeners was only 0.04. Chairs with type G listeners were less absorptive at 1000 and 2000 Hz than chairs occupied with type A listeners. Adding type G model listeners to the chairs decreased the absorption coefficients at 1000 and 2000 Hz more than those found for chairs occupied with type A model listeners. The differences were 0.09 and 0.12 at 1000 and 2000 Hz respectively. Type F listeners, a
1.0 0.8 0.6
Unoccupied model chairs Type A+chairs Type E+chairs Type F+chairs Type G+chairs
0.4 0.2 0.0
125
250
500
1000
2000
4000
Frequency, Hz Fig. 6. Measured absorption coefficients of occupied chairs with varied forms of model listeners compared with the absorption coefficients of the unoccupied chairs.
125
250
500
1000
2000
4000
Frequency, Hz Fig. 7. Absorption coefficients of model listeners with varied dimensions of the legs of the model listeners.
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Y.-J. Choi / Applied Acoustics 75 (2014) 52–58 0.6
1.8
(a)
1.4
0.4
1.2
Slope
Absorption coefficient
1.6
1.0
Type F-Fe+chairs Type F-F+chairs Type F-PF+chairs Type F-PP+chairs Type F-33FePP+chairs Type F-27FePP+chairs Full scale type E chairs, occupied Full scale type G chairs, occupied Average, full scale types E and G chairs
0.8
0.6
0.2
Type E chairs Type G chairs Model chairs
0.0
0.4 125
250
500
1000
2000
4000
125
250
Frequency, Hz
500
1000
2000
4000
Frequency, Hz
Fig. 8. Measured absorption coefficients of type F form model listeners with varied added absorbing materials.
1.8 1.6
(b)
1.4 1.2
Intercept
modified human form of model listener having shorter upper legs and narrower lower legs, reduced the increase in the absorption coefficients at 125 Hz to 0.07. Therefore, model listener type F was selected as the best form of model listeners. Fig. 8 shows the measured absorption coefficients of type F form model listeners with varied added absorbing material. The measured absorption coefficients of full scale occupied theatre chairs are also plotted for comparison in Fig. 8. Adding 0.5 mm felt absorbing material to the model listeners reduced the absorption coefficients of the occupied chairs at most frequencies. That is, the felt did not increase the absorption coefficients of the occupied chairs as desired. Because the expanded PVC pieces and the felt had similar absorption coefficients (see Fig. 4), adding the 0.5 mm felt absorbing material to the model listeners covered the expanded PVC pieces and reduced the total absorption of the occupied chairs at most frequencies. Adding 0.5 mm fabric absorbing material to the model listeners led to increases in absorption coefficients at most frequencies and the increase in absorption coefficients at 125 Hz was the largest among the six model listeners with varied added absorbing materials. Adding 0.5 mm pineapple sheet absorbing material to the model listeners slightly reduced the absorption coefficients at 125 Hz and increased the absorption coefficients at other frequencies. Adding 1 mm felt absorbing material to the Ø33 mm head of the model listeners increased the absorption coefficients at most frequencies and was found to be the most successful model listener with similar absorption coefficients to the full scale occupied theatre chairs. At 125 Hz, model listener type F-33FePP had a slightly higher absorption coefficient than the full scale occupied type G chairs with a difference of 0.06. The occupied model chairs with model listeners type F-33Fe-PP showed good agreement of their absorption coefficients with the occupied full scale type G chairs. The difference between the absorption coefficients, averaged over
1.0 0.8 0.6 0.4 Type E chairs Type G chairs Model chairs
0.2 0.0 125
250
500
1000
2000
4000
Frequency, Hz Fig. 9. (a) Slopes and (b) intercepts of regression lines of absorption coefficients versus P/A for occupied full scale theatre chairs and model chairs.
frequency, for model chairs occupied with the most successful model listeners, and those measured for the full scale occupied type G chairs, were no more than 0.1.
3.2. Comparison of estimated absorption coefficients for samples of occupied model chairs with P/A = 0.5 m1 with those for full scale occupied chairs Linear regression lines were fitted to plots of absorption coefficient versus P/A for various blocks of chairs measured in the model reverberation chamber. Theses analyses were performed using the Origin plotting and analysis software [17] to perform standard least squares linear regression fits to the data. The slopes and intercept values of the regression lines were included in Table 4 as well as the statistical significance of the results. Fig. 9(a) plots the slopes
Table 4 Summary of: the standard errors of the estimate of the octave-band data about the regression lines (r), the statistical significance of the octave-band regression results (p-value), The values of slopes (b), and intercepts (a1) for high absorption model chairs (ns = not significant, p < 0.2, p < 0.05, p < 0.01). Chair type
N
Model chairs, occupied
7
Frequency, Hz
r p b
a¥
125
250
500
1000
2000
4000
0.052 0.106 0.664
0.040 0.307 0.686
0.049 0.264 0.953
0.061 0.317 0.949
0.047 0.394 0.868
0.043 0.470 0.786
58
Y.-J. Choi / Applied Acoustics 75 (2014) 52–58 1.8 1.6
Absorption coefficient
1.4 1.2 1.0 0.8 0.6 0.4
Full scale type E chairs ,occupied Full scale type G chairs ,occupied Average, full scale types E and G chairs Model chairs ,occupied
0.2 0.0 125
250
500
1000
2000
4000
Frequency, Hz Fig. 10. Comparison of absorption coefficients versus frequency for samples of occupied model chairs with P/A = 0.5 m1 with Bradley et al.’s [13] estimates of the absorption coefficients of two types of full scale occupied chairs as well as the average of all two types of full scale chairs.
of the regression lines versus frequency for occupied theatre chairs and model chairs. The intercepts of the regression lines are plotted versus octave band frequency in Fig. 9(b). The slopes and intercepts of the regression lines for the type G and model chairs in Fig. 9(a) and (b) are very similar at most frequencies. The type E chairs have higher absorption coefficient values at most frequencies. The model chairs are more representative of the more intermediate characteristics of the type G chairs than the more highly absorptive type E chairs. The absorption coefficients of model chairs for larger sample blocks of chairs with P/A = 0.5 m1 were calculated and compared with those estimated for full scale types E and G occupied chairs shown in Fig. 10. This gives more reliable comparisons by using the mean trend over several measurements of blocks of chairs. The estimated absorption coefficients of full scale type G occupied chairs were good agreement with those calculated for model chairs over frequency. Although the measured absorption coefficients from one single sample blocks of chairs for full scale chair E type were very similar to those measured for model chairs (see Fig. 5), the estimated values for larger sample blocks of chairs were quite different at most frequencies. This is likely due to the different slope values over frequency for both full scale type E and model chairs shown in Fig. 9(a). It is needed to understand the differences in characteristics of different types of chairs. 4. Conclusions The process of modeling chairs and occupants resulted in absorption characteristics that closely approximated the sound absorption of full scale occupied theatre chairs and the details of the successful model listeners were reported. A modified form of model listener (type F), having shorter upper legs and narrower lower legs than a typical human form of model listener (type A), reduced the increase in the absorption coefficient at 125 Hz and achieved better agreement with the absorption coefficients of occupied full scale chairs. The differences between the absorption coefficients, averaged over frequency for model chairs, occupied
with the most successful model listeners (type F-33FePP), and those measured for the full scale occupied theatre chairs were no more than 0.1. The measured absorption coefficients from one single sample block of chairs for model chairs with or without model listeners showed good agreement with the measured values for both full scale types E and G chairs. However, the estimated values for larger sample blocks of model chairs representative of chairs in an auditorium (P/A = 0.5 m1) were closer to the measured values for full scale type G chairs than to the type E chair results. That is, the model chairs and occupants were a better model of the P/ A effect for the type G chairs. The present results demonstrate that the model chair and listener that were developed accurately simulated the sound absorption characteristics of the type G full scale chairs. As the type G chairs are representative of many more absorptive full scale chairs, the model chairs and listeners should be more widely useful. The effects of P/A on the incremental effects of adding occupants have not been fully explored and the incremental effects of carpet under chairs have also not been considered. These are issues that also need to be better understood so that we can better estimate their effects on the total sound absorption in an auditorium. They will be discussed in a forthcoming companion paper. Acknowledgements I would like to thank Prof. Dae-Up Jeong at Chonbuk National University for his help with the development of the model listeners. I also would like to acknowledge Dr. John S. Bradley at National Research Council in Canada for his advice and discussions with the preparation of the manuscript. This work was partly supported by National Research Foundation of Korea Grant funded by the Korean Government (2012R1A1B5000491). References [1] Brebeck D, Bucklein R, Krauth E, Spandock R. Acoustically similar models as an aid in space acoustics. Acustica 1967;18:213–26. [2] Day BF. A tenth-scale model audience. Appl Acoust 1968;1:121–35. [3] Hegvold LW. A 1:8 scale model auditor. Appl Acoust 1971;4:237–56. [4] Cremer L, Muller HA, Schultz TJ. Principles and applications of room acoustics, vol. 1. Applied Science Publishers LTD.; 1982; p. 179. [5] Tahara Y, Shimoda H. 1/16 Scale model experiment for room acoustics. J Environ Eng AIJ 2006;608:1–7. [6] Tahara Y, Shimoda H. 1/16 Scale model experiment for room acoustics physical properties and auralized sound quality. Proceedings of 19th ICA; 2007. [7] Nagata Acoustics. News 12-04 (No.292); 2012. [8] Barron M, Coleman S. Measurements of the absorption by auditorium seating – a model study. J Sound Vib 2001;239(4):573–87. [9] Choi YJ, Bradley JS, Jeong DU. Effects of edge screens on the absorption of blocks of theatre chairs. Appl Acoust 2012;73:470–7. [10] Bradley JS. Predicting theatre chair absorption from reverberation chamber measurements. J Acoust Soc Am 1992;91:1514–24. [11] Martellota F, Cirillo E. Experimental studies of sound absorption by church pews. Appl Acoust 2009;70(3):441–9. [12] ISO 354-Acoustics. Measurement of sound absorption in a reverberation room; 2003. [13] Bradley JS, Choi YJ, Jeong DU. Understanding chair absorption characteristics using the perimeter-to-area method. Appl Acoust 2013;74:1060–8. [14] Beranek LL, Hidaka T. Sound absorption in concert halls by seats, occupied and unoccupied, and by the hall’s interior surfaces. J Acoust Soc Am 1998;104(6):3169–77. [15] von Gierke HE. Sound absorption at the surface of the body of man and animals. J Acoust Soc Am 1949;21(1):55. [16] Treeby Bradley E, Pan Jie, Paurobally Roshun M. An experimental study of acoustics impedance characteristics of human hair. J Acoust Soc Am 2007;122(4):2107–17. [17] OriginLab data analysis and graphing software.