Study of Wenger Audience Seating Chair Absorption Ron Freiheit January 22, 2002
Study of Wenger Audience Seating Chair Absorption Purpose The purpose of this project was to evaluate the acoustical performance of Wenger Audience Seating Chairs. Results from testing will provide absorption numbers (absorption coefficient) that can be integrated into the acoustics design of various spaces utilizing the Wenger Audience Seating Chair. History The acoustic characteristics of performance spaces are of paramount importance especially when used to support music performances. In the desire to create the best possible acoustics, specialists in the study of sound (acousticians) have spent much time researching how this can be successfully done. One of the most important factors in designing the space for good acoustics is realizing the impact that the construction materials and furnishings will have on the acoustics. Wenger Corporation conducted testing of the new Wenger audience-seating chair to investigate its acoustical characteristics. The results from the testing provide information on the acoustic absorption of the chair. With this information, acousticians will be able take into account the acoustic impact of the audience seating. Products The Wenger Audience Seating Chairs that were tested were Model 981100. Each chair is 32 high, 23 wide and 26 deep (20 deep with the seat in the folded position). The seat position was in the folded position for testing. The seat cushion is 2.5 high resiliency polyurethane foam with a 5-ply 3/8 molded hardwood plywood structure. The back cushion is 1.5 high resiliency polyurethane foam on a 5 ply, 3/8 molded hardwood plywood substrate. The seat and back cushions are covered with 100% polypropylene fabric. The armrests are solid hardwood with rounded corners and a black finish. The chairs are free standing with a built-in ganging device. Test Equipment The test equipment used was the Larson-Davis dual channel 2900B, S/N 2900B0891. The microphone capsule was a Larson-Davis Model 2559 Random Incidence Type S/N 2224 and a Larson-Davis M900B microphone preamp, S/N 2826. The calibrator used was a Larson-Davis Model CA 250, S/N 0298 which produces a 114 db SPL @ 250 Hz. A B&K rotating boom was utilized for the measurements. The speaker was Page 2
a Community XLT43 S/N 26568. The source was internal from the Larson-Davie 2900B. Filtering on the source signal was done with a Rane ME 15 two-thirds band octave equalizer, S/N 00226 and amplified by a Crown Micro-Tech 1200 (320W/channel @ 8 ohms) amplifier S/N A178753. Test Environment Testing was carried out at the Wenger acoustic test lab located at the Wenger Corporation in Owatonna MN. Temperature was 23 C (73 F) and a relative humidity of 55%. The reverberant room that was used has a cubic volume of 3153 cu. ft. The dimensions of the room are 14.4 ft. x 14.6 ft. 14.9 ft. Due to the low cubic volume and nearly equal room dimensions the modal density is not high enough to support accurate measurements below 200 Hz. Reverberation Measurements Reverberation times (T 60 ) were made from 20 measurements (averaged) using a rotating microphone boom set to two revolutions per minute. The analyzer was set to measure and curve fit from 5dB point to the 25dB point to establish the slope of the sound decay for frequencies from 125 Hz to 4000 Hz. Background noise measurements were also taken. Audience Seating Testing Testing Empty Room The absorption of the room was calculated in one-third octave bands from 125 Hz to 4000 Hz. A = Empty room absorption S = Surface area of the reverberant room α sab = Average absorption of the empty test room R = Room constant V A = 0.161 T60 S = 119.8m 2 α sab = A S R A = 1 α sab Page 3
Calculations for total absorption (A), average Sabine absorption coefficient (α sab ) and room constant (R) are shown by one-third octave band frequencies (f) in the following table: Empty Test Room f A (sab) R T 60 125 16.26 0.14 18.81 0.884 160 13.70 0.11 15.47 1.049 200 7.79 0.06 8.33 1.846 250 5.51 0.05 5.78 2.609 315 5.23 0.04 5.47 2.749 400 4.54 0.04 4.72 3.168 500 3.81 0.03 3.93 3.776 630 3.74 0.03 3.86 3.84 800 3.84 0.03 3.97 3.743 1000 3.97 0.03 4.11 3.618 1250 4.43 0.04 4.60 3.245 1600 4.44 0.04 4.61 3.235 2000 5.01 0.04 5.23 2.871 2500 5.36 0.04 5.61 2.684 3150 5.80 0.05 6.09 2.479 4000 6.87 0.06 7.29 2.092 The mean absorption coefficient for the empty test room is 0.052. Using the recommended limits for uniform absorption (0.5 to 1.5 of the mean value) show a range of 0.026 to 0.078. The 125Hz and 160 Hz absorption falls outside this limit and should not be used (highlighted in the above table). Testing with Audience Seating Chairs Eight Audience Seating Chairs were arranged in two rows of four each. The approximate area covered by the chairs was 48 sq. ft. The average α sab for each audience-seating chair tested was calculated from the formula: 0.161V 1 1 ( ) ( ) α = + sab α 2 sab 1 Sa T2 T1 T 2 = Reverberation time (T 60 ) of the test room with the chairs T 1 = Reverberation time (T 60 ) of the test room without chairs S a = Surface area of the chairs (estimated) V = Volume of the test room Page 4
(α sab ) 1 = Absorption coefficient of the empty test room (α sab ) 2 = Absorption coefficient of the chairs From the reverberation measurements the absorption of the audienceseating chair shown below: Chair Absorption f empty room chair 125 0.13 0.52 160 0.11 0.50 200 0.06 0.64 250 0.05 0.53 315 0.04 0.57 400 0.04 0.62 500 0.03 0.72 630 0.03 0.84 800 0.03 0.83 1000 0.03 0.82 1250 0.03 0.74 1600 0.04 0.77 2000 0.04 0.81 2500 0.05 0.85 3150 0.05 0.92 4000 0.06 0.92 Chair Absorption Coefficient 1.00 Absorption (chair) 0.80 0.60 0.40 0.20 0.00 125 160 200 250 315 400 500 630 800 1000 1250 1600 2000 2500 3150 4000 Chair Absoprtion 0.52 0.50 0.64 0.53 0.57 0.62 0.72 0.84 0.83 0.82 0.74 0.77 0.81 0.85 0.92 0.92 Frequency Limitations of Testing: Page 5
Due to the small size and nearly square dimensions of the test room, modal density is deficient below 200 Hz. Data highlighted in yellow is shown for information, but should not be used for performance analysis. Statistical certainty was not calculated as part of this experiment. The effects of temperature and humidity were not included in the calculations. Conclusions: The absorption of the Wenger Audience Seating chair modestly compares to the same absorption of a person seated in the chair. Typical absorption coefficients given for a person seated in an upholstered chair 1 are: 125 Hz 250 Hz 500 Hz 1000 HZ 2000 Hz 4000 Hz.39.57.80.94.92.87 From an acoustics point of view this allows the space to be fairly consistent even with variations in the number in the audience. 1 M. David Egan, Architectural Acoustics, Mc-Graw Hill Publishing, 1988 Page 6
Data Plots Experiment Pictures and Test Measurements Page 7
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