Acoustical Testing 1

Transcription

1 Material Study By: IRINEO JAIMES TEAM ANDREW MILLER SAM SHROYER NATHAN NEGRU ERICH PFISTER Acoustical Testing 1 Dr. Lauren Ronsse, Dr. Dominique Chéenne 11/05/2014

2 Table of Contents Abstract. 3 Introduction....3 Test Method and Modifications.. 4 Measurement set up and Equipment Room Modal Frequency Analysis..10 Results.. 11 Equations..13 Conclusion 14 Appendix...15

3 ABSTRACT The purpose of this experiment was to find absorption coefficient of Sonofiber Auralex panels. Parameters presented in The Standard Method for Sound Absorption and Sound Absorption Coefficients by the Reverberation Room Method (Designation: c423-00) paper were followed but limitations due to a pre-existing pillar and unavailable equipment required modifications to the parameters. INTRODUCTION The main focus of this experiment was to determine the absorption coefficient of Sonofiber Auralex absorption panels from 100Hz to 5000Hz using parameters set forth by The Standard Method for Sound Absorption and Sound Absorption Coefficients by the Reverberation Room Method (Designation: c420-00) paper. The team was able to calculate the absorption coefficients of the panels with some modifications to the parameters to best fit the property of the CCC chamber and to accommodate for a pre-existing pillar in the room. Columbia College Chicago s reverb chamber is located in the basement of 33 East Congress Parkway. The 72ft^2 (6.69m) area of the room dimensions, Sonofiber panels, and microphone positions are depicted in Figure 1.

4 TESTING METHOD AND MODIFICATIONS Figure 1: plan view of CCC reverb chamber Table 1 has been created in order to compare the standard vs. implemented parameters. In the right column (Test Standard) are the parameters, on the right column (CCC reverb Chamber) is the application of the standard and adjustments made. TEST STANDARDS 1) Volume - Room should be no less than 125 m^3 or ft^3 2) Diffuse Sound Field - Satisfactory approximation can be achieved with sound reflectors hung or distributed with random orientations about the volume of room. - Experience has shown that a satisfactory approximation can be achieved with a number of sound-reflective panels hung or distributed with random orientation. CCC REVERB CHAMBER Volume CCC reverb chamber is m^3 or ft^3 Reflection panels were not available, concluded that the present diffuse sound field in the CCC reverb chamber was close to desired diffuse sound field. *Pre -existing pillar prevent the microphones from receiving equal amounts of energy from all directions by causing unwanted early reflections

5 3) Background Noise Level - Level shall be 15dB below lowest level used to calculate decay rate. 4) A-Mounting for Specimen - The specimen can be placed on floor but should avoid placing in center of the room or avoid specimen sides being parallel to any other walls. 5) Distance From Surfaces - No part of the specimen shall be closer than 2.5ft (.75m) to a reflective surface. 6) Mic Measurements - Fixed microphone needs 5 measurements at 35 different positions which are at least 5ft (1.524m) apart. 7) Mic Distance From Specimen - Fixed microphone shall be at least 1.5ft (.0457m) from surface of the test specimen. 8) Relative Humidity - Relative humidity in the room shall be greater than 40% during the test. 9) Room Construction - The room is best constructed of massive masonry or concrete materials, but other materials, such as well-damped steel, may be used. Lighter construction may be excessively absorptive at 200Hz. 10) Microphone - The microphone or microphones used to measure decay rate shall be omnidirectional with a flat ( + 1 db within any one-third octave band) 11) Measurement of sound absorption coefficient - The average absorption coefficient of the room surfaces at each frequency, determined by Absorption of the empty room area of the room surfaces Highest level was 115dB *Lowest background noise level was at 33dB at 100Hz. Figure 1, shows the placement of the specimen. The Sonofiber Auralex absorption panels were not placed in the center and avoided and avoided the specimen s sides from being parallel to any wall. The closest distance the specimen was to a reflective surface was approximately 4ft well above 2.5ft (.75m). Figure 1 shows microphone positions, and their relative position to one another. All microphones were positioned 5ft (1.524m) above specimen. Relative humidity was at 45% *Temperature was at 73% Previously the CCC reverb chamber was a bank vault and later converted. Walls made up of composite steel with an air gap behind it followed by a concrete wall. ECM800 ( specifications of microphone located in appendix) Absorption Absorption - Empty Room coefficient

6 Shall be less than or equal to.05 12) Test Signal - The test signal shall be band of random noise with continuous spectrum covering the range over which measurements are made. The frequency range of the measurements shall include the onethird octave bands with mid-band frequencies, as defined in ANSI S1.0 from 100Hz to 5000Hz Using pink noise generated by SpectraPlus, measurements from 40Hz to 20000Hz were gathered. For test purposes, absorption coefficients from 100Hz to 5000Hz were the focus. Table 1: Conditions to qualify a room as a diffuse field compared with the conditions present/followed in the CCC reverb chamber tests Measurement set up and Equipment For test microphone an ECM8000 was used as stated in Table 1. A Motu Interface (828MK3) was used to provided a high quality signal. Figure 2: Motu interface converts analog to digital, providing good quality single

7 Four test speakers were used to produce sound in the reverberation chamber. The speakers where placed facing the corners of the reverberation chamber to create diffusion. Figure 3: Mackie speaker facing room corners The test specimen was Sonofiber Auralex Panels. The configuration of the specimen is shown in Figure 4. Figure 4: Sonofiber (72ft^2) panel, made up of 18 (2ft x 2ft) panels Aviom An-16/I were used to connect the inputs and outputs from the modeling lab to the reverberation chamber. - Figure 5: visual representation of Aviom An-16/i

8 SpectraPlus tone generator was used to excite the room with pink noise. The test microphone would then pick up the excitation then redirected back into SpectraPlus where its algorithms calculated decay time for specific frequencies. Figure 6: Computer connected through patchbay in modeling lab to patchbay in reverb chamber. Figure 7: Signal chain flow diagram of test

9 db Figure 7 is the signal chain diagram. A single tone was generated. SpectraPlus in this case generated pink noise. The tone went through signal processors, then played through the speakers in the reverberation chamber. The microphone in the reverberation chamber picked up the frequency energy content and then went back into signal processors and into SpectraPlus, where SpectraPlus calculated T-60 Values. Prior to testing, background noise levels was measured to satisfy condition 3 from Table 1. LEQ levels were measured in the CCC reverb chamber for 2 minutes. Longer LEQ could have been achieved, however, the CCC reverb chamber is located in the basement of Columbia College Chicago. The only outside interference that could have created drastic changes in background noise level would have been the CTA train, located in close proximity to the building. The low frequency boost would have occurred below 100Hz and would not be a factor in the final test result if the train happened to run by while testing SPL (db re 20uPA) Freq (Hz) Figure 8: LEQ levels for the given frequencies [numerical values are located in appendix]

10 The highest LEQ value was measured at 100 Hz at approximately 33 db. With the standard only requiring the lowest level used to calculate decay rate to be 15 db below the highest value of LEQ, the test signals generated in the chamber was at 115 db. With decay rate levels reaching 95dB at the lowest, well above the noise floor present. Satisfying the background noise level condition, microphone positions were chosen based on condition 7 & 10 from Table 1. For each microphone position, (Mic-[B thru F], 5 measurements were calculated by SpectraPlus. T-60 values for both CCC reverb chamber empty and CCC reverb chamber with specimen+, with a total of fifty T-60 values. Taking the average of each microphone s T-60 values, decay rate, absorption of room, absorption of material, and absorption coefficient of the specimen can be derived. Calculations located in Table 2.1 in appendix. Refer to the section Equations in report to review individual formulas. ROOM MODEL FREQUENCY ANALYSIS To get a better understanding of possible sound pressure level differences, the lowest modal frequencies were calculated. Mode # (Length, Width, Height) Frequency 1, 0, , 1, , 0, , 1, , 0, , 1, , 0, , 0, , 1, , 0, Table 2: under the equations section, Equation 1 was used

11 Seconds (s) When audibly testing mode (0, 1, 0), by walking throughout the room, a sound pressure level different did occur mid-way in the width (x) dimension, as predicted by the calculation. However, throughout the room unpredictable sound pressure level drops were audibly heard. Reviewing T-60 values for the same frequency at different microphone positions yields decay rate discrepancies from one microphone to the next. RESULTS The unexpected sound pressure level drops can be better understood by graphing T-60 values vs frequencies Figure Mic-B Mic-C Mic-D Mic-E Mic-F Freq (Hz) Figure 9: T-60 over each frequency tested in the CCC reverb chamber The chart, in a perfectly diffuse sound field with no interference from axial modes would have all 5 of the microphone curves layered on top of one another and theoretically just be one plotted curve. Figure 9 shows just how much T-60 differed throughout the room. The outlined

12 section covering roughly 1 2 khz, has peaks that varied when they should have been similar. The differences in T-60 values at different parts of the room were taken into account when reviewing final results. 1.2 Abs. Coefficient Figure 10: absorption coefficients of Sonofiber panels from 100Hz to 5000Hz, numerical values in Table 1.1 in Appendix Reviewing the Absorption Coefficients from Figure 10, the Sonofiber ability to reduce T-60 between 1 khz and 4 khz is high, with a roll-off happening out of this range. The expectation was below 500Hz the effectiveness of the material to reduce T-60 values would decrease due to low frequency properties. With these findings it was concluded the Sonofiber panels are great to place within an environment requiring T-60 values to be as low as possible around 1 khz to 4 KHz, in many cases would be a vocal performance space. Surprisingly, with varying T-60 values throughout the room the results did not produce skewed final absorption coefficient results.

13 EQUATIONS Equation 1 calculates axial modes, given the dimensions of the room. Equation 2 calculates the speed of sound when given temperature in Fahrenheit. Equation 3 calculates the decay rate, time it takes a signal to decay 60 db. Equation 4 calculates total absorption of the room when decay rate, speed of sound, and volume are known. Equation 5 calculates the total absorption due to the specimen. Equation 6 calculates the absorption coefficient of the specimen. Equation 1: Modal Frequencies = C ( l 2 Lx )2 + ( m Ly )2 + ( n Lz )2 l = Length integer w = width integer h = height integer [Hz] C= Speed of Sound Lx = length of room Ly = width of room Lz = height of room Equation 2: Speed of Sound = Temperature(Fahrenheit) [ft/s] Equation 3: Equation 4: Decay Rate = 60dB T60 [db/seconds] Total Absorption for both empty & with specimen = C = Speed of sound.921 (V)(Decay Rate) C V= Volume [Sabine] Equation 5: Specimen Total Absorption = [Total Abs. w/specimen - Total Abs. empty] [Sabine]

14 Equation 6: Absorption Coefficient of Specimen = Specimen Total Absorption Area of Specimen [Sabine/ft^2] *Area of specimen (Sonofiber Auralex panels) = 72 ft^2 or 6.69 m^2 *Volume of CCC reverb chamber = m^3 or ft^3 *Temperature = 73 degrees Fahrenheit CONCLUSION The focus of the paper was to find the absorption coefficient of the Sonofiber Auralex panels by averaging calculated T-60 values throughout the reverb chamber. With a less than ideal diffuse sound field modifications to the The Standard Method for Sound Absorption and Sound Absorption Coefficients by the Reverberation Room Method (Designation: c423-00) still yielded reliable applicable absorption coefficients. Finally Sonofiber Auralex absorption panels are great for vocal performance spaces. For future studies an addition of reflection panels to the reverb chamber capable of eliminating axial modes would yield more accurate results. Expanding the test method to include a frequency sweep and an impulse response can offer more useful data to analyze. Gathering data about the modal properties of the CCC reverb chamber can prevent poor microphone placement.

15 Appendix * LEQ Values (numerical values for figure 8) Freq. (Hz) SPL (db re 20uPA) Freq. (Hz) SPL(dB re 20uPA)

16 Table 1.1 Table 2.1

17 Frequency response and directionality of microphone EMC-8000 Behringer