Tina Famighetti

Advisor: Dr. Kenneth Cunefare

Integrated Acoustics Laboratory

George W. Woodruff School of Mechanical Engineering

Georgia Institute of Technology

A new reverberation chamber in the Integrated Acoustics Laboratory at The Georgia Institute of Technology must be configured and qualified for sound absorption measurements. It is desirable to make chamber reconfiguration easy and safe, and thus, the chamber is equipped with lightweight fiberglass diffusers. This research involves exploration of their performance.

Objectives

The objectives of this research are to optimize the configuration of a reverberation chamber and qualify it for sound absorption measurements according to ASTM C423 Appendices X1 and A3. Specifically, this will include:

• Exploratory testing to determine the optimum chamber configuration
• Experimental determination of the effectiveness of lightweight diffusers as a replacement for the heavy diffusers recommended in the standard. This will make chamber reconfiguration easier and safer.
• Qualification testing to demonstate the acceptable performance of the chamber

Background

For sound absorption testing, the parameter of interest is the decay rate of sound. From the decay rate, the sound absorption coefficient of a specimen can be calculated. The sound absorption coefficient is a ratio of the acoustic energy absorbed by a sample and the acoustic energy incident on its surface.

The following procedure is used to measure the decay rate of sound and thus calculate the absorption coefficient. Noise is generated in the chamber for several seconds, and then abruptly turned off. The decaying pressure level is recorded in 1/3 octave bands, and the slope of the decay is calculated using linear regression. This is performed with and without the test specimen in the chamber. The decay rate, d, for each case is then inserted into the following equations, to yield the sound absorption coefficient of the test specimen.

Qualification Requirements and Recommendations
To qualify as a reverberation chamber for sound absorption testing according to ASTM C423 Appendix A3:

• Sound decay rate must be sufficiently uniform throughout chamber
• Sound decay rate must not change significantly with test specimen location

The standard further recommends that diffusers be added to the chamber to increase diffusion. It recommends that the diffusers be made of damped sheets of a material with low sound absorption and weight at least 5 kg per square meter. Also, the diffuser-to-chamber surface area ratio should be 15-25%. To increase diffusion, rotating diffusers may also be added. They cause the room to appear to constantly change shape, which breaks up standing waves in the room.

Schematics of Chamber Setup

Methodology for Qualification
To determine the optimum chamber configuration (ASTM C423 Appendix X1):

• Experimentally measure the sound absorption coefficient of a highly absorptive specimen with no diffusers in the chamber.
• Add diffusers, one at a time, and repeat sound absorption tests.
• The diffuser configuration that produces the highest average absorption coefficient is the optimum configuration.

To qualify the chamber for sound absorption measurements, (ASTM C423 Appendix A3):

• Experimentally determine the normalized standard deviation of d over 5 microphone positions and over 3 specimen positions.
• Compare the normalized standard deviations to the maximum allowable values tabulated in C423 Appendix A3. To qualify, the standard deviations must be lower than the allowable values in all 1/3 octave bands between 100 and 5000 Hz.

Fiberglass Diffusers

• Lightweight (1 kg/m2)
• Safe for handling
• Easily reconfigured
• Inexpensive
• Corrugated to promote randomness

Results of Qualification Testing

As outlined in Appendix X1 of ASTM C423, it is expected that the mean absorption coefficient of a test specimen will increase with increasing number of diffusers (expressed as a ratio of diffuser surface area to chamber surface area), until it reaches a maximum, and thereafter, it will remain constant or decrease. As shown in the figure above, the maximum coefficient was measured when the surface area ratio of fiberglass diffusers was 19% .

Appendix A3 of C423 limits the amount of spatial variation of sound allowed in the chamber. The standard deviation of the decay rate across five microphone locations was calculated. The figure above shows that the deviation was too high in several of the frequency bands, especially at 125 Hz, 500 Hz, and above 6300 Hz.

Although the sound field within the chamber was not sufficiently diffuse according to the requirements of ASTM C423, the chamber is able to accurately measure the sound absorption of a test sample. A round robin test conducted in 2003, comprised of absorption data from 17 qualified laboratories, provides a confidence interval for the measurement of the sound absorption of a 72 sqft sample of CertaPro Insulation Board. When this test was duplicated in the reverberation chamber at Georgia Tech, the measured absorption coefficient was within the confidence interval in all frequency bands, as shown in the figure below. This result causes one to question the requirements of ASTM C423. Are they more stringent than necessary?

Future Work

The effectiveness of the fiberglass diffusers will be investigated further. Also, alternative types of diffusers will be tested to note the effect of their characteristics on the diffusion in the chamber.