The most critical parameter in characterizing the capability of a material of how well it can absorb sound or noise is called the acoustical absorption coefficient (AAC). A sound wave carries certain amount of the energy called sound energy. When a sound wave hit a material, portion of the sound energy will be reflected or "bounced" back. Simplistically speaking, from the noise reduction point of view, the lesser sound energy being bounced back, the better the effect of noise reduction. A value AAC=0 means sound energy being reflected completely, and a value AAC=1 means that all the sound energy is absorbed by the material, which is the best in noise reduction. American Society of Testing and Materials (ASTM) issued a standard in how to conduct the test of acoustical absorption coefficient on a specimen made by a specified material and how to determine the value of AAC based on the test result. The standard is ASTM C423-90a.
As part of this study, a testing specimen for ASTM C423-90a is fabricated (Figure-2). The mix design is 0.8 (crumb rubber) to 1 (bonding agent) with a mixed size of rubber particles using the second and third spray devices referred in above. The dimension of the specimen is quite large. It consists of four panels and each panel is 48 inches by 48 inches by 1 inch. Then, the specimen is shipped to Riverbank Acoustical Laboratories (RAL) at Geneva, Illinois, and the test is performed there in September, 1999. RAL is a highly reputable laboratory in conducting acoustic related tests. The testing result of the acoustical absorption coefficient versus frequency is obtained and is plotted in Figure-3. For the comparison purpose, also displayed in Figure-3 are the same coefficient for concrete and the aforementioned Carsonite noise barriers. It can be seen that the crumb rubber based specimen shows superiority in acoustical absorption.

Figure-2 Crumb rubber based test specimen for ASTM C423-90a

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Figure-3 Acoustic absorption coefficient versus frequency. Square symbols represent
for concrete noise barriers, circular symbols for Carsonite noise barriers,
and diamond symbols for the crumb rubber mix.