Reverberation room method absorption coefficient

Test Methodfor Sound Absorption and Sound Absorption Coefficients by the Reverberation Room Method, ASTM C423-90a, ASTM, Philadelphia, Pa., 1990. Standard Practices for Mounting Test Specimens During Sound Absorption Tests, ASTM E795-92, ASTM, Philadelphia, Pa., 1992. 

ASTM C423-09, 2009. Standard test method for sound absorption and sound absorption coefficients by the reverberation room method. American Society for Testing and Materials. ASTM C522-03, 2009. Standard test method for airflow resistance of acoustical materials. 

McGrory, M., Cirac, D.C., Gaussen, O., Cahrera, D., 2012. Sound absorption coefficient measurement Re-examining the relationship between impedance tube and reverberant room method. In Proceedings of Acoustics 2012—Fremantle, Australia. 

Because the reverberation room test method approximates many real-world conditions, it is used to derive sound-absorption coefficients for evaluating the effect of most actual appHcations of sound-absorbing treatments. Sound-absorption coefficients pubflshed in acoustical textbooks and by manufacturers of acoustical materials are almost exclusively from reverberation room tests, and this may be assumed unless specified otherwise. 

When sound waves come in contact with a boundary obstacle, such as a wall or door, a portion of the sound wave energy is reflected, a portion is transmitted through the obstacle, and the rest is absorbed by the obstacle. One of the standard methods of measuring the effective sound absorption coefficient of an acoustical material is by finding its effect on the reverberation time, or decay rate, of the sound pressure level in the sound chamber. The total sound absorption in the receiving room is required to determine the noise reduction of the specimen being tested. The key here is to rule out the absorption of sound waves within the chamber that may be attributed to the door being tested. 

Sound absorbers are usually employed for modilying the sound reverberation in a room, suppressing undesired sound reflections from remote walls (echoes), and reducing the acoustical energy density and, hence, the sound pressure level in noisy rooms. There are two standard methods of measuring absorption coefficients. One is for measuring normal incidence absorption coefficients in an impedance tube and the other is for measuring random incidence absorption coefficients in a reverberation room (Bies and Hansen, 2009). 

Fig. 5.8 shows the setup for measuring the random incidence scattering coefficient in a reverberation room. The random incidence absorption coefficient of the test specimen can be obtained using the method mentioned in Section 5.4.2. The specular absorption coefficient is obtained by rotating the test specimen while phase lock averaging the reflected pulses. A circular test specimen is placed on a turntable. While the turntable is rotated, the impulse response is repeatedly measured. The initial parts of the impulse responses corresponding to the direct and specular components are highly correlated and remain unaltered as the test specimen is rotated, while the later parts in... 

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