Absorption coefficient impedance tube method

In order to evaluate the acoustic properties of the various formulated composites, the sound absorption coefficients were determined by the impedance tube method according to ASTM E 1050-12. A soxmd absorption coefficients and impedance measurements instruments (Figure 16.3) made by Walen Audio Technologies, Maharashtra, India was adopted. The sample diameter was 100 mm, and each value represented the average of six samples. The soxmd absorption coefficients were measured in six frequencies 100, 200,400, 800,1600 and 3200 Hz. 

One advantage of the impedance tube test methods is the small (usually <10 cm (4 ia.) dia) size of the test samples. For these tests sound impinges on the test sample only at normal iacidence to the surface, and the sound-absorption coefficients derived ia this manner are vaUd only at this angle. 

One of the drawbacks of the two-microphone transfer function method is that the absorption coefficient determined may not be a true representation of the material s characteristic. In the case of a porous material, such as silica aerogels, the reflected wave from the rigid wall could contribute to a rise in the absorbed energy by the material. To account for this uncertainty, the four-microphone impedance tube setup is usually used to determine the transmission loss (TL) and absorption coefficient (Feng 2013). In the absence of additional microphones downstream of the specimen, a sound meter could be used instead to measure the TL of the specimen under test. However, the sound meter picks up discrete transmitted signals at periodic interval, which could result in a mismatch with the generated signals from the source. 

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. 5.6 Setup for normal incidence absorption coefficient measurements within an impedance tube based on the standing wave method.

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. 

ISO 10534-2, 1998. Acoustics— Determination of sound absorption coefficient and impedance in impedance tubes— Transfer function method. International Organization for Standardization. 

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