Transfer Function Method 2-microphone

Equation (7.3) is the reflection coefficient for the two microphone transfer function method. When the specimen is backed by a rigid back waU, there wiU be no transmitted waves and thus by conservation of energy, all the incident waves are reflected and absorbed. Thus the absorption coefficient ( i), can be expressed as in... 

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. 

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