The acoustics group uses a two- microphone Bruel & Kjaer 4206 impedance tube to measure the absorption coefficient and surface impedance of material samples in agreement with ISO 10534-2.
In addition, an extension with two additional microphones is used to determine the transmission loss as well as the characteristic impedance and wave number within a material.

The system consists of a 100 mm tube for a freqeuncy range of 50 - 800 Hz (1.6 kHz with a reduced microphone spacing) and a 29 mm tube for frequencies of 500 - 6400 Hz.
Data aquisition and processing is done with B&K PULSE.

With two additonal microphones behind the sample, information about the wave components travelling in both directions is available for the cavities in front and behind the sample - as long as plane wave propagation can be assumed.
Based on the transfer matrix formualtion of the problem, the properties of the sample, especially the transmission loss, can be deduced [1].
If the sample is furthermore considered to be homogeneous, the wave number and characteristic impedance (or the complex speed of sound and the bulk density) can be determined [2].

Two examples are provided in the following figures. In the first row, the transmission loss of a 100 mm diameter disc of 1" Basotect is shown: the material is almost transparent to low frequency sound.
One the other hand, a 5 mm thick, lightweighted panel made from a stiff foam core covered with impregnated paper demonstrated a very high transmission loss. However, one has to keep in mind that this result is for a sample of finite size, with simply supported edges, and that an extended sample will show a much lower loss.

[1] B. H. Song and J. S. Bolton, “A transfer-matrix approach for estimating the characteristic impedance and wave numbers of limp and rigid porous materials”, J. Acoust. Soc. Am. 107, 1131-1152 (2000) [2] Allard JF., “Propagation of Sound in Porous Media: Modelling Sound Absorbing Materials.” Kluwer Academic Publishers (1993)