An extension of the transfer function method described in ASTM E 1050-90 for its use in semianechoic conditions

Abstract

The wave tube described in the above standard requires a rigid end characterized by a very high impedance, ensuring the correctness of the measurement of the sample-specific acoustic impedance. This method can also be employed for the measurement of other characteristic parameters associated with fibrous materials, more specifically, its dynamic flow impedance. If the rigid end is removed and the tube is enlarged with a piece containing absorbing material, the tube termination impedance is obtained when measuring without the probe. This impedance is affected by the tube modal response, which is a function of the tube’s length. If a fine slice of the sample (as compared to the wave length inside the tube) is then used, and again an impedance measurement is performed, the values obtained are in phase with the first measurement (without sample) and differ by the amount of the dynamic flow impedance of the sample. Measurements have been performed with Rockwool samples of different densities giving good results, as shown in a recent J. Sound Vib. paper. The advantage of this method is that measurements can be done with a nonideal end, as its effects are taken into account in the evaluation procedure.