ABSTRACT
This chapter covers a variety of methods used to measure and characterize acoustic absorbers. For many practitioners, the only important measurement is to gain the random incidence absorption coefficient in a reverberation chamber. While this may be the absorption coefficient that is needed for performance specifications in room design, other measurements are needed to understand and model absorptive materials. This is particularly true, because the prediction of random incidence absorption coefficient is problematic, and consequently it is necessary to measure materials in a more controlled environment to allow direct comparison between theory and measurements. The more controlled environment that is often used is the impedance tube, which
allows normal incidence impedance and absorption to be determined. Less often, but nevertheless valuable, are the free field measurements on large area samples done in hemi-anechoic spaces. The most common free field method uses a two-microphone approach, but this is often only applicable to isotropic, homogeneous samples. Consequently, attention has recently turned to using more than two microphones; however, the measurements appear to be problematic and very noise sensitive. The next subject for this chapter is in situ measurements for obtaining acoustic
absorption and impedance. There has been recent interest in these techniques to allow the true absorption properties of materials after construction and in their final in use condition to be determined. There is also a need to be able to characterize the propagation within the absorbent
material, to enable theoretical modelling of the absorption properties. For instance, the well-known Delaney and Bazley empirical model outlined in Chapter 5 requires the flow resistivity and porosity of the absorptive material to be known. For this reason, methods to measure key parameters that characterize the propagation within the absorbent are outlined. The intention of the chapter is to provide information on the different measurement
techniques. It is not intended that each description is a comprehensive standard with a foolproof description of how to carry out the measurements. What is intended is that the reader should be able to make an informed decision about the different techniques described, understand the advantages and disadvantages, and supplement the descriptions given here with that available in the referenced literature.
