Determination of the Dilatational Wave Speed in Elastomer layers by Means of an Interferometer

In recent years, TNO has developed a water-filled interferometer, which is able to determine acoustical properties of a sound absorbing material or a compliant layer like the Alberich coating. The frequency range of the measurements is 1.5 to 20 kHz. The main results are mostly in the form of absolute reflection losses of the material mounted onto an air-backed stiff, massive brass block. The transmission loss TL of the material mounted, for example, onto a semi-submerged steel plate cannot be obtained directly from these data. This difficulty can be circumvented by considering the complex impedance of the layer. This quantity yields much more information about the acoustical properties of the material itself, like the sound speed for dilatational waves. If this speed is known, it is in principle feasible to calculate transmission losses for a layer on a steel plate under various configurations. Wider applications are also possible, such as determination of compressional wave speeds of rubbers with cavities. Recently, a novel method has been developed in order to measure the complex (amplitude and phase) reflection coefficient as a function of frequency using the interferometer. From these data the complex impedance of a layer can be calculated. Corrections can be included for the fact that the terminator block has a finite impedance. The complex dilatational wave speed as a function of frequency can be determined by using a rootfinding algorithm in the complex impedance curves. Measurements of the dilatational wave speed in various rubbers with cylindrical cavities are presented. It is again shown that the introduction of air cavities in rubber (mounted on stiff material) reduces dramatically the compressional wave speed, but the novelty lies in the possibility of directly comparing experimental wave speeds with theoretical data.