A STUDY OF RESONANT ABSORBERS FOR UNDERWATER SOUND.

In the first part of the work a simple model of a resonant absorber is proposed from which is derived an expression relating the acoustic impedance at the surface of the absorber with the dynamic modulus of the material from which it is made. Results are presented of measurements of the impedance and dynamic moduli of resonant absorber samples having rectangularly shaped holes, to which the model presumably applies. In Part II a theoretical study is made of a resonant absorber consisting of a thin rubber membrane cemented or clamped over a metal disc in which circular holes have been bored. An expression for the specific acoustic admittance of this membrane type absorber is derived in terms of the tension in the membrane, the surface density, the damping constant, the radius of the holes, the number of holes per unit area, and the mass and thickness of the disc. The anechoic properties of such a surface is investigated for various values of the different parameters involved. Experimental procedures and apparatus used for the various measurements are also presented.