Sound Transmission Characteristics of a Layered Wall Construction

Abstract

It is well known that the transmission loss of most solid, isotropic walls is seriously limited by the high dynamic stiffness and the correspondingly fast speed of bending waves in a stiff wall. On the other hand, while a relatively limp wall will have a significantly higher transmission loss due to the lower bending wave speed, it may not be acceptable to the architect because of the low static rigidity. A new construction has been found which has high static stiffness but drastically lowered dynamic stiffness, thus permitting the simultaneous satisfaction of both acoustical and architectural requirements. The change in stiffness is effected by establishing a “shear-wave” motion of the core of a layered construction. A shear wave has a speed which is independent of frequency. Thus, the speed of transverse waves is constant over the frequency range in which this type of motion predominates. If the shear wave speed is chosen to be suitably low, the wall impedance is essentially massive in this frequency range. This construction is expected to have important applications especially in the field of architectural acoustics. Measured data on some particular embodiments of the idea will be presented.