Naval Res. Lab., Washington, DC 20375-5000
An open end axisymmetrical cavity is used to study the generation of implosion sound. The cross section of the cavity's profile has a shape which will distort the wave front of a plane shock wave converging it into a single point. Hence, a spherical collapsing phenomenon can be approximately achieved when a shock wave propagates into the opening of this finite size artificial cavity. Through the shock and shock interference mechanism, a strong impulse therefore is generated at the focal point caused by the pressure difference between the inside and outside of the cavity. This highly localized impulse then is radiated out from the opening of the cavity as a sound projector. A prototype model of 0.5 l size has been constructed and tested in the laboratory. The preliminary experimental results indicated that this device can reach a peak sound pressure level of 146 dB//20 (mu)Pa in air and 185 dB//30 (mu)Pa in water, with an ambient pressure of 1 atmosphere (1 bar). The 50% pulse width is about 20 and 30 (mu)s for air and water, respectively. Analysis and application of this type of sound projector will be discussed.