Underwater low frequency subwoofer systems

Experiments with a gas-filled adiabatic bubble resonator of large diameter have shown that it is an efficient emitter of low-frequency seismic waves. The resonator has a single resonant frequency with a narrow bandwidth. The internal acoustic resonant structure changes the frequency response of the entire source and makes it possible to expand its bandwidth. An internal Helmholtz resonator converts the system into a double resonant system with a wide bandwidth. The bass-reflex resonator separates the pressure in front and behind the speaker cone, and the speaker works as an internal monopole driver. Any other known loudspeaker enclosure design can be used to create the desired multi-pole frequency response. A specific of the underwater enclosure is that it is loaded into the inner chamber of the bubble resonator. The bubble resonator works as an efficient impedance converter, matching the impedance of the internal resonant system to the water radiation impedance. Examples of various systems and the results of their tests in water are considered. Experiments have shown that a gas-filled underwater bubble covered with an elastic membrane and driven by an audio subwoofer can achieve source levels of 180–185 dB re 1 μPa@1 m over a wide frequency band.