Underwater subwoofer systems for sound emission in the low frequency range

A large gas-filled adiabatic bubble resonator is an efficient source of low-frequency and ultra-low-frequency seismic waves. The bubble resonator has a single resonant frequency with not very large bandwidth. Its resonant frequency is difficult to change without changing the size of the original system, which can be very large for an ultra-low frequency band. The internal acoustic resonant structure can change the frequency response of the entire source and expand its bandwidth. Internal resonant systems can be designed so that the bubble resonator can be tuned over a wide frequency range. Other systems may add one or more internal resonances and spread the emitted spectrum over a very wide frequency band. An internal Helmholtz resonator convert the system from a single resonant to a double resonant system with a wide bandwidth. By lowering the resonant frequency of the Helmholtz resonator, it is possible to expand the bandwidth of the system below the principal resonance of the bubble resonator to the ultralow frequency range. This approach can be extended to more complex structures. A subwoofer in a bass-reflex enclosure works like a monopole inside a bubble resonator. The bass reflex resonator separates the pressure in front and behind the subwoofer cone and prevents waves with opposite phases from canceling each other out. Any other known audio loudspeaker enclosure design can be used to create the demanded frequency response of a multi-pole underwater subwoofer. The specific of the subwoofer underwater encloser resonator is that it is loaded not into the open air, but into the inner chamber of the bubble resonator. The underwater bubble resonator works as an efficient impedance converter, matching the impedance of the internal resonant system with the water radiation impedance. Impedance matching improves efficiency and source level. The proposed approach opens the way to the design of simple and inexpensive low-frequency broadband seismic sources. Examples of various systems and the results of their tests in water are considered in this paper. Experiments have shown that a gas-filled underwater bubble driven by a standard audio subwoofer can achieve source levels more than 185 dB re 1 µPa per 1 m over a wide frequency band. The results of the experimental verification are discussed.