Underwater sound source with tunable resonator for ocean acoustic tomography

Abstract

A high Q tunable resonator makes possible the construction of an underwater sound source with a very high efficiency. Tuning this narrow-band resonator to match the instantaneous frequency of a reference signal enables control of sound projection over a wide spectrum width. At any given time the source has one fundamental resonant frequency equal to the instant frequency of the frequency modulating signal and the sound radiates with maximum efficiency. Such sound sources find application in ocean acoustic tomography and deep-penetration seismic profiling. The analysis of the simplest known Helmholtz resonator shows its ability to instantaneously switch its frequency or to quickly adjust to a changing reference frequency. From this simplest lumped resonant source the consideration then turns to a complicated distributed system, such as a tunable organ pipe. The analysis shows that the organ pipe can also be quickly swept and instantly switch the radiated signal frequency. The 200–300-Hz swept frequency sound source was designed for ocean acoustic tomography. Test results for this sound source are presented showing that it has a high efficiency, up to 50%, 3 dBi directivity gain, and depth independent parameters.