The significance of high-order resonances of spherical bubbles to the acoustic response of fish with swimbladders.
The high-order resonant response of spherical gas bubbles in seawater, when insonified by an acoustic pulse of 12 cycles of 38 kHz, used for target strength measurements, shows substantial variation in shape with only small changes in bubble radius. In contrast, the amplitude response of a gas bubble near its fundamental resonance changes predominantly in magnitude as the radius changes. The fundamental response is very similar to that of a more rigid target such as a tungsten carbide sphere. It might initially be thought that the high-order resonant response of spherical gas bubbles in seawater would have little relevance to the response of a gas-filled spheroidal swimbladder immersed in a viscous fish body. However, target responses similar to those predicted theoretically for gas bubbles have been found in data collected in situ for estimating target strength from normal fish populations. With further theoretical and experimental work, high-order resonances could be a useful aid to estimating target size and possibly target species.