Nonlinear coupling between the surface and volume modes of an oscillating bubble

An experimental study was conducted of the nonlinear coupling between the surface and volume modes of an oscillating bubble. Through this coupling, the surface modes of a bubble can produce monopole sound radiation, which has a higher radiation efficiency than the surface modes. A hydrophone and a hypodermic needle, connected to an air supply, were submerged in water in a small (7.1×7.1×7.7 cm3) sealed cell that was also connected to a regulated vacuum system for control of the ambient pressure. The sound radiation produced by releasing a bubble from the needle was monitored by a hydrophone and displayed on a digital oscilloscope. Two high‐speed video cameras simultaneously recorded the radial motion of the bubble and the sound radiation trace as displayed on an analog oscilloscope. At low ambient pressures (i.e., a few cm Hg), it was observed that after the bubble creation sound died out, a sound of the same frequency but with a lower and relatively constant magnitude followed. Therefore, it is believed that these subsequent acoustical emissions are evidence of the coupling between the surface and the volume modes of the bubble. Observation of a constant phase angle between the surface oscillations of the bubble and the radiated sound suggests a causal relationship.