Measurements of the Resonant Frequency of a Bubble near a Rigid Boundary

The resonant frequency of an air bubble in water near a plane rigid boundary was measured by varying the frequency of a sound source immersed in the water. The bubble was observed through a microscope and judged to be resonant when standing waves just appeared on its surface. The values of resonant frequency measured in this way, for bubbles having a radius in the range 1/10–3 cm (frequency range 100–3000 cps), were found to be in close agreement with a theory due to Strasberg. Another set of measurements over the same frequency range, for a bubble trapped in a hole that was drilled in the rigid surface, was also in close agreement with theoretical predictions. Some additional observations were made at higher amplitudes, where it was found that the presence of the hole affected the motion of the bubble in some ways. In the absence of a hole, a long time-scale oscillation of the bubble shape was observed.