Anisotropy in the sound field generated by a bubble chain

A vertical chain of rising bubbles represents a transition from individual to continuum behaviour in a compressible gas–liquid flow. Experiments on the distribution of acoustic pressure around a bubble chain revealed a strong anisotropy in the acoustic field in the frequency band generated by individual bubbles. Sound appeared to propagate much more efficiently along the chain than normal to it. A simple theoretical model using a linear coupled-oscillator approximation was developed to explain this result. Although comparison with experiments is inherently qualitative, the model clearly demonstrated the anisotropy. The model also reproduced the change in pulse waveform along the chain. The results suggest that the enhancement of sound intensity along the chain can to some extent be explained by bubbles acting as resonant amplifiers re-transmitting vibrations.