The speed of sound in a bubbly liquid is strongly dependent upon the volume fraction of the gas phase, the bubble size distribution, and the frequency of the acoustic excitation. At sufficiently low frequencies, the speed of sound depends primarily on the gas volume fraction. This effect can be audibly demonstrated using a one-dimensional acoustic waveguide, in which the flow rate of air bubbles injected into a water-filled tube is varied by the user. The normal modes of the waveguide are excited by the sound of the bubbles being injected into the tube. As the flow rate is varied, the speed of sound varies as well, and hence, the resonance frequencies shift. This can be clearly heard through the use of an amplified hydrophone and the user can create aesthetically pleasing and even musical sounds. In addition, the apparatus can be used to verify a simple mathematical model known as Wood’s equation that relates the speed of sound of a bubbly liquid to its void fraction.

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The field inside the waveguide is spatially dependent according to Eq. (16). One can accentuate or suppress a mode by placing the hydrophone near an antinode or a node, respectively. Since the excitation is broadband, a number of modes are excited, but the relative amplitude of a particular mode received on the hydrophone depends on the hydrophone’s location inside the waveguide.

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