Measurements of the Attenuation of Sound by a Warm Air Fog

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  • 1 Department of Mechanical Engineering, Tufts University, Medford, Mass
  • | 2 Division of Engineering, Brown University, Providence, R.I
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Abstract

Measurements of sound attenuation by a warm air fog are presented for dimensionless frequencies, ωτ/Cm (where τt is the thermal relaxation time for the droplet, ω the circular acoustic frequency, and Cm the liquid mass fraction), near unity where the effects of mass transfer are dominant. The tests are made in a Wilson cloud chamber by measuring the rate of decay of the fundamental mode of acoustic oscillation which is excited during the operation of the chamber. Measurements of pressure and volume are made continuously during the expansion. Droplet size and concentration of the monodisperse fog are determined from time-resolved measurements of the optical transmission at two wavelengths of light. The range of ωτ/Cm from 0.5– 16 is scanned by varying droplet size and concentration through the controlled addition of condensation nuclei. This range includes the Napier frequency (frequency for which attenuation per unit wavelength is maximum), and the resulting data indicate that a maximum dimensionless attenuation occurs for a value of ωτ/Cm≈1, as predicted by the theory of Cole and Dobbins. The magnitude of the maximum dimensionless attenuation is approximately 100 times that which would be obtained for a corresponding nonvolatile aerosol.

Abstract

Measurements of sound attenuation by a warm air fog are presented for dimensionless frequencies, ωτ/Cm (where τt is the thermal relaxation time for the droplet, ω the circular acoustic frequency, and Cm the liquid mass fraction), near unity where the effects of mass transfer are dominant. The tests are made in a Wilson cloud chamber by measuring the rate of decay of the fundamental mode of acoustic oscillation which is excited during the operation of the chamber. Measurements of pressure and volume are made continuously during the expansion. Droplet size and concentration of the monodisperse fog are determined from time-resolved measurements of the optical transmission at two wavelengths of light. The range of ωτ/Cm from 0.5– 16 is scanned by varying droplet size and concentration through the controlled addition of condensation nuclei. This range includes the Napier frequency (frequency for which attenuation per unit wavelength is maximum), and the resulting data indicate that a maximum dimensionless attenuation occurs for a value of ωτ/Cm≈1, as predicted by the theory of Cole and Dobbins. The magnitude of the maximum dimensionless attenuation is approximately 100 times that which would be obtained for a corresponding nonvolatile aerosol.

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