Acoustic measurements of the sound-speed profile in the bubbly wake formed by a small motor boat

In situ measurements of the bubble field within wakes generated by a small motorboat show that the bubble field, shortly after the initial turbulent generation period, consists mainly of bubbles with radii between 20 and 200 microm. The subsequent dispersion of the wake field can be described using a model that includes bubble buoyancy and dissolution only, and the air volume fraction within the wakes decay exponentially with an e-folding time of between 40 and 60 s. Simultaneous measurements of sound propagating through the bubbly wake exhibit spectral banding due to waveguide propagation. Inversions using the inverse-square theory developed by Buckingham [Philos. Trans. R. Soc. London, Ser. A 335, 513-555 (1991)] show that this acoustic inversion technique provide a viable means of estimating the low-frequency sound-speed profile in an upward refractive bubble layer when dispersion can be neglected.

The effect of temperature on sound wave absorption in a sediment layer

The effect of temperature on sound velocity, absorption, and reflection coefficient in the seabed sediment layer is investigated. Experimental measurements of sound speed, absorption, and the reflection coefficient in a sandy sediment layer have been carried out at several temperatures. An absorption reduction of 75 dB/m and a velocity increase of 65 m/s have been measured at a frequency of 1 MHz when the temperature increases from 5 to 25 degrees C. Because of the absorption temperature dependence the amplitude of the reflected wave from the back surface of the sub-bottom layer after going back and forth across the layer increases with the temperature.