A laboratory study of the effect of internal waves on acoustic propagation

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Abstract

For calculating the acoustic pressure due to sound propagation at sea using usual methods (pressure variations signals), knowing the density distribution and consequently, changes the speed of sound in the environment is very important. Many environmental factors affect the distribution of the density at sea, depending on environmental conditions and geographical location and the weaknesses of each of them are different. One of them is internal waves which usually cause temporal and spatial changes and consequently affect the acoustic wave propagation in the ocean. Internal waves can be generated by tidal currents over sea floor sloping that is very common in the stratified oceans. Results of study in the some researches showed that internal waves can effected on sound waves in two ways: 1-Internal waves can be decrease sound level up to 25 dB due to sound mode coupling in an exact frequency. 2- Internal waves can fuscous and defocus sound waves because of sound speed fluctuation.
The purpose of this study is a laboratory investigation of internal waves caused by fluctuation of a cylinder in a stratified glass channel with 3 meters long, 0.5 meter width and 1 meter height, on the sound waves propagation. In this study, using the double bucket and filling box method for generating stratification that stratification can be measured by one pair of salinity and temperature meters fixed on a rail moved up and down. Using the usual methods of setting up internal waves and using acoustical transducers in 53 KHz frequency, internal wave's effects on the propagation of sound waves, were investigated. In this study with usual optical method (Synthetic Schlieren) internal waves generated in the tank can be detected. In this method Internal wave generated in the glass tank change optical index of water layers and cased deviation of Straight lines designed on the back of tank. Laboratory results showed that sound waves can be focused and defocused due to the normal modes of internal waves. Some 9 experiments were done mainly in cases withvertical linear density stratified fluid. As the modal structure of internal waves in the water tank change due to the waves, constant density surfaces change slopes, hence changing the sound ray's paths and the amount of signals reaching the receivers. Similar results of numerical simulation also show similar behavior in the strength of the acoustic signal. Numerical simulation modeled by AcTUP v2.2L software that use KERAKENC method based on normal mode method. The acoustic signal can be weakened up to 54 per cent depending on the degree of sound ray divergence. We can conclusion that in the laboratory tank in this study internal waves effects on sound waves by focusing and defocusing and not by mode coupling.
Similar behaviors can be expected in the open ocean as the existence of internal waves is ubiquitous. For this goal dimensionless numbers should be use. Bowen (1993) showed that for simulating a sound waves interaction with a phenomenon in laboratory scale we can use ka = k'a'. With this formula we can compare laboratory results with real results on oceans.

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References

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Journal of the Earth and Space Physics
Volume 43, Issue 1
May 2017
Pages 181-192

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