The Evaluation Method And The Equipment Design On Measuring The Sound Isolation Effect From Acoustic Cover Layers In Low-frequency Range

With the demand of low-frequency sound protection from underwater navigators,the more requirement of acoustic cover layers’ performance has been proposed.In the low-frequency range,the sound waves from the pressure hull of underwater navigators are at the random incident angle to the acoustic cover layers.Therefore,the sound isolation coefficient calculated by the normal plane wave incidence and reflection can not be applied into practical engineering.In the airfield,there are mature methods and standards of the sound isolation materials,for example,ISO140-3:1995,and GB/T19889.3-2005.In these methods and standards,the test environment is normal pressure,not the high pressure.Since underwater navigators are usually running at high depth,we must consider the sound isolation performance of acoustic cover layers at high pressure.If we establish a sound isolation test equipment at 3 million Pa by the steel,16 MnR,the wall thickness of the reverberation chamfer at least should be 1200 mm.Under this thickness,the coupling between the two reverberation chamfer cannot be removed.The manufacture expense cannot suffer.Based on the methods and standards of the sound isolation in the air,an equipment has been proposed to measure the sound isolation from the acoustic cover layers on the condition of high water pressure.The equipment has included a sound source reverberation tank,a test material tank,and a sound recieving reverberation tank.Through the pressure added in the test material tank,the high pressure can be placed on the sound isolation materials.Since the normal water and high pressure water have the similar properties of the density and the sound speed,this equipment has simplified the requirement of the experimental test,such as the high pressure design of the transducers,the easily spatial averaging of the sound pressure.The numerical simulation models have been established by the finite element method.The analysis of the test method on measuring sound isolation from acoustic cover layers in the low frequency range and normal pressure has been carried out.The frequency is in the range from 10 Hz to 1000 Hz.In order to test the low frequency sound isolation at the water tank of relatively small dimension,we have proposed the method of force excitation to extend the measured frequency of the tank.This method has also been validated by numerical simulation.The materials of sound isolation are the steel,the compound structure of the steel and the rubber.The law has been summarized and is in good accordance with the sound isolation rules of the materials.After that,a small glass experimental equipment has been built up.Firstly,the method of extending the measured low-frequency by the force excitation has been validated.Secondly,the silicon rubber and the normal rubber have been measured.The results agree well with the sound isolation rules of the materials.All these tests has shown that the equipment and the method in this paper are feasible in practical engineering.In addition,the paper has designed the mechanical structure for the sound isolation effect comparison of acoustic cover layers under high water pressure.The research in this paper has provided a method to measure sound isolation from underwater acoustic cover layers under high water pressure.The results can also provide the tehnique support for the low-frequency acoustic stealth of underwater vehicles.