Traveling wave tube measurements for low-frequency properties of underwater acoustic materials

A traveling wave tube measurement technique is developed for measuring acoustic properties of underwater acoustic materials. In the laboratoryr water temperature and pressure environments of the ocean can be simulated in the water-filled tuber and the acoustic parameters of samples are measured in range of low frequencies. A tested sample is located at central position of the tube. A pair of projector is separately located at both ends of the tube. Using active cancellation technique, transmitted sound wave of the sample is hardly reflected by the surface of secondary transducer. So the traveling sound field is built up in the tube. By separately calculating the transfer functions of every pair of hydrophone in the both sound fields separated by the sample, its reflection coefficients and transmission coefficients are obtained. In the measurement system, the inside diameter of the tube is Ф208 mm, the working frequency range is 100 N4000 Hz, the maximum pressure is 5 MPa. The reflection coefficients and transmission coefficients of water layer and stainless steel layer samples are measured actually and calculated theoretically. The results show that the measured values are in good agreement with the calculated, and the measurement uncertainty is not greater than 1.5 dB.