Theory and design of acoustic dyadic sensor

An acoustic vector sensor can directly measure particle velocity or acceleration under water. Its directivity is a cosine pattern in the working frequency band. And its array gain can achieve 4.8 dB to 6 dB. An acoustic dyadic sensor can be used in order to make the directivity of the acoustic vector sensor sharper, and to measure the gradient of velocity or acceleration under the water. And its directivity is the quadrupole's directivity; and, the directivity function is the square of cosine or cosine multiplied by sine. Its array gain can achieve 6 dB to 9.5 dB. Usually an acoustic dyadic sensor is made of a vector hydrophone array, or a number of neutral buoyancy velocity sensors or accelerators are mounted at a certain positional relationship to form an acoustic dyadic sensor. The gradient of velocity or acceleration can be obtained by subtracting signals from two velocity sensors or accelerators. When designing an acoustic dyadic sensor, the distance of velocity sensors or accelerators must be taken into consideration because this influences the working bandwidth. When an acoustic dyadic sensor is made of accelerators, the slope of the sensitivity is 12 decibels per octave. So the working frequency is lower, and the sensitivity becomes smaller. Acoustic dyadic sensors usually cannot work at a very low frequency because of the low signal tonoise ratio. are very small compared to the wavelength. The acoustic pressure-gradient can be calculated by subtraction of signals from the two hydrophones. So it is an indirect measurement of acoustic vector signals under the water. The second type is usually the co oscillating vector hydrophone. This kind of acoustic vector sensor is usually built by mounting a velocity sensor or a accelerometer in a spherical or cylindrical shell body, or in a neutrally buoyant materials which is made by fIlling an amount of hollow glassmicroballoons in a base material of epoxy resin. The suspension system is usually needed to suspend the hydrophone. And it can directly measure the acoustic vector signals under the water. No matter which kind of acoustic vector sensor is used, its directivity is a cosine pattern in the working band. Its array gain can achieve 4.8 dB-6 dB. An acoustic dyadic sensor can be regarded as an advanced form of acoustic vector sensor. It can measure the second order gradient of acoustic pressure; and the gradient of velocity or acceleration under the water. Its working principle is similar to the acoustic vector sensor. However, it takes advantage of finite difference approximation. Usually an acoustic dyadic sensor is comprised of an acoustic vector sensor array, or a number of neutral buoyancy velocity sensors or accelerators which are mounted in a certain positional relationship. Such an array gain can achieve 6dB to 9.5dB. In this paper an acoustic dyadic sensor is designed comprising accelerators, and it can be directly installed on the underwater platform. The accelerators are mounted in a spherical shell; and, acoustic transparent rubber is filled in the space between the accelerators and the spherical shell. The directivity of the acoustic dyadic sensor was measured under water. From the results, the design method of an acoustic dyadic sensor in which the accelerators are inside is shown to be feasible. It can measure the acoustic gradient of acceleration.