ABSTRACT
In this section, we will focus on the spatial directional characteristics of underwater radiated noise of ship targets in the vertical section at a specified part. Based on the data acquired by the vertical linear array, the corresponding mathematic model and the sound pressure directional distribution analysis method will be elaborated in detail. As shown in figure 1, an xyz-coordinate system is set up with the target’s geometric center O as the origin. The x-axis is set in the fore-and-aft direction of the target, with the positive side pointing to the bow. The y-axis is
normal to the x-axis, with the positive side pointing in the direction of the target’s port. The z-axis is normal to the xOy-plane, with the positive direction pointing upward. The linear array is located at a certain distance from the target’s port. At the time of data recording, the target is passing by the linear array at a constant speed v, which is equivalent to the condition of treating the target as the reference while the linear array is moving in the opposite direction at a constant speed –v. The resulting scenario is coincident with the concept of spatial scanning and synthetic aperture, such that the rectangular plane passed by the measurement array is defined as “the eective measurement plane.” The measurement array comprises N equally spaced hydrophone units. The total length is L , and the unit’s spacing distance is d. The array body is vertical with the horizontal lengthways section plane of the target under test, and the distance between the first array unit and the plane is H. The target track is parallel with the eective measurement plane, and the distance between them is D. The target track is coplanar with the eective measurement plane. Under normal measurement conditions, =x xb and =x xe represent the target’s initial and terminal positions, respectively. The precise values are given by the ranging and positioning system. −x xe b is considered the eective maneuver distance. Qn x, is the nth spatial position (rn x, , θn) on the linear track passed by the nth hydrophone unit, n=1, 2,……, N;
≤ ≤x x xb e, and Qn is the spatial position θ( )r ,n n at the vertical section of =x xW. Within a certain eective measurement time interval ∆T , ( )p tn is the transient sound pressure obtained in position Qn at time t.
