Grating lobe reduction in transducer arrays through structural filtering of supercritical plates

Abstract

The effect of placing a structural acoustic filter between water and the transducer elements of an array to help reduce undesirable grating lobes is investigated. A supercritical plate is mounted to transducer elements with a thin decoupling polyurethane layer between the transducers and the plate. The plate acts as a radiation/incidence angle filter to pass energy at angles near normal incidence, but suppress energy at large incidence angles. Grating lobe reduction is achieved at the expense of limiting the available steering of the main lobe. Within this steer angle limitation, the main lobe can be steered as normal while the grating lobe level is reduced by the plate's angular filtering. The insertion of a plate structural filter provides, in principal, an inexpensive and easily implemented approach to extend usable frequency bandwidth with reduced level grating lobes, without increasing the number of array elements. Even though the data match theory well, a practical material has yet to be found that possesses optimal material properties to make the proposed idea practical. This work represents the first attempt to advantageously utilize a plate above its critical frequency to provide angular dependent sound transmission filtering.

Loudspeaker line array educational demonstration

“…If the actual sources cannot be considered simple sources, and are instead of a finite size, then one may use the so-called First Product Theorem to better estimate the directivity pattern. [10][11][12][13][14] The First Product Theorem allows one to estimate the directivity function of a line array of identical sized sources where ka ( 1 no longer holds. The total directivity function then is determined by multiplying the directivity pattern of a single source by the directivity function of a line array of point sources (where the point sources are located at the positions of the finite sized sources).…”

Grating Lobe Reduction in Transducer Arrays Through Structural Filtering of Supercritical Plates

“…The directivity pattern, H͑͒, without a plate may be calculated from typical line array theory ͑first term in curly brackets͒ multiplied by the single element directivity ͑second term in curly brackets͒, [1][2][3]35…”

Extraction of plate bending stiffness from coincidence angles of sound transmission measurements

“…The purpose of this paper is to present a method to extract the bending stiffness of submerged or airborne plate samples using the array beam forming insertion loss measurement. 4,5 This method is used to measure sound transmission through the plate under test as a function of angle at various frequencies, similar to the work of Luukkala et al For thin plates, above the critical frequency of the plate, there exist angles of maximal sound transmission mirrored about normal incidence, called coincidence angles. By extracting the coincidence angle as a function of frequency from measurement and fitting it to a theoretical expression, the bending stiffness can be calculated, assuming the mass density is determined.…”

“…By extracting the coincidence angle as a function of frequency from measurement and fitting it to a theoretical expression, the bending stiffness can be calculated, assuming the mass density is determined. The advantage of the technique used here, developed by Anderson et al 4 and Shaw and Anderson, 5 over that proposed by Luukkala et al, is that a line array of sensors is used as a beam former to increase the sensitivity of the receiver towards the sound source and reduce the sensitivity to diffracted waves, thereby allowing testing of smaller plate samples.…”