Discriminating between the nearfield and the farfield of acoustic transducers

Abstract

Measurements of the performance of acoustic transducers, as well as ordinary measurements made with the same, may require discriminating between the farfield, where the field is spherically divergent, and the complementary nearfield, where the field structure is more complicated. The problem is addressed for a planar circular piston projector, with uniform normal velocity distribution, mounted in an infinite planar rigid baffle. The inward-extrapolated farfield pressure amplitude p f is compared with the exact nearfield pressure amplitude p n , modeled by the Rayleigh integral, through the error 20 log jp f /p n j. Three sets of computations are performed for a piston with wavenumber-radius product ka ¼ 10: normalized pressure amplitudes and error versus range at angles corresponding to beam pattern losses of 0, 10, 20, and 30 dB; error versus angle at three ranges, a 2 /k, pa 2 /k, and 10a 2 /k, where k is the wavelength; and range versus angle for each of two inward-bounded errors, 1 and 0.3 dB. By reciprocity, the results apply equally to the case of a baffled circular piston receiver with uniform sensitivity over the active surface. It is proposed that proximity criteria for measurements of fields associated with circular pistons be established by like modeling, and that a quality factor be assigned to measurements on the basis of computed errors.

Support for the beam focusing hypothesis in the false killer whale

“…In the near field, the acoustic pressure is not well described by the product of a range term and an angular term as it is in the far field (Foote, 2014;Urick, 1983). As a result, in the near field there is no clearly defined beamwidth, and the intensity of the sound does not obey 1/r spreading seen in the far field.…”

Calibration of a broadband acoustic transducer with a standard spherical target in the near field

“…(A5)]. For a transducer of finite size, the radiation field can be calculated by integrating the active element surface (Zemanek, 1971;Nyborg and Steele, 1985;Van Buren, 1989;Foote 2014). In this application, the scattering involves both transmitter and receiver, the single 2-D integral over the transmitter for radiation filed only given by Zemanek (1971), Nyborg and Steele (1985), Van Buren (1989), and Foote (2014) is extended to two double integrals: One is over the transmitter surface (A T ) and the other is over the receiver surface (A R ).…”

Characteristics of Sounds Emitted During High-Resolution Marine Geophysical Surveys

“…The acceleration time series were then integrated using the fractional step digital filter provided as equation (16) and illustrated in figure 48 to yield the three-dimensional acoustic particle velocity. The acoustic pressure at the center of the array was estimated as the average of the pressures observed at the vertices of the tetrahedron as indicated in equation (15). figure 50a.…”