The transducer is the same as the one considered in an accompanying paper by the same authors [Part I, J. Acoust. Sue. Am. 72, 1056-1061 ( 1982}]. It has a diameter of 182 era, and is composed of 364 elements, 4.8 X 6.4 cm each, which are distributed in 13 equally spaced rings. It is used to generate difference frequency sound at 0.5-5 kHz in water. Amplitude and phase of the generated sound are measured at various distances in the nearfield of the transducer. The results are compared with calculations based on the quasilinear approximation of the governing equations. Simplifications are obtained by utilizing the parabolic approximations [J. Naze Tj•tta and S. Tj•tta, J. Acoust. Sue. Am. 67, 484-490 ( 1980}]. This also makes it possible to use a realistic model for the primaries (see the accompanying paper} when evaluating the nonlinear source term, such that all effects of the nearfield oscillations in amplitude and phase are fully accounted for. The analytical results obtained also apply to the case of an amplitude and/or phase shaded source.
Results are reported of an investigation of the reflection of parametric radiation from a finite planar target. The interference of the difference frequency sound generated after reflection with the reflected difference frequency sound is considered. Amplitude and phase were measured before and after reflection from a polyfoam target. Target size ranged from slightly larger than the beamwidth to smaller than the beamwidth. The results are compared with calculations based on equations describing the linear radiation and the parametric radiation. These equations are derived using the parabolic approximation.
The theory presented in two previous papers [Part I, J. Acoust. Soc. Am. 72, 1056–1061 (1982) and Part II, J. Acoust. Soc. Am. 74, 1013–1020 (1983)] is further developed and generalized to the case of no axisymmetry. Emphasis is given to a study of the parametric generated sound. Asymptotic formulas are presented, showing the development of the difference frequency sound from the source (transducer) into the farfield. New numerical results for the axisymmetric case are given for the linear and the parametric field of a source which is not radiating uniformly. New experimental results are reported and compared with the theory.
Results are reported of an investigation of the nearfield of a large circular transducer (182-cm diameter, and composed of 364 smaller elements operating in water at 11–16 kHz, and with parametric generated difference frequencies in the range 0.5–5 kHz). Linear radiation is considered in this paper (Part I), whereas parametric radiation is discussed in a related paper (Part II). Amplitude and phase are measured at various distances and compared with calculations based on a simplified integral representation of the field obtained by Naze Tjo/tta and Tjo/tta [J. Acoust. Soc. Am. 68, 334–339 (1980)], as a solution of the parabolic approximation of the Helmholtz equation. Comparison is also made with an exact on-axis calculation. The main results are (1) the transducer radiates as a piston with a uniform velocity distribution, (2) the integral solution represents an adequate description of the observed sound field. It provides a simple method to obtain general properties of the field as well as numerical results by computing a simple (line) integral. (3) Within the same approximation scheme, formulas are presented for the case of axial symmetry, from which the field of an amplitude or phase shaded source can be calculated from a simple (line) integral. (4) From this, it is further shown how the farfield (amplitude and phase) can be calculated readily from the nearfield measurements.
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