Abstract
The acoustic field of a parametric focusing source is experimentally examined. It is found that the theoretical beam sharpness and focusing of the difference frequency sound are primarily due to a less-cumulative sound generated near a strong virtual source located at the focus. Since the amplitude of the difference frequency sound on the focal plane is very weak compared to the primary sound, the signals received by a hydrophone may be dominated by false signals due to the nonlinear response of the hydrophone. Employing the primary sound of frequencies as high as 5 MHz to attain high conversion efficiency, the characteristic of the narrow beam at the focus is demonstrated by observing the signal reflected by a small reflector set at the focal plane. When a highly dissipative medium covers a target to be detected by the parametric focusing source, the difference frequency sound is markedly reduced and the beam sharpness is significantly degraded at the range of the target.