Near-field measurements of a parametric source

In order to characterize nonlinear interactions within granular materials, measurements were made in the near-field of a 50 kHz transducer (an ITC 5270). High-resolution beam patterns were observed which reveal significant diffraction effects. Observed beam patterns were used to predict the difference frequency signal and this resulted in more than a 10 dB difference from that predicted by assuming constant amplitudes. This points to an inadequacy of the typical treatment of the parametric field as that due to collimated beams of constant amplitude in the near field, e.g. as in the Moffet and Mellen model. [This work was supported by ONR.]