Experimental evaluation of pseudo-sound in a parametric array

Parametric arrays have been widely investigated due to their high directivity in the low-frequency region when measuring the acoustic characteristics of materials, such as sound transmission and reflection properties, in the limited space of acoustic water tanks. Because a parametric array utilizes nonlinear interaction across the frequency components, nonlinear phenomena arise in both the acoustic medium and the hydrophone system. The unwanted nonlinear sound generated in the hydrophone and instrumentation is termed pseudo-sound. This study devised an experiment to measure the pseudo-sound without a truncator to assess the effect of the pseudo-sound in the near field. In order to separately measure the pseudo-sound, the hydrophone was placed close to the projector so that the pseudo-sound was dominantly generated. The method is based on the fact that, if we can suppress a parametric signal by minimizing the propagation path length in the medium, the resulting measured signal can be attributed to the pseudo-sound. The resultant signal levels were matched to the levels at corresponding distances based on the primary levels of a (separately conducted) distance-varying experiment. Finally, the suitability of the proposed technique was evaluated by comparing its results with the pseudo-sound levels measured using a truncator.