The influence of waveform distortion on hydrophone spatial-averaging corrections—Theory and measurement

Abstract

A significant source of error in measurements of the acoustic output of medical ultrasonic equipment can arise from spatial averaging of the acoustic pressure over the active area of the hydrophone. Although criteria exist that quantify the maximum permissible effective hydrophone diameter, these are frequently violated for measurements on medical diagnostic systems, even for 0.4-mm-diam hydrophones that currently constitute the smallest commercially available device. In such circumstances, corrections have to be applied to the measured acoustical parameters. In the past, these correction procedures have been based on idealized models of the transducer pressure field distribution and have not taken account of finite amplitude effects which lead to nonlinearly distorted waveforms. This paper presents a systematic study of the influence of nonlinear distortion on spatial-averaging corrections. A 5-MHz focused transducer has been used to generate a range of acoustic waveforms suffering from varying degrees of nonlinear distortion. The important acoustical parameters have been determined using a group of hydrophones with effective diameters, determined at a frequency of 5-MHz, of between 0.3 and 4 mm. For each of the acoustical parameters, measurements were used to derive estimates of the spatial-averaging correction for the larger hydrophones used. The derived corrections were compared with existing correction procedures and the predictions of a new model which explicitly includes waveform distortion. The implications of the model to acoustic output measurements of medical ultrasonic equipment are discussed and correction curves presented which allow the magnitude of the spatial-averaging correction in many measurement situations to be estimated.