Temperature dependence of diffuse field phase

Diffuse fields, which have scattered from microstructure or reflected from walls so much as to prohibit conventional analyses, are usually examined by means of the time evolution of their ultrasonic spectral energy density. The phase information is usually discarded as resisting analysis. The phase, while unpredictable is, however, robust; according to theory it remains constant if source and receiver are not disturbed. Nevertheless, in practice we do observe slow drifts of phase over time scales of minutes. Here we examine the hypothesis that the phase drifts are due to temperature fluctuations. Temperature changes on cooling from 40 degrees C to room temperature were monitored and compared with changes in diffuse field phase. It was found that the reverberant ultrasonic field in a 7 cm aluminum block evolves with temperature in a manner that is in accord with published data on the temperature dependence of the ultrasonic velocities. Our 1 MHz transient source gives rise to a complex waveform that is observed to undergo an almost pure dilation. The precision with which this shift can be measured approaches 20 ns. This is remarkable when compared with the 100 ms travel time of the signal. Thus the temperature dependence of elastic wave speed is measured with a precision limited by the precision of one's thermometer. The signal is also found to suffer some distortion which, it is suggested, is related to the different rates of change of longitudinal and shear speeds. The corresponding prediction for the degree of distortion is found to be in accord with measurements.