Ultrasonics without a Source: Thermal Fluctuation Correlations at MHz Frequencies

TLDR

Noise generated in an ultrasonic receiver circuit is usually ignored or treated as a nuisance. The study argues that acoustic thermal fluctuations with ~3 fm displacement encode substantial ultrasonic information, with the noise autocorrelation function reproducing the waveform of a direct pulse/echo measurement. Experiments comparing direct measurements to correlation functions demonstrate this relationship. The thermal nature of the elastodynamic noise is confirmed by an absolute measurement of its strength, effectively measuring the sample temperature.

Abstract

Noise generated in an ultrasonic receiver circuit consisting of transducer and amplifier is usually ignored, or treated as a nuisance. Here it is argued that acoustic thermal fluctuations, with displacement amplitudes of 3 fm, contain substantial ultrasonic information. It is shown that the noise autocorrelation function is the waveform that would be obtained in a direct pulse/echo measurement. That thesis is demonstrated in experiments in which direct measurements are compared to correlation functions. The thermal nature of the elastodynamic noise that generates these correlations is confirmed by an absolute measurement of their strength, essentially a measurement of the sample temperature.