Ultrasonics without a Source: Thermal Fluctuation Correlations at MHz Frequencies

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.

Deep ocean sound speed characteristics passively derived from the ambient acoustic noise field

“…The main underlying assumption is that the ambient noise field is equipartitioned, that is, that the energy current is equal in all directions (Weaver & Lobkis 2001;Wapenaar 2003;Snieder 2004;Roux et al 2004). Fortunately this strong assumption can be relaxed in many situations.…”

Retrieving the Green's Function From Cross Correlation in a Bianisotropic Medium

“…For example an ordinary reciprocal anisotropic lossless medium becomes bianisotropic when it is moving with velocity v A and the magneto-electric tensor elements are given by ξ ik;A = (ε A µ A − ε 0 µ 0 ) ijk v j;A and ζ mn;A = ξ nm;A [35]. It is clear that if v B = −v A the two media are each other's adjoint and the volume integral vanishes from Equation (4). If state A is a gyrotropic plasma, then the adjoint state B has a DC magnetic field in the opposite direction [36] and a moving gyrotropic plasma has an adjoint medium with both the DC magnetic field and the velocity in the opposite directions [22].…”

“…Over the last seven years much attention has gone to Green's function retrieval from cross-correlations of acoustic ambient noise recordings and to use the results to form acoustic images of the surrounding medium. Weaver and Lobkis [4] retrieved Green's functions from natural thermal field fluctuations. Campillo and Paul [5] applied Green's function retrieval to the diffuse parts of seismic coda waves.…”

Earth science: Harnessing the hum

“…As a proof of principle, Weaver showed that ambient ultrasound vibrations in aluminium could be used to determine the material's elastic properties 1 . Then Campillo applied the same technique to the earthquake coda; the noise seemed to be correlated just as he hoped it would be 2 .…”