Elastic wave thermal fluctuations, ultrasonic waveforms by correlation of thermal phonons.
It is widely recognized that acoustic degrees of freedom coupled to a thermal bath have amplitudes which fluctuate with a mean square proportional to temperature; this is the basis for the Debye theory of the heat capacity of insulating solids. It is shown here that these elastic wave thermal phonons have correlation functions identical to the system's ultrasonic Green's function, and furthermore that thermal noise in ultrasonic detectors should have correlation functions equivalent to conventional waveforms obtained by active transmission and reception. This suggests the possibility of doing ultrasonics without a source. Theory for the identity is presented, and several room temperature laboratory confirmations are conducted in the frequency range 0.1-1.0 MHz. The thermal nature of the origin of these correlations is established by comparing their strength with theoretical expectations. Applications are discussed.
Weaver
Richard L
RL
Theoretical and Applied Mechanics, University of Illinois, 104 South Wright Street, Urbana, Illinois 61801, USA. r-weaver@uiuc.edu
Lobkis
Oleg I
OI
D008962:Models, Theoretical; D013696:Temperature; D014159:Transducers; D014465:Ultrasonics