Absolute calibration of the helium gas jet noise source

A helium gas jet produces a convenient and reproducible localized (point source) displacement spectrum over the frequency range 0.2–1.0 MHz. The calibration spectrum has a maximum value of 10 Å at 200 kHz and decreases approximately logarithmically with increasing frequency to about 0.2 Å at 0.8 MHz. Comparison of a detected voltage spectrum for an acoustic emission event with the helium gas jet system spectrum permits the displacement amplitude spectrum of the acoustic emission source event to be determined using the calibration spectrum. The source event spectrum thus obtained can be transferred between different systems via the helium gas jet system spectra.The knowledge of the actual localized displacement spectrum associated with an acoustic emission source event should permit quantitative deductions to be made about the relation between the observed acoustic emission signals and the micro-failure processes that give rise to the acoustic emission events.