Final Assessment of the Feasibility of a System for Acoustically DetectingStationary Gas Bubbles Using Time Delay Spectrometry

摘要

The Jet Propulsion Laboratory swept-frequency bubble detector has been discussedin previous reports (Albin et al., 1991 and 1992). Physically similar to the familiar ultrasound scanning devices for medical imaging, it was designed to nondestructively detect stationary resonant bubbles of < 7 um diameter using time delay spectrometry (TDS), but proved insufficienfly sensitive to do so. This report is a discussion of whether that system could be redesigned successfully. Techniques to improve the signal/noise (S/N) ratio are explored, including exciting larger bubbles at lower frequency. Analysis indicates that the new system should indeed be sensitive enough to detect resonant bubbles in the 40-400 um diameter range. However, the shift to lower frequencies brings with it a problem peculiar to TDS systems: there are multi-path signal arrivals, which in TDS systems translates into a loss of resolution in the time domain. Because identifying the resonance frequencies depends upon the temporal coherence of the signal, the technique cannot work at low frequencies. Various strategies for maintaining time resolution at lower frequencies are discussed, along with the reasons why they will not work. Adequate time resolution is realized at much higher frequencies, but the S/N ratio at those frequencies would be satisfactory only if the acoustical scanning signal were of such high amplitude that the measurement would be destructive to the bubbles and their milieu. It does not appear possible at present to design a TDS system that is simultaneously sensitive enough for the proposed application and nondestructive--such a device will become possible in the future if acoustic transducer sensitivities are improved by several orders of magnitude.