Fig. 4.

Illustration of bandlimiting due to spatial averaging over the detector field-of-view. At low concentrations, the absorbers within the tube are relatively sparsely distributed; there are rapid time-varying fluctuations in the detected photoacoustic (PA) waveform, and the acoustic frequency spectrum is broad. This permits accurate velocity measurements to be made. At high concentrations, the fluctuations in the PA waveform are smoothed out and the frequency spectrum is downshifted; the absorber distribution is perceived to be less heterogeneous and thus it is difficult to track the absorber motion and accurately measure the flow velocity.