FIG. 4.

(a) (Color online) A combination of a band-pass filter (2.5 –7.5 MHz) and a notch-shaped comb filter applied to each PCD signal in the frequency domain to suppress the harmonics and ultraharmonics of HIFU backscattered by the gel, yet retain the broadband noise emitted by inertially collapsing bubbles. (b) Theoretically estimated distribution of the PCD sensitivity over the frequency range of the filter and relative positions of the focal regions (at the null level) of the three HIFU transducers. The thick black contour denotes the −6 dB level in the PCD sensitivity map. X = 0 corresponds to the geometric focus of the PCD.