Fig. 1.

(Color online) (a) Representative configuration of the pitch-catch technique employing a confocal transducer pair of unequal geometry with source focal length $f_{\mathrm{S}}$ and PCD focal length $f_{\text{PCD}}$. (b) Corresponding configuration of the pulse-echo technique employing a reflector and image source. (c) Simulated pressure spatially averaged over a concave PCD surface, $\left|⟨P_{\text{PCD}}(f)⟩\right|$, pressure averaged over a planar hydrophone measurement surface, $\left|⟨P_{\mathrm{H}}(f)⟩\right|$, and pressure magnitude averaged over the same planar surface, $⟨\left|P_{\mathrm{H}}(f)\right|⟩$. Pressures are plotted normalized to the nominal surface pressure amplitude of the source as a function of ka for a pitch-catch transducer pair with respective f-numbers of 1.32 and 3.35. (d) Corresponding simulated, spatially averaged pressures for an equal-geometry pitch-catch transducer pair with f-number 3.35. (e) Decibel-scaled ratios of $⟨\left|P_{\mathrm{H}}(f)\right|⟩$ to $\left|⟨P_{\text{PCD}}(f)⟩\right|$ for unequal transmit-receive transducer pairs representing the pitch-catch technique. (f) Corresponding pressure magnitude ratios for matched transducer pairs representing the pulse-echo technique.