source: Â© 2011 IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control

The frequency response of ultrasonic detectors is commonly calibrated by finding their sensitivity to incident plane waves at discrete frequencies. For certain applications, such as the emerging field of optoacoustic tomography, it is the response to point sources emitting broadband spectra that needs to be found instead. Although these two distinct sensitivity characteristics are interchangeable in the case of a flat detector and a point source at infinity, it is not the case for detectors with size considerably larger than the acoustic wavelength of interest or those having a focused aperture. Such geometries, which are common in optoacoustics, require direct calibration of the acoustic detector using a point source placed in the relevant position. In this paper, we report on novel cross-validating optoacoustic methods for measuring the frequency response of wideband acoustic sensors. The approach developed does not require pre-calibrated hydrophones and therefore can be readily adopted in any existing optoacoustic measurement configuration. The methods are successfully confirmed experimentally by measuring the frequency response of a common piezoelectric detector having a cylindrically focused shape.

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Fig.1 The different 2-D configurations analyzed in this paper for measuring the frequency response of acoustic detectors. The acoustic sources are equal to one in the gray areas and to zero outside them; the detectors are on the far right (point, flat, and curved). a and b denote the lateral and axial dimensions of the acoustic source, respectively; c denotes the distance from the source to the detector; d denotes the vertical length of the detector; and v denotes the speed of sound. (a) A source with a smooth boundary and similar dimensions on the axial and lateral axes and a point detector. The dashed curves represent the two extreme arcs over which the integral in (2) is nonzero. (b) A heuristic description of the integral in (2) and of (c) the spectrum of pÎ´(r,t) for the geometry in Fig. 1(a). (d)â€“(f) A rectangular optoacoustic source with point, flat, and curved detectors, respectively. The curved detector is focused on to the middle of the proximal edge of the source. The dashed lines represent the longest distances from any point on the detectors to any point on the proximal edge of the sources.