Reciprocity calibration of hydroacustic transducers using swept-sines (chirps)

The three hydroacoustic transducers reciprocity principle is the base for a widely used and well known technique for sensitivity calibration in the frequency range between 1 kHz and 1 MHz. For practical reasons, at least one, and not necessarily all, of the transducers should be reciprocal to comply with the method requirements. In a typical setup, in which the excitation signal are discrete frequency tone bursts, free and far fields are mandatory; otherwise signal interferences, such as reflections on surfaces and diffractions due finite aperture width, would preclude trustworthy results. However, an alternative approach, combining signal processing and the basic theory for reciprocity, minimizes the necessity of both free and far fields. Further, using this technique, calibration can be performed in a quasi-continuous and broad frequency range in a single shot, avoiding the time-consuming stepped-sine excitation signal. The approach is based on the use of a non-conventional chirp-signal (or swept-sine) used as excitation, which is developed under a frequency spectrum magnitude prescription. In this paper it will be presented the description, employment and comparison of the conventional calibration by reciprocity and non-conventional technique using chirps.