Spectral control of sparker acoustic sources

Impulsive acoustic sources are important in a variety of industrial, academic medical, environmental and military applications. By their nature impulsive acoustic sources have a broadband acoustic spectrum and spectral control is limited. The source level, peak frequency of the acoustic spectrum and the ambient pressure are inter-related. For instance any change in an impulsive source's depth (i.e. ambient pressure) shifts the acoustic spectrum. The author reports on three approaches for partially controlling the acoustic spectrum. In the first an enclosure is placed around the impulsive acoustic source to shift the peak in the acoustic spectrum to higher frequency. The presence of the pod increases the pressure and reduces the bubble oscillation period. This shifts the acoustic spectrum to higher frequency. The pressure, and hence peak frequency, depends on the pod's size, thickness and material. The second tailors the electrical power pulse to alter the spectral width of the acoustic spectrum. For instance increasing the electrical pulsewidth narrows the acoustic spectrum. The third uses multiple pulses (i.e. a pulse train) to "synthesize" an acoustic spectrum by producing peaks and nulls to shape the spectrum. A train of pulses from one or more impulsive acoustic sources can generate a wide range of spectral shapes. This paper is intended to make sparker source capabilities more widely known so they can be used in underwater acoustic applications to meet requirements where existing acoustic sources are inadequate.