Flextensional transducers are generally small compared to the wavelength of sound in the usable frequency range, usually near their first resonance frequency. The resulting acoustic radiation pattern is nearly omnidirectional resembling an acoustic monopole. The “double-driver” cymbal, a directional class V flextensional tranducer like the Moonie and Cymbal transducers, maintains the advantageous of conventional cymbal transducers, including thin profile, small size, simplicity, ease of fabrication and low cost, while introducing a new mechanism to project sound unidirectionally. The double-driver cymbal transducer is a promising candidate for directional underwater sound projectors in conformable arrays mounted on underwater vehicles.
Researchers constructed a 3×3 double-driver element array, which demonstrated that under optimal conditions the array can provide a directional beam pattern with a front-to-back ratio of more than 20dB. In the dipole mode of the double-driver cymbal, the Transmit Voltage Response (TVR) showed two maxima in opposite directions (front and back), but the phase of the TVR output from one lobe is opposite to that from the other. When combined with the omnidirectional mode, this can be used to generate a directivity pattern which has only one maximum. If the output from the dipole mode is added to the output from a monopole of equal transmitting voltage response, the resulting beam pattern is a cardioid curve with a single maximum.
The finite element analysis code ATILA was used in modeling the performance of the double-driver cymbal transducer, particularly the vibration modes and the calculation of the TVR and beam pattern. A nearly omnidirectional and a dipolar beam pattern were obtained for the monopole and the dipole mode and are consistent with the ATILA prediction. The driving voltages and phases for the cardioid mode at 20kHz was not a perfect cardioid pattern but shows a very directional beam shape. When the driving amplitude and the phase was slightly adjusted, a nearly perfect cardioid beam pattern was obtained.
The researchers demonstrated that a directional beam pattern can be obtained from a double-dipper transducer which is much smaller than a wavelength. With this method, a directional pattern can be obtained at virtually any frequency. However, the TVR amplitude and phases of the double-dipper fluctuates dramatically with frequency. As a consequence, the calculated voltage ratios (amplitude and phase) at different frequencies are dramatically different, suggesting unique driving conditions at each frequency or a narrow working bandwidth. This may complicate the driving electronic circuits, if the double-dipper is used over a wide frequency range.

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