Acoustic transducers made from piezoelectric ceramic cylinders usually exploit the breathing or omnidirectional (omni) mode of vibration. However, with suitable voltage distribution, higher order extensional modes of the cylinder can be excited which produce directional radiation patterns. These modal radiation patterns can then be combined to synthesize desired beam patterns which may be steered by incrementing the excitation. This paper describes a model for the combined acoustic response of the extensional modes of vibration of a piezoelectric ceramic cylinder, a method of synthesizing a desired radiation pattern, and an experimental implementation of a directional transducer that uses these techniques. This tri-modal transducer is broadband and directional with a frequency independent beam pattern yet simple, small, and lightweight.
A unique low-frequency ͑900 Hz͒ class IV flextensional transducer that produces an enhanced far-field pressure on one side and canceled far-field pressure on the other side has been developed. The transducer radiating surface consists of a thick-walled elliptical aluminum shell and a U.S. Navy type III piezoelectric stack along its major axis with two active sections and one inactive section. The directionality is achieved by simultaneously exciting the shell into an omnidirectional and dipole operation by driving stack into both extensional and bending modes. Both measurements and modeling on this device show a front to back pressure ratio of more than 30 dB, producing cardioid-type radiation patterns over an octave band, for a single transducer element. The transducers measured mechanical Q is 8, coupling coefficient is 0.25, and electroacoustic efficiency is 80% and produced a source level of 215 dB re: 1 Pa at 1 m when driven at a field limit of 394 kV/m ͑10 kV/in.͒ at resonance. The uniqueness of this transducer is its directional beam patterns ͑directivity indexϭ3.4 dB͒ and high acoustic output power from a small ͑less than a third of a wavelength͒ single element. Six of these transducers were placed in a closely packed line array two-wavelengths long. The array successfully produced narrow directional sound beams ͑directivity indexϭ8.7 dB͒ with a front to back ratio greater than 30 dB and a source level of 225 dB re: 1 Pa at 1 m.
The superposition of acoustical modes radiated from a transducer can provide directionality from a compact projector. This paper presents the foundation, as well as examples, of piezoelectric ceramic, piezoelectric single crystal, and magnetostrictive versions of this "modal projector." Measured and analytical, as well as finite element analysis modeled, results show good agreement and establish this transducer as a viable source of intensity with the advantages of an improved directivity index, reducing the power needed from a compact source.
A self-tuned magnetostrictive/piezoelectric hybrid Tonpilz transducer which produces enhanced motion at one end and canceled motion at the other is presented. The particular hybrid Tonpilz transducer described here consists of two 5.4 in. (13.65 cm) diameter circular pistons with a central mass separating a Terfenol-D magnetostrictive tube and a Navy type I piezoelectric ring stack driver. The in-water mechanical resonance and hybrid electrical self-tuning occurs in the vicinity of 4.25 kHz where a 15-dB front-to-back pressure ratio was obtained under array loading conditions. As a result of the shared electrical stored energy, an improved effective coupling coefficient is obtained.
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