IMPROVEMENTS IN EFFICIENCY AND BANDWIDTH OF PARAMETRIC LOUDSPEAKERS

Abstract

A parametric array is a nonlinear conversion process that can generate a highly directional sound beam with a small aperture. Directional sound generated by the parametric array can be applied for the parametric loudspeaker that transmits sound only to listeners on the acoustic axis in a quiet place. For mobile application, directional sound is expected to provide private listening in video telephony without ear/headphones. To generate directional sound by the parametric array in air, high-intensity sound is required for nonlinear interactions. Moreover, if a wide bandwidth of directional sound is specified for the application such as the parametric loudspeaker, the bandwidth of the transducer should also be considered. However, in air, it is not easy to generate a high-intensity sound, moreover, at wideband frequency. The difficulties arise from the large impedance difference between the transducer and air. This large impedance mismatch suppresses the efficient transmission of acoustic energy from the transducer to air and limits bandwidth of the transducer. In general, an array type transducer with a large number of unit drivers has been used to generate high-intensity ultrasound in air. However, the large volume and substantial power consumption of such transducers limit their practical applicability beyond mobile devices. Therefore, in this study, a study to improve the efficiency and the bandwidth of the airborne transducer was investigated for a source transducer of the parametric array. A flexural mode transducer with a thin radiating plate was proposed to overcome the impedance mismatch problem in air. Through theoretical analysis, it was verified that a micromachined ultrasonic transducer, which has a radiating plate of micron-size thickness, could achieve the theoretically maximum efficiency and bandwidth of a flexural mode transducer. The electrical problems of the micromachined ultrasonic transducer, which reduce the electromechanical efficiency, were also identified and resolved via an appropriate design scheme. To extend the frequency bandwidth, two types of unit drivers with different resonance frequencies (f1 = 100 kHz, and f2 = 110 kHz) were arranged in the transducer array and were driven by out-of-phase signals. By adopting the proposed design schemes, the piezoelectric micromachined ultrasonic transducer array was designed, fabricated, and evaluated for parametric loudspeakers. The fabricated transducer array was 7 cm in diameter and consisted of 476 unit transducers. The experimental evaluation showed that the theoretically estimated maximum efficiency and bandwidth of a flexural mode transducer could be achieved. The electroacoustic efficiency of the unit driver was 71% at its resonance frequency. By driving two types of unit driver out-of-phase, the flat frequency band without the null and peak could be achieved within the 3dB-frequency bandwidth of 15 kHz. The difference frequency wave was also generated in the audible frequency range from 100 Hz to 20 kHz. The beam patterns and propagation curves of the difference frequency wave were measured and compared with the computed data. The fabricated transducer array consumed 1 W of electric power while generating a 10 kHz-difference frequency wave with sound pressure level of 85 dB. This type of small, thin, high-efficiency transducer array shows potential as a parametric loudspeaker in mobile devices.