Loudspeaker systems for low-frequency radiation driven by rotational piezoelectric ultrasonic motors

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The authors are developing a completely new direct-radiator loudspeaker as an alternative to the conventional electrodynamic loudspeaker. It is driven by the continuous revolution of piezoelectric ultrasonic motors, and is useful for the radiation of very low-frequency signals because it shows almost flat phase-frequency characteristics in the low-frequency region. A preliminary model, named the dual-motor, de-spin (DMDS) model, includes two coaxial ultrasonic motors. The stator of one motor is fixed to the base and that of the other is connected to the cone radiator. Velocity modulation of either motor induces driving force to the cone radiator. The low-frequency sound (for example, 30–120 Hz) output by this model was excellent because it has no significant resonance in this frequency region. However, the output sound was occasionally poor. In this paper, a highly improved model, named the quad-motor, de-spin (QMDS) model, is presented. It is based in two coaxial DMDS mechanisms. The experimental model has a cone radiator of 46 cm diameter and an enclosure volume of 268 l. Its working frequency range is the same as that of the DMDS model. Harmonic distortions included in the output signal are improved to be less than 10 percent of DMDS. Its sound quality is excellent.

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