Limitations of low frequency sound reproduction ability of the conventional direct-radiator loudspeakers is essential because the diaphragm of them must be driven in a mass-controlled range for a flat response. This paper proposes a novel direct-radiator loudspeaker suitable for low-frequency signal radiation. It utilizes a ultrasonic motor (USM) including a piezoelectric transducer. A velocity modulated continuous revolution is better than a reciprocal motion to avoid distortion in wave form due to the difference between dynamic and static frictional forces. A few fundamental ideas, a continuously revolving flat radiator, an air-flow modulation type without any mechanical radiator and a conventional radiator actuated by a revolving mass are compared to investigate merit of the loudspeaker proposed here.
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