Two driving constructions of loudspeakers for low-frequency range by piezoelectric ultrasonic motors

The loudspeaker driven by piezoelectric ultrasonic motors is characterized by a precise very-low-frequency reproduction due to its high-driving mechanical impedance. It has a lot of merits comparing to the conventional electrodynamic loudspeakers. One of the reason will be that this loudspeaker is a power flow modulator, not a transducer. In this presentation, two sorts of ultrasonic motors are compared as driver elements of the loudspeakers. One is an ordinary revolution-type motor and the other is a reciprocal linear motion type actuator. The authors constructed and improved practical low-frequency-range loudspeakers by using continuous revolution of ultrasonic motors. Its final model uses combination of two motors with same axis, which drive two cone radiators moving oppositely. This model shows a satisfactorily large output sound pressure and stable reproduction. However, its complicated elements for connection of the motors and the cone radiators cause a mechanical weakness. The authors, therefore, propose another new, completely different construction to avoid this defect. It applies linear motion of two ultrasonic actuators. A moving piece driven by piezoelectric ultrasonic vibrators is connected directly to a cone radiator. Comparison at various viewpoints and practical performance of these two constructions are presented at the meeting.