Loudspeaker using continuous revolution of a piezoelectric ultrasonic motor

Volume displacement of a direct radiator loudspeaker diaphragm is inversely proportional to the square of its signal frequency. The lowest frequency of a flat response is restricted by the diaphragmfs fundamental resonance frequency. Therefore, a loudspeaker diaphragm for radiation of a low-frequency signal shall have large amplitude and low resonant frequency. The authors propose the application of a piezoelectric ultrasonic motor for this purpose. It produces a completely controlled large output force because its output mechanical impedance is much greater than any conventional transducer or motor. An ultrasonic motor whose stator is connected to a direct-radiator loudspeaker cone diaphragm by a rod and whose rotor is loaded by a heavy metal ring rotates with a constant velocity. Modulation of the velocity by the audio signal causes driving force to the diaphragm because a heavy ring with large inertia tends to rotate with constant velocity. This paper reports construction of a practical experimental model and its output sound pressure characteristics.