PURPOSE: To make stable acoustic transducing at high sound pressure irrespective of the fluctuation of an external load by selecting the resonance frequency of a hollow formed in front or back side of a diaphragm lower than the resonance frequency of the diaphragm.

CONSTITUTION: A yoke 4 having an iron core 3 wound with a coil 2 is attached to the bottom of cylindrical case 1 of an acoustic transducer. A cylindrical permanent magnet 5 surrounding an electric magnet made of the coil 2 and iron core 3 is provided in the yoke 4. A discoid diaphragm 6 is mounted at the upper opening of the case 1 via a cavity through the tip 3a of the iron core 3 and tip face 5a of the magnet 5. A hollow 83 is formed in front or back of the diaphragm 6. The inherent resonance frequency f1 of the diaphragm 6 and the hollow resonance frequency f2 of the hollow 83 are selected to form the frequency relation of f1>f2. Thus, the stable acoustic transducing at high sound pressure is performed regardless of the fluctuation of the external load etc.