PURPOSE:To obtain a dynamic sound reducing device of high sound reducing effect in spite of being comparatively compact by emitting control sound of opposite phase to original sound but of the same sound pressure level from the control speaker of each of plural intake chambers, and performing the space composition of the original sound and the control sound so as to cancel the sound pressure level mutually to reduce the sound. CONSTITUTION:Air flows into the impeller 1 of an axial fan through a suction silencer 5, a suction duct 2 and an intake chamber 3. In this case, the intake chamber 3 is partitioned by a partition plate 4 into two horizontal chambers, and each intake chamber 3 is provided with an independent control speaker 7 and an independent control microphone 8. Each control microphone 8 is connected to the speaker 7 through an amplifier 9, waveform synchronization controller 10 and an amplifier 11. The waveform synchronization controller 10 is further connected to a rotational pulse transmitter 13 provided in proximity to the shaft of a fan driving motor 12. The control sound emitted from the control speaker 7 to original sound collected by the control microphone 8 is controlled to be of opposite phase to the original sound but of the same sound pressure level, and the composite sound of each intake chamber 3 is mutually cancelled to perform sound reduction.
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