To provide an apparatus that resolves the problem that large loudspeaker and amplifier are required if low-pitched sound is added to sound of the loudspeaker, and amplifies low-pitched sound not using electricity with a low weight and without the need to choose places.

It is utilized that atmospheric pressure causes flow from a high place to a low place. A pipe is made with its inlet ports being narrowed to make an outer shroud (1) to be pushed by acoustic pressure and thereby to raise an internal pressure thereof, so as to pick out only low-pitched sound from a band bass port (3).