PURPOSE: To set the volume of an internal air chamber and to change performance on acoustic characteristic by making an adapter mobile.

CONSTITUTION: An adapter 12 can be moved back and forth by the screw part 5c of a case 5a and the screw part 12a of the adapter 12 by rotating the adapter 12 engaged with the screw part 5c of the case 5a and thereby the volume of an internal air chamber 11a can be changed. Acoustic characteristic when the volume of the internal air chamber 11a is made small is shown by a curve 13 and that when the volume of the internal air chamber 11a is made large is shown by a curve 14. Thus, the acoustic characteristic can be changed easily, and lowering of cost can be realized.