PURPOSE: To improve frequency characteristic especially at a high frequency by providing an air chamber at the back side of a diaphragm.

CONSTITUTION: An opening leading to an acoustic resistor is provided to a yoke 3 in common use for an outer circumferential case and the lower position to which voice coils 71, 72 are placed is closed tightly. Further, a cavity case forming an independent air chamber is provided and an opening 10 leading into the yoke 3 is provided under the edge part of the diaphragm. In setting properly the volume of the air chamber 12, dips are formed in the characteristic with a parameter of 180° as shown in Fig., thereby improving the directivity characteristic. Since the dips are changed because of the volume, it is required for the setting to produce the dips.