PURPOSE: To achieve the beam uniformity of an expansion transducer driven resonantly by fastening the longitudinal end periphery of a slender diaphragm more firmly relative to the base than to the side periphery in its width direction.

CONSTITUTION: A conical diaphragm 12 of a transducer is set on the base 14, and secured by an integral clamping member 16. The base 14 provides an inner elliptical lip 26, and the circumference of the diaphragm 12 is mounted thereon. In the end peripheral area of the diaphragm 12, the lip 26 is formed aslant in the whole, and a complementary surface 30 is formed on the member 16 so as to form a firm clamp combination. Contrastively, in the side peripheral area, since the lip 26 has a more sharp contour shown in its surface 30, the clamp is not so firm. In this manner, it is valuable in a diaphragm having its length at least 1.2 times the width, and it is desirable that rigidity in the longitudinal direction is within 50% of the rigidity in the width direction.