PURPOSE: To control the transmission sensitivity by generating large transmission sensitivity in the lower band than a resonance frequency of vibration in the diameter direction of a cylindrical type piezo-resonator, in the transmitter-receiver of a pressure balance structure.

CONSTITUTION: In the transmitter-receiver of a pressure balance structure in which the inside and the outside of a cylindrical type piezo-resonator are filled with insulating oil 6, this transmitter-receiver generates a primary vibration mode so that it becomes a loop in one and which is opened, and a node in a closed mass 3. It is what is called a Helmholtz resonance, and by this resonance, not only the resonance of vibration in the diameter direction of the cylindrical type piezo-resonator 1, but also a band which becomes large transmission sensitivity are generated. A columnar spacer 7 performs an action for changing length of an acoustic tube for which the piezo- resonator 1 is utilized, therefore, by changing height of the columnar spacer 7, this Helmholtz resonance can be adjusted freely. Accordingly, the resonance frequency can be set freely in accordance with a use frequency. In this regard in the case the columnar spacer 7 is provided length of the acoustic tube is shortened, comparing with the case when it is not provided, therefore, the Helmholtz resonance is shifted to the higher direction than this measured data.