PURPOSE:To allow a speaker to execute a specified performance, by subjecting an internal water pressure to follow after an external water pressure even if the depth is remarkably changed. CONSTITUTION:A transmitter 18 is provided with a speaker consisting of a diaphragm 13, a magnet 17, a coil bobbin 16 and a coil 14 and driven with a signal from a cable 19. The internal part of the transmitter 18 is connected to an air chamber 4 with a link tube 12. When a diaphragm 11 is displaced up and down with an external water pressure and it is located between magneto-sensing elements 21 and 22, the pressure in the air chamber is equal to the external pressure. Thus, the pressure in the transmitter 18 is equal to the external pressure. If the diaphragm 11 is located above the element 11 or below the element 22 with a large external pressure, the internal pressure can not follow the external pressure. If the external pressure is larger, a valve 3 is controlled to supply air in a high compressed air tank and if smaller inversely, a valve 8 is controlled to blow out air externally, allowing to set the position of the diaphragm 11 to the position of an element 20.
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