1. Field of the Invention
This invention relates to sound generating transducers, more particularly but not exclusively to so-called "Flextensional" sound generating transducers.
2. Description of the Related Art
One Flextensional sound generating transducer is disclosed in European patent specification No. 215657-A. It comprises an elliptical shell with end cover plates defining a cavity containing a stack of piezo-electric elements, the stack being compressed between portions of the shell wall at opposite ends of its major axis.
If the cavity is air-filled, it must be provided with pressure-resistant sealing, for example between the shell and cover plates, and the service depth of the transducer is in any case limited due to the increasing water pressure which acts on the sides of the shell to elongate it and reduce the stack compression. Compensation mechanisms, for example controllable wedges included in the stack, have been proposed but are complex and expensive.
On the other hand, if the transducer cavity is allowed to flood with liquid, the apparent stiffness of the shell increases along with its resonant frequency. This may be a disadvantage because the usefulness of a Flextensional transducer mainly is its ability to operate in a particular, relatively low, frequency range.