Protection by underwater sound waves

DETAILED DESCRIPTION One object of the invention is to produce a field or area traversed by sound waves and including a receiving device which is activated by the sound waves and which will respond abnormally when an extraneous object traverses the field or area.
For example, the presence of a swimmer, who is carrying a bomb or other means of destruction, would be detected as he passes through a field laid out for the protection of a warship or raft or other floating object, or a bridge pier.
By setting up a field of sound waves across a harbour or ship channel, the receiver will be abnormally activated whenever a ship proceeds through the channel.
The shore crew, which may for example be armed or provided with missiles, may thereby be alerted to destroy a hostile submarine passing otherwise unobserved through the channel into the harbour.