hydrophone noun. ['hʌɪdrǝfǝʊn] M19.
[from HYDRO- + -PHONE.]
1. Medicine. A bag of water placed between a stethoscope and the patient's chest in order to intensify the sounds heard. Now rare or obsolete. M19.

2. A transducer which detects sound waves transmitted through water and produces corresponding electrical signals which can drive a loudspeaker etc. L19.
■ P. Ziegler Mountbatten claimed to have picked up traces of the U-boat on his hydrophone.

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