Piezoelectric speakers use an expanding and contracting crystal to vibrate the air and produce sound. Piezoelectric speakers are limited in frequency response therefore they are only used as tweeters or in small electrical devices like watches/clocks to make simple sounds. It may be possible in the future that this technology may improve, providing a speaker with good sound characteristics and durability, however it remains up to tomorrow's engineers to make this possible.
Piezoelectronics are a solid state technology which makes them durable and good for use as a microphone underwater. These speakers are used as microphones in submarine warfare; they can detect other microphones and hear sounds of other vessels.
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