Pneumatic earphones: The air pump and air valve are used to control the air flow, and the air pressure and flow rate are directly controlled to make the air vibrate. Sometimes the gas valve is replaced by a high-powered speaker. Such headphones are commonly used on airplanes, which are actually just air ducts. The advantages are no electric drive, unlimited parallel connection and high efficiency. The disadvantages are large distortion, narrow frequency response and noise.
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