An ionophone is a transducer for converting acoustic vibrations in plasma into an electrical signal, or for converting an electrical signal into acoustic vibrations in plasma. It can be used for "generation of subsonic or ultrasonic waves as well as a microphone or loudspeaker".
Ionophones are to plasma what microphones and loudspeakers are to air. Because the acoustic impedance of air and plasma are similar, an ionophone will work well as a microphone or loudspeaker when appropriately coupled to air.
Ionophones are more commonly used as loudspeakers than as microphones. Unlike a loudspeaker having solid matter (cone) that moves to push air (or water in the case of an underwater speaker or hydrophone) sound is produced by plasma (ionized gas) in a spark gap. Typically a high frequency (ultrasonic) electric arc is established, and then modulated by audio frequencies.