Plasma speakers or ionophones are a form of loudspeaker which varies air pressure via an electrical plasma instead of a solid diaphragm. The plasma arc heats the surrounding air causing it to expand. Varying the electrical signal that drives the plasma and connected to the output of an audio amplifier, the plasma size varies which in turn varies the expansion of the surrounding air creating sound waves.

The plasma is typically in the form of a glow discharge and acts as a massless radiating element. The technique is a much later development of physics principles demonstrated by William Duddell's "singing arc" of 1900, and can be related to modern ion thruster spacecraft propulsion.

The term ionophone was used by Dr. Siegfried Klein who developed a plasma tweeter that was licensed for commercial production by DuKane with the Ionovac and Fane Acoustics with the Ionofane in the late 1940s and 1950s.

The effect takes advantage of several physical principles: Firstly, ionization of gases causes their electrical resistance to drop significantly, making them conductive. This resulting plasma can be made to vibrate sympathetically with alternating electric fields and magnetic fields. Secondly, the involved plasma, itself a field of ions, has a relatively negligible mass. Thus the air remains mechanically coupled with the massless plasma allowing it to radiate a potentially ideal reproduction of the sound source when the electric or magnetic field is modulated with an audio frequency signal.