The STL‐ionophone consists of a dc discharge in air. The discharge emits sound when an ac current is superimposed on the dc current. It represents a small sound emitter with high internal impedance and constant source strength from low to ultrasound frequencies. It can also register air motions as a superimposed voltage and works in this function from zero to ultrasound frequencies. It has been employed as a sound source for measurements on woodwind instruments and models of the human vocal tract. As a receiver, it has been employed to study the sound field at the embouchure of flutes. Fundamental properties registered and applications will be discussed.