A microphone that consists of a very thin ribbon of aluminium alloy a few millimetres wide loosely fixed in a strong magnetic flux density parallel to the plane of the strip. A sound wave incident on the ribbon causes a pressure difference to be established between the front and back edges of the ribbon and it therefore experiences a force. The resultant motion causes a corresponding e.m.f. to be induced in the ribbon (see electromagnetic induction).

If the acoustic path difference across the ribbon is much smaller than a quarter wavelength, the pressure on the ribbon (and hence the e.m.f.) is proportional to the particle velocity and to the frequency. If the resonant frequency of the ribbon is made smaller than the frequency of the sound waves the frequency dependence becomes negligible and the induced e.m.f. is proportional to the particle velocity. The microphone has strong directional properties since sound waves that originate in the plane of the ribbon arrive at the front and back edges in phase, and therefore no resultant force is produced.

The ribbon microphone may be used to measure sound intensities. An alternating current of the same frequency as the sound wave is passed through the ribbon and the phase and amplitude are varied until the force due to the current just balances that due to the sound wave. The sound intensity is then calculated from the current amplitude and the magnetic field strength.