Sennheiser HD 25 Light - headphones

A distinction is made between headphones which are worn on the external ear and those which surround the ear. Open headphones have foam ear pads that rest on the ears or ring pads that surround the ears. Closed headphones, on the other hand, nearly always have circular ear pads.

Impedance expresses the AC resistance of a microphone or a set of headphones. It is dependent on frequency and is given at 1 kHz as the so-called nominal impedance. Impedance is measured in ohms. In recent years, an industrial standard has developed, setting the impedance of headphones at 50 or 600 ohms. Sennheiser headphones are manufactured according to this standard, thus practically ruling out connection problems. With microphones, the input impedance of the following microphone amplifier should have at least three times the value of the nominal impedance, in order to prevent it from unnecessarily attenuating the microphone signal.