The Sennheiser MD 421-II Microphone is a cardioid, dynamic microphone popularized by the recording and broadcast industry. The MD 421-II is favored for its ability to handle a variety of applications due to its flexible bass control. The microphone features a 5-position bass roll-off switch and frequency response of 30 to 17,000 Hz. The cardioid polar pattern effectively rejects off-axis noise due to room acoustics and noise, and also provides effective feedback rejection often caused by monitors positioned within close proximity.

The MD 421-II features a dynamic capsule, allowing even signal response whether close-up or at a reasonable distance. This is particularly useful for broadcast applications, where announcers and DJs can refrain from remaining absolutely stationary. The MD 421-II also handles very high sound pressure levels, making it suitable for miking instruments high in transient (peak) response.