The MD 421 is a well-known Sennheiser microphone of proven reliability and per­form­ance for many years now. Because of its excellent quality it is being used for al­most all kinds of applications. The broadcasting stations use it for radio and TV field work, the musician prefers it for stage work as well as the recording amateur for home and outdoor recordings. The studio cardioid micro­phone MD 421 features a wide frequency response from 30 to 17,000 Hz. Its high sensitivity and slight increase in response towards high frequencies are responsible for its brilliant sound. Each microphone is custom cali­brated and delivered with its own per­form­ance chart which may be com­pared with the published standard per­form­ance plot printed in this leaflet. Each individual frequency plot has to fall within the published limits of the standard per­form­ance curve. The cardioid direction­al characteristic of the MD 421 with its front-to-back ratio of approximately 18 dB is an important feature making the microphone suitable for use in sound rein­forcement systems and all situations where it has to be used near loudspeakers. To prevent hum pickup by mag­netic stray fields the IVID 421 is fitted with a compensa­tion coil.