Introduction 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.
Types There are different types of MD 421, the main difference between them being the type of connector ﬁtted. The models are:
MD421 N The MD 421 N is ﬁtted with a three-pin plug according to DIN 41524 which is symmetrically wired according to DIN 45594. This mic is ﬁtted with a roll-off ﬁlter.
MD 421-2 The MD 421-2 is ﬁtted with a large 3-pin Tuchel plug as preferred by most European Broadcasting Authorities which is also wired symmetrically. The IVID 421-2 is a low-impedance microphone. A roll-off ﬁlter is not provided with this model.
MD421-U The MD 421-U is provided with a 3-pin Cannon plug (XLR). It is, as the other two models, a low-impedance mic which is symmetrically wired. A roll-off ﬁlter is provided. Two versions of the MD 421-U are available: MD 421-U-4 with adapter for ⅜-inch threads and MD 421-U-5 with adapter for ⅝-inch x 27G threads. The MD 421 N and the MD 421-2 are available in light grey heavy-duty ﬁnish. The models MD 421-U-4 and MD 421-U-5 feature a matt-black housing.
Directional Properties The MD 421 has a cardioid directional characteristic which means that the maximum insensitivity to unwanted sound sources is at about 180°. The directional index of the MD 421 at 1000 Hz and 180° is at least 18 dB. This means that sound from the rear with a frequency of 1000 Hz produces an 18 dB lower signal as sound of the same intensity reaching the microphone from the front. As can be seen from the polar diagram, the MD 421 should therefore be spoken into directly from the front.
Adjusting the Roll-off Filter When a directional microphone is spoken into from a close distance, the lower frequencies are over-emphasized. If this effect is not desired it is possible to counteract or compensate for it by means of a roll-off ﬁlter. From the diagram showing the inﬂuence of the roll-off ﬁlter, it can be seen that in position M (music), the frequency response curve is not altered in any way. In position S (speech), however, the frequencies below 500 Hz are reduced by approximately 6 dB/octave. Between the positions M and S, the frequency response can be altered in three deﬁned steps. See Figures 4 and 5.
Listen to a comparison of the ﬁve Roll-off Filter positions.
Text and illustrations are from the Sennheiser Users Guide for the MD 421.
The two photos above were taken at WYAY, Eagle 106.7, Atlanta in 1985.