by David Lane Josephson
MOST READERS of this article will no doubt be trying to achieve a stereo pickup in their recordings. This is one of the toughest problems in making good tapes--getting the "stereo effect"-because, especially in live recording, stereo is not "two monaurals." Rather it must be a complex aural experience leading to the listeners' illusion that they are actually where the recording took place. The jump from mono to stereo is much more difficult than the jump from stereo to quadraphonic, since four-channel, in this context, is an extension of the stereo effect.
Fig. 1--Single mic pickup of piano. Single mic shown may be replaced with
two for stereo; they should be placed one on either side of the mono mic
position and about 18 in. from where the mono mic would be. Distance between
piano rim and mic(s) may be varied from six to 12 feet to achieve a balance
between direct and reflected sound.
Fig. 2--Stereo pickup for upright piano. The main mics are placed about
four feet apart, equally spaced from the two ends of the piano and an equal
distance (about 18 in.) from the piano back and the wall. The ambience mic
may be recorded on a separate track and mixed into left and/ or right later,
or may be mixed into left and right during recording. Some mixers have a
panpot or "blend" channel and this is how it is used.
Recording engineers have arrived at two basic means to obtain the stereo effect. One is an imitation of the way human ears perceive this depth-and-position sensation, known as binaural; the other uses a great number of microphones, each placed quite close to one particular sound source and then mixed to create a synthetic stereo illusion, known as multi tracking. Since the amateur recordist usually doesn't have the huge number of high quality mics and mixing gear required for multi-tracking, this guide to microphone placement will stress variations on the binaural technique, using at most two microphones per recording channel.
Pickup of a single voice is probably the easiest microphone placement problem a. recordist will run into, but it is by no means simple. Any good quality microphone may be used. A velocity mic will provide an extra fullness, another intangible characteristic that isn't reflected in a microphone's specifications. Most recordists mike much too close to the speaker--this may give a bassy "studio sound" that some appreciate, but it is not an accurate pickup. Some mics, such as the Neumann U87 and Electro-Voice RE 15, incorporate bass rolloff switches that reduce the response below 150 Hz or so by as much as 10 dB, to compensate for the bass accentuation that occurs when any microphone, especially a unidirectional one, is used close to the sound source. Sometimes, in voices lacking depth and assurance (most amateur announcers), this proximity effect will be beneficial.
Omnidirectional dynamic mics, such as the Shure SM-60 or Electro-Voice 635A, are often used for voice pickups since they are relatively insensitive to wind and breath noise, and because of their lack of proximity effect, usually sound smooth and natural. Additional noise reduction-to the almost complete exclusion of all wind, breath and handling noise-may be had with the isolated mics similar to the SM-60 and 635A, the Shure SM-61 and EV RE-50. With this type of mic, the announcer or singer may be placed as close as six inches from the mic without any severe proximity effect or breath noise. Unidirectional mics, unless equipped with bass rolloff and very effective windscreens, must be used no closer than 12 inches--preferably two feet--from the speaker. Bidirectional velocity mics must be placed no less than two feet from the announcer, for accurate pickup.
Placing the microphone(s) directly on the table or lectern, rather than on a stand, will reduce reflections that can add a tinny quality to the recording. Covering the table with felt or any similar sound-absorbing material will also reduce this form of aural multipath distortion.
Miking Pianos
Piano pickups are another very simple type of setup. Grand pianos should be placed in the one position they were designed to be used in--with the lid at full stick. Some engineers advocate leaving the lid closed, or opening it to half-stick, or even removing the lid entirely. But the piano was designed to be used at full stick, and the most accurate and natural-sounding pickup almost always results when it is recorded this way. In mono, a single microphone, preferably capacitor or velocity, should be placed about six feet off the floor, on a line parallel to the keyboard but centered in the middle of the sounding board, eight feet out from the rim of the piano. If the bass is too prominent, the mic may be moved toward the front (keyboard) of the piano, still maintaining the eight foot distance from the rim. Conversely, if the treble is prominent and the sound is harsh or bright, the mic may be moved toward the rear of the piano. The distance from the mic to the rim should be decreased only if the room is too reverberant, and then .only to a minimum of four feet. Unidirectional miss are usually used to eliminate extraneous noises. A stereo effect may be created using two microphones, placed about three feet apart (each about 18 inches from the center line). Height and distance from the rim of the piano should not be changed. Two pianos may be miked with one omnidirectional or bidirectional mic as shown. Two unidirectional mikes are used for stereo.
Upright pianos present a sort of diamond-in-brass problem. They were not designed for use except in places where grand pianos could never fit. The compromises made to reduce the size usually render them less fit for music recording--but it can be done. Most of the sound is produced from the back of the piano and reflected from the wall. The piano should be spaced about two feet from the wall, and the mic placed above the piano. The lid (if you can call it that) may be raised for a little more brilliance. The distance between mic and piano should be between four and eight feet but experimentation is called for. Player pianos may be miked the same way, except that the mic may have to be raised further to eliminate mechanical noises. A zero-reflectance pickup with the mic on the ceiling might work well for this sort of pickup.
Electric organ pickups have been described as "duck soup." Simply place one mic directly in front of the unit's speaker, about four feet out. Since these units are entirely electronic, the microphone may as well be dispensed with, except for pickup of room acoustics and/or vocals. Most electric and electronic organs have tape recorder outputs that tap directly into the amplifier. Pipe organs present an entirely different miking requirement. Good capacitor mics are almost a necessity for pipe organ pickups, although good velocity and omnidirectional dynamics have been used. The main criteria is that the frequency response must be as flat as possible down to 30 Hz and up to at least 16 kHz. In this type of pickup, since the acoustics of the room have probably been designed for organ, the natural ambience is important. This calls for mic placement further away than would be expected. Church organ pickups generally may have a great deal more reverberation than might be desired in an electric organ pickup. Walking around the church during a rehearsal will help find the best spot for a single-mic pickup for the organ, which should be employed if at all possible. An accent mic should be used for the choir. This may be a unidirectional capacitor or dynamic unit.
Fig. 3--Acoustic guitar pickup using a bidirectional velocity mic, in this
case the RCA SK-66. Most velocity mics have a bass rolloff of some sort,
either fixed or variable, that can be used to counteract the proximity effect
to be expected when the mic is used this close.
Guitars and Strings
Guitar pickups are another subject of acute controversy among professional recording engineers. Generally the preferred technique is to use a good unidirectional capacitor mic one to two feet out from the sound hole of the instrument and directly out from it. A good omnidirectional dynamic may also be used. The main problem with guitar pickup is the great bass accentuation when the mic is any closer than two feet. This is, of course, especially troublesome when a unidirectional mic is used, and if a bass rolloff switch is available on this type of mic, it should be switched in. Any acoustic guitar, lute, mandolin, ukelele or banjo may be miked this way. With banjos, only capacitor mics should be used because of the necessity of preserving the "twang" and sharp attack time that characterizes this instrument.
Electric guitars may be miked in one of two ways. Either the output from the unit itself may be fed into the recorder, or a single good dynamic or capacitor mic may be placed in front of the amplifier speaker. If any sort of effects (fuzz, reverb, tremolo, etc.) are used, the latter method must be employed. Since fuzz tone depends on good square wave response, capacitor mics should be used if this effect is predominant. Electric basses may be miked the same way, although any decent dynamic or velocity mic may be used.
Contact mics should be avoided like the plague, both for electric and acoustic guitars. In most cases, these are simple crystal units, with response dropping sharply below about 200 Hz. The standard electric guitar pickups depend only on changes in a permanent magnetic field caused by movement of the steel strings, and this usually provides an accurate pickup.
Classical stringed instruments such as violin, cello and bass should always be miked with capacitor mics if they can possibly be obtained. Accuracy in pickup of bowed stringed instruments depends mainly on the untarnished rendition of the complex harmonics of the basic note. Violin harmonics extend past 18 kHz, and it is important to use mics that will respond linearly to this high frequency. Omnidirectional capacitor mics are ideal for this type of pickup. Only two need be used in achieving a very realistic and accurate stereo effect.
Addenda
The Revox Corp., American distributor for the Beyer line of microphones, complained about the contention in the December, 1973, Audio that velocity microphones were heavy and susceptible to damage due to strong wind or breath noises. The firm's representative stated that Beyer mics, pressure gradient ribbons though they may be and not velocity units, were "tarred with the same brush." Revox very kindly loaned this writer three samples of the Beyer pressure gradient ribbon mics for testing.
First, let us be clear that not all mics that use a ribbon element are velocity microphones. In order to be a true velocity mic, the ribbon must be open to the air front and back for its whole length. These mics are usually very heavy and always very susceptible to strong coughs and wind. They are also certain to be damaged by high-level impulse noise, such as a close-range shot from a timer's gun at a sporting event. The steep rise and fall of the pressure from this sort of noise simply snaps the ribbon.
On the other hand, pressure gradient ribbon mics, such as the entire Beyer line of ribbon mics, the RCA BK-58 and 77-DK (in unidirectional position), the Shure SM-33, can be made much lighter (they require less magnet) and much smaller, some as small as small dynamic units. Since more than half of the ribbon is not exposed to the air in the rear but rather ducted to an acoustical labyrinth which is sealed to outside air, the pressure gradient ribbon mics are much less sensitive to damage caused by breath noise or wind. The mics tested, the Beyer M-160, M-260 and M-500, and the Shure SM-33, all seemed to be impervious to damage from these causes, when subjected to extremely loud feedback and repeated breath and wind noise over a five-day period by an electric guitar class at the Middletown, Calif., High School. In short, the pressure gradient ribbon mics aren't nearly as fragile as their velocity ancestors.
-Ed.
(Audio magazine, Jul. 1974)
Also see:
Microphones. Vital Link In the Recording Chain--Part 3 (Aug. 1974)
Microphones--The Vital Link in the Recording Chain--Part 1 (Dec. 1973)
A Microphone Primer: Basic Construction, Performance, and Applications (Dec. 1972)
A Microphone Primer: Conclusion (Feb. 1973)
Mike Technique and Sound Effects (Dec. 1974)
Recording in 2 and 4 Channels (Dec. 1973)
Electro-Voice microphones (Jan. 1973)
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