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Microphone placement techniques
We have designed this page to offer suggestions for non-convention microphone placement techniques. We realize that these are notable deviations from industry standard methods but we have found them quite useful when trying to achieve a listenable recording with a stereo feel in a less than desirable setting. If you are seeking information about common techniques, we highly recommend a visit to the Microphone University at www.dpamicrophones.com. While there, please take note of the standard stereo techniques they present and note that the do NOT address recording from a PA system at a live concert.
With all due respect to the folks at DPA and all other recording industry professionals, we do not find that many of the standard techniques work particularly well in concert recording applications where the recordist does not have a choice in where to place the microphones. Techniques like ORTF and X/Y originate in symphonic recording and in studios. Under the right circumstances, they produce fantastic results that portray a stereo image beautifully.
ORTF, a near-coincident technique, 170mm apart with and included angle of 110 degrees, has its genesis in symphonic recording. Properly placed, the stereo pair would be suspended above the conductor's head and each mic would point to its own respective end of the orchestra. Such placement utilizes the polar characteristic of the capsule. Since a first order cardioid is 3db down at 55 degrees off-axis, sounds that originate further to the side of the other microphone will then sound louder on the side of that microphone and consequently result in an accurate stereo image. When placed as described above, the microphones encapsulate the performers and are close enough to the source that the peripheral characteristics of the room do not compromise the sound. They merely provide a texture and feeling for the venue.
Conversely, an X/Y pair, a coincident technique, with its technically proper time alignment (phase), will yield a tape with a very narrow stereo image. All sound will seem to eminate from a small region between the speakers with very little actually sounding like it is coming from the speakers themselves. While this is excellent for mono compatability, and panning a stereo instrument in a studio, it does not make for a very exciting listening experience that represents the concert hall feel. Another interesting anomaly of X/Y is the relative reduction of low frequencies compared to a near coincident pair.
Identify the goal of the recording:
If the recordist intends to capture the sound of "being there" just like it was, then an ORTF, or a stereo pair of omnis could be perfect. In a pristine recording environment, this is wonderful. However, as many of us want to make a recording that sounds great in our living room and maintains a balance between the concert hall feel, and the presence of a studio or soundboard recording, we must begin to compromise.
As concert tapers record in environments that many professionals would never even consider, tapers must realize that we are on our own. We must find out what works best for us in our own unique situations. Once we have accepted that compromise is necessary, we can consider how to mutate the common techniques to best suit our unique application. As we are often recording music from a distance that prevents realization of stereo panning (off-center, too far back in the room, out beyond the delay towers, under the balcony...), we must determine how we want to create a feeling of stereo. To return to the concept of ORTF placment, the mics are separated widely enough that there is limited overlap of the capsule pickup patterns. Moving them too far apart would yield a tape that suffers from the notorious "hole-in-the-middle." All sound would appear to be coming from the loudspeakers without any portrayal of information in between. An appropriate deviation of this technique can yield excellent results whereas misapplication of the "right technique" can still lead to unfavorable results. A pair of microphones in ORTF position do portray accurate stereo imaging, and replicate what the recordists ears heard in the given location, but given the undesirable qualities of many venues, the recording can be riddled with the same negative component that compromised the sound at the performance.
The compromise can take on the following form. By aiming each microphone to the outer edges of its own respective side of the P.A. system (left to left and right to right), and spacing the microphones 8-12 inches apart on the horizontal plane, the feeling of stereo can still be captured while reducing the ambient characteristics of the room and improving clarity of the recording. The included angle will naturally be a result of the distance from the source and will usually end up between 70 and 90 degrees. As this narrower angle deviates from the optimal angle of incidence addressed in the ORTF technique, the spacing from left-to-right must be wider to prevent overlap of the polar pattern of each microphone. Otherwise, the image will become too narrow.
Although this type of recording is not technically accurate according to standard stereo microphone techniques, the results will be very listenable to most listeners.. The sense of stereo image will be created due to thetime arrival differences from each respective side of the sound source arriving at the opposite mic marginally behind the time it arrived at the other. This negligeable time delay will be perceived as time-based stereo separation and will be complemented by any instrument and effects panning utilized by the front of house mixer.
In addition to the matter of microphone angle, height of the stereo pair should be considered. As tapers, we seldom have the opportunity to suspend microphones, and our stands are seen as an intrusion. Although it seems appealing to get our microphones high above the crowd, this has a couple negative impacts in our recording. Of primary importance, we must recognize that a Front of House (soundguy) Engineer's job is to get sound throughout the house and have it sound best where we will all hear it. Consequently, the system engineer should optimize the PA (set EQ's and delays) for head level. Many P.A. systems, especially line array technology, like V-DOSCtm, now being used by many popular bands, are designed to have the most neutral tonal, qualities at head-level. The PA system, when focused properly will throw sound over the heads of those not intended to hear a given section of the PA, and place it, with best tonal balance and desired volume, on the intended recipients. Going higher with microphones will cause them to pick up from a section of the PA that has not reached its balance yet. This will potentially sound brittle, edgey, harsh, and mid-range dominant. At the very least, the results will be unpredictable to a listnener without measurement equipment.
Another potential malady of running a microphone stand too high, results from aiming over the PA or into seams where the interaction of the cabinets is quite unpredictable. Keeping the microphones just above head level (7-9 feet) assures that we are capturing the sound that we heard, as it was intended to be heard, assuming the system was installed by a skilled system engineer. As well, without being able to accurately hear what the microphones are picking up, we may place the microphones directly in the path of a horn (part of a speaker cabinet that can be particularly bright and harsh, especially up close, until it mixes with the rest of the sound). The best rule of thumb is to simply bring the mics a couple feet above head level and hope for the best. Anything else is guess work that has no gaurantee, or reason, to sound any better than what we are already hearing.
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