Because of these factors, panels used to disperse the sound can sometimes be the safest bet. The farther away from a sound source a microphone is placed, the more ambient sounds in the form of ambient room noise, or “bleed” from other instruments, will be picked up. A long studio reverb time can cause conflicts when recording dialog. The additional sound can not only causes smearing, but can also actually garble the talent’s words. You would never think that a short room reverb time could disturb the tempo of a piece until you actually hear it. Variable acoustic panels along the walls of a large room solve a host of potential problems for the audio recording artist. Critical distance refers to the position where the sound source and the reverberant field will be equal in level. A mic positioned at any point farther than this distance from the source will render a sound that can lack definition, sound blurry from excessive reverb, or sound muddy due to phase cancellation. A Rule of Thumb One “rule of thumb” is to place a directional (non-omni directional) mic within one-half of the critical distance:
Try this method just once and you’ll agree that it’s a natural. You can count on it to always work because it is not based on anyone’s subjective opinion, but on the laws of physics. Pretty soon you’ll be able to judge the correct distance without all the stop, listen, and go movement. It’s faster and easier to listen to the source while you move away with one ear blocked. But unfortunately, half the critical distance may be too far away in terms of other room noise, heating, ventilation, air conditioning (HVAC), or instrument bleed. It is fairly easy to place go-bos around acoustic instruments, but often excessively loud combo amplifiers are best set up in the corner of the room that has absorptive surfaces. In some cases the amp will need to have an enclosure of go-bo panels assembled all around it, or be placed in an isolation booth to prevent its output from “bleeding” onto other instrument’s tracks by leaking into their mics. Reflections can cause what sounds like utter havoc at the microphone’s diaphragm. This is because the direct sound can combine with the reflective sound to produce any or all of the following three distinct and undesirable results. If two sounds arrive at exactly the same time, there will be a doubling of the output. If two arrive out of phase in timing by 180 degrees, they can cancel each other out. Anything in between these two extremes can result in lower or higher output levels at narrow frequency bands, the cancellation of certain frequencies, the smearing of frequencies with low frequencies being affected more at greater distances than high frequencies, tube- or tunnel-like sound effects, muddy sound, or just about anything else. The difference between all of these results can equate to as little as a one-quarter inch change in the mic’s position. So using two or more microphones on a single instrument, or in a reflective environment, can be bad news. Another Rule of Thumb Called the “3-to-1” Rule This says that when using two omni directional microphones it is best to keep the second mic at a distance from the first that equals three times the distance the first is placed from the sound source. This means that if the primary mic is 1.5 feet from the instrument, you must place the second mic 4.5 feet from the first one. Okay, the problem when multi-micing an instrument is obvious. I like to place a mic 1 to 2 feet in front of a saxophone’s bell. This is because I don’t like a sound that only comes from the bell. From this position, I can aim the mic higher up toward the neck and pick up all the sound coming off the upper sound holes while still getting some of the raspiness from the bell. But it is also essential to mic woodwind instruments at the player’s right-hand side to pick up its lower frequency content. The 3-to-1 rule presents no problem when micing a baritone, but for a curved soprano it’s plain ridiculous. It could equate to a reduction in phase cancellation problems, but the mic that’s more distant by three times will have an output that’s 12 decibels lower. Even though this distance can be lessened for directional mics, it exemplifies how the addition of a second microphone can introduce a whole set of unpredictable problems. Combining two mics together that are picking up the same sound from different distances will always result in some amount of phase cancellation. The 3-to-1 rule does not apply to microphones placed very close together (a coincidence pair), as sound waves will reach both mics simultaneously. The 3-to-1 distance can also be reduced for microphones with cardioid pickup patterns if the two mics are pointed away from each other (near coincidence pair), as this will not expose them to the exact same sounds. Spaced omni directional microphones make wonderful environmental ambience pickups. Luckily, there are other ways around this problem besides the 3-to-1 rule. Using a directional mic and taking advantage of its off-axis rejection characteristics (as per its polar pattern chart) to eliminate pickup from unwanted sound sources is a recording engineer’s mainstay, especially around a drum kit. If you mic the back as well as the front of an open cabinet guitar amp (like a Fender twin reverb) to boost the low-frequency response, the two signals will be close to 180 degrees out of phase with each other. The same goes for dual micing a kick drum, a snare, and even double skinned mounted toms. So hitting the console’s (180-degree) phase reversal switch may rectify the situation. You can locate 180-degree out-of-phase positions on things like Leslie speaker cabinets by summing the two mics to a mono output and moving their positions until the sound just about cancels out. Now hit the console’s phase reversal switch and the signal will jump from nothing to twice the output of the single mic.
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