Audio production has long included choir miking and that’s where the problem started. Miking a choir requires proper microphone placement and the 3-to-1 rule is often referenced. It’s also where the myth was birthed.
The myth is this:
The 3:1 microphone placement ratio extends to ALL microphones placed on the stage to ELIMINATE one sound from being detected in multiple microphones.
The 3-to-1 rule is for specific situations and choirs are number one on the list. Examine why it’s true:
- Vocal frequencies are being projected in many directions. This can be from turning of the head to the shape of the choir.
- All vocals are equally loud – not exactly, but with the number of vocalists on stage, many sing at the same volume.
- Choir microphones likely use cardioid pickup patterns, so they detect frequencies on the sides as well as in the front. Not a lot, but enough.
- The choir microphone channels have the same channel gain level.
Combine these factors and choir miking is the time for the 3-to-1 rule because each microphone should pick up a group of people within the choir. The choir microphones should not be so close as if it was two microphones on the same source from slightly different distances. If so, there would be problems with sounds detected out-of-phase between the two microphones. Combining the same sound from two sources but at different points on the sound wave negatively impacts the new combined sound.
Practical miking
Working with instruments and solo vocals is quite different:
- They produce directional sound, low-end frequencies aside.
- They produce different signal levels – thus have different gain settings.
- They are typically close-miked.
There might be a little leakage where the cymbals can be heard in a vocal microphone but due to the significantly lower volume level of the signal in that vocal microphone, the combining of cymbal signals has no negative consequences.
Perfect sound isolation can’t be achieved in the live environment and that’s ok. There are a few steps that will get you close.
Steps for near-isolation miking
Focus on close miking. For example, a guitar amp is miked from an inch away from the amp, a violin has a mic on the pickup instead of a condenser two feet away, and vocalists have the mic up to their lips. All of this being in place, spill over can still occur.
Consider the following:
- Don’t assume all spill is bad. Just because the drums are heard in the vocal microphone when the channel is soloed AND they aren’t singing AND you’re wearing headphones, it doesn’t mean the spill negatively affects the mix.
- Eliminate spill through proper gain structure. The biggest mistake seen here is when wireless microphones haven’t had the gain set in the wireless pack (or handheld mic) and therefore the audio console has the gain cranked all the way up just to get a little signal.
- Use gating. Drums and bass amps will flood the stage with sound. There is no reason for a vocal microphone to broadcast any sound when the vocalist isn’t singing. By using gating on the vocal channel, the signal is not passed when it falls below a certain dB level.
- Cut frequencies. Cut low end from vocal microphones so drums and bass frequencies are minimized. Cymbals can easily be heard in all stage microphones. Cut those highs in other channels if possible.
- Watch for reflective surfaces. Whether it’s floor monitor placement or microphone placement, a reflective surface can put extra sound into the microphones.
Drums
Across the stage, microphones are located far enough away so phase and comb filtering problems shouldn’t occur. The big exception here is the drums. Due to the nature of close-miking a drum kit, it’s possible that microphones are located at a distance less than the 3:1 rule.
Take the case of the snare drum and the hi-hat. These are two kit pieces positioned next to each other. Their associated microphones could be next to each other. Looking ONLY at the 3:1 rule, the microphone for the hi-hat would need to be 3x the distance from the snare compared to the snare mic distance.
When it comes to miking drums, things do get complicated. For instance, a mic with a cardioid polar pattern would not pick up any sound behind it. So using the snare and hi-hat scenario, the 3:1 rule shouldn’t matter, right? The hi-hat mic wouldn’t pick up the snare so it shouldn’t matter. WRONG. There are so many reflective surfaces, from the hi-hat itself to the cymbals to the drum cage, that the snare sound will be bounced right into the on-axis area of the hi-hat microphone.
I’m not saying that polar pattern doesn’t matter. When it comes to drums, use every advantage you can get. The drum kit is a place where a kit piece can easily be detected by multiple microphones but through using microphones with the right polar pattern and obeying the 3:1 rule, you can minimize (and eliminate) problems.
A simple trick for checking the drums is switching the polarity of a microphone. Some mikes have this option and some audio consoles have this option on the channel level. Doing so switches the waveform of the sound, so the peaks become troughs. Listen for the polarity option that produces the better tone.
The Take Away
The 3-to-1 rule isn’t about sound isolation, it’s about same-source miking. For near-sound isolation, use gating, proper gain structure, frequency cuts, and improved microphone placement. A little spillover is ok. It’s a natural part of live production.
John Lawton says
November 30, 2014 at 3:52 amOne often misunderstood and little used (at least in ‘terms’ of online explanations such as the above) “3:1 Rule Explanation” is that the 3:1 Rule applies–in theory–to a single sound source being picked up by a single microphone (in free space), i.e. a soloist singing into a stand-mounted microphone. By keeping the distance from the singer’s mouth to the mic equal to 1/3 or less of the total “reflection-distance”–the distance from the singer’s mouth, to the floor, and back up to the mic–comb-filtering is reduced to an acceptable level, on that single source. Thus, (ignoring “the proximity effect”), the closer to the mike one stands, the better.
But, you might legitimately ask, “What about the 3:1 Rule as applied to any one singer in a chorus where 2 or more mikes are used, due to the ‘reflection-distance’? Doesn’t that cause comb-filtering problems for each singer” And, the simple answer is “Yes–in theory and in practice.” In theory (and I haven’t semi-formally studied the theory for 25+ years), anytime you violate the 3:1 Rule, some comb-filtering ensues. But, most people only understand the simple part of the theory and ignore a concept called “The Directivity Factor”. However, I digress–first, one has to understand that when you have multiple sound sources being picked up by multiple microphones properly placed–as close to the 3:1 Rule as practically applicable–the multiple comb-filtering responses all sum to give a more even response for the group of sources in all the mikes. If they didn’t, miking a choir would be practically impossible, for either live sound reinforcement or for recording.
Furthermore (I un-digress), most people who apply–or think that they have properly applied–the 3:1 Rule fail to take into account the Directivity Factor of microphones. Here, the “inverse-principle” applies: just as a mid- to high-frequency horn loudspeaker is much more directional than a low-frequency direct-radiator (think “woofer”), microphones (even so-called “unidirectional” mikes) have varied polar response patterns at different frequencies. In theory, (if memory serves, over 1/4 a century and premature onset of old age–I’m 58), the 3:1 Rule only applies (as mentioned above)–using perfectly omnidirectional mikes) in whole/free-space, not in half-space, not in one-quarter space, nor in one-eighth space (the latter two being the most common approximations to indoor acoustical reality). (So-called) Omnidirectional mikes are rarely used outside a recording studio, rarely used for live sound reinforcement, because of their lack of directivity. Thus, when one attempts–in practice, not in theory–to apply the 3:1 Rule to a live stage, using cardioid or even more directional mikes, the 3:1 distance from mikes can be and often is reduced, without noticeable phase-shift/comb-filtering problems, because of high-pass filtering (if you’re not high-pass filtering your vocal mikes, you’re not doing your job properly) and because of the increased directivity of the mikes with increased frequencies (in general–see your particular mike’s response graphs for specifics).
Finally, being “old-school” (with distinct “new-school” behavioral tendencies, like actually measuring equidistantly placed mikes’ responses using EQed pink-noise through a single speaker, to set levels), I don’t use “issue” when I mean “problem” and neither should you (any or all!). But, being new-school, I do spell “mic” with the phonetically correct “mike” and so should you.
(All suggestions, “so should you”, may be ignored at your peril–time to put this prematurely old body back to bed. [chuckle])
John E. Lawton
Dave Stagl says
November 6, 2014 at 11:41 pmAlan is correct on the reason for the rule, but I don’t think your explanation of the 3:1 rule is clear. The 3:1 rule has nothing to do with the distance between mics. The 3:1 rule states when you have a microphone on a source, any additional mics need to be at least 3x the distance away from the source as the first mic is from the source.
For example, if you have a microphone on the snare that’s 2″ off the drum, an additional mic that picks up that snare such as a hi-hat mic or a tom mic needs to be at least 6″ away from the drum. And if we have a mic that’s 6″ away, a 3rd mic added would need to be at least 18″ away from the drum.
6 dB down is thought in some circles to be the point where comb-filtering doesn’t matter. Remember that the Inverse Square Law says every time we double the distance from a sound source we lose 6 dB of level. Of course, that’s assuming we’re in a perfect, reflection-free environment, but we’re not so 3x is thought to be the minimum distance for our 2nd mic to get the level of the source down at the mic in order to minimize comb-filtering as Alan explained.
If you close-mic instruments on a stage, you shouldn’t be worrying about the 3:1 rule because additional mics getting any bleed are probably going to be far enough away to not matter with the exception of drums. Drums get complicated because we often turn things up and compress things a bunch. For example, an overhead mic is almost always over 3x the distance away from the snare as the snare close-mic. However, when that overhead gets pushed up in level to use in the mix, there can be phase issues.
The 3:1 rule is a good rule-of-thumb for multi-mic’ing things, but it’s not a guarantee you won’t run into phase problems. What’s better is to get educated on phase and how to avoid phase related issues.
Oh, and one last thing. Gating vocals is dangerous if you don’t have the right gates to do it. Expansion is better, but still requires the right tools.
Chris Huff says
November 7, 2014 at 7:21 amDave, thanks for the comments. You’re right, I should have been much clearer in explaining the 3:1 rule. I’ll be updating the post.
I’ve dealt with those overhead issues. Because of the variety of drummers of we have (some play surprisingly soft), we got rid of the overheads and now close-mic every kit piece. Get a much better sound.
Alan Hardiman says
November 6, 2014 at 9:29 pmThe 3:1 rule is there to ensure that the next closest microphone’s output to the intended source will be at least 9 dB lower than the main mic, and therefore comb filtering and phase problems will be avoided. It was never intended as anything other than a minimum. With close miking, the ratio is much higher than 3:1. I can’t remember a single instance where anyone thought the 3:1 ratio was anything other than a minimum, let alone a recommended practice for all situations. But I guess some folks will believe the strangest things!
Chris Huff says
November 7, 2014 at 7:24 amAlan, comb filtering and phase problems…”but they told me audio production would be easy!” LOL.