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 says when a microphone is on a sound source, any additional microphones that pick up that same source should be at least three times the distance away from the sound source. This distance eliminates problems related to phase and comb filtering, due to the significant drop in volume. It’s not about isolation.
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.
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.
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 three times 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.