When I record guitar or most other instruments, I will typically employ multiple mics. Most times these mics perform a specific function like a tube and small condenser in an acoustic guitar. The tube captures the body of the instrument while the small condenser captured the detail and clarity.
Some engineers also employ this same technique when recording vocals which allows you to use different mics to highlight key frequencies. A ribbon mic will capture some frequencies way better than a dynamic. Others will record two separate takes and double the performance. And while this is an excellent method for enhancing vocal tracks, in my experience, it can be a recipe for phase.
The primary problem you will encounter with multiple mics or doubled vocal tracks is phase cancellation. When I setup two mics on a guitar cabinet or drum overheads, usually ensuring spacing from the source like the snare is equal for both, any phase that results is usually manageable.
I won’t go into all of the techniques for managing phase on a drum kit, a lot of what it takes is mic placement and phase flipping. But for vocals, I find phase issues are very difficult to overcome. You can try to flip phase, move the waveforms, or any number of other tricks that usually work for instruments, and yet the phase issue will persist.
There isn’t a lot of information out there regarding vocal phase. But I think the primary reason you hear phase either from double-miking or doubling tracks is the result of how the voice works, and more specifically how some vocalists perform.
FREQUENCIES AND VOCALS
I typically see phase issues when a vocalist is particularly strong in certain frequencies or when they are pushing their voice. The phase will generally occur between the two different tracks and on specific notes or phrases.
If the vocalist is really strong in certain frequencies like 3k, you should use mics that don’t boost this region. An SM7B has the ability to boost this zone, and for some singers this can be an awesome setting. I have found, however, a boost in this zone is better for spoken word, not singing.
For a vocal mic, and particularly with a strong vocalist, my preference is to employ mics that are either flat in the 1-3k range or employ a cut.
The R84 by AEA is considered to be one of the most outstanding vocal mics on the market. Interestingly, there are dips at 2k and 3k and a gentle roll-off of highs after that. There is something to be learned here:
If you have a mic that has multiple polar patterns, I have found most mics that have a boost around 1-3k in Cardioid mode will dip this boost in Omni directional mode.
The Rode K2 in Omni mode is pretty flat in this zone:
In Cardioid mode, however it actually boosts after 1k, quite a bit!
The same goes for the Twin87 by United, but with an additional cut at 4-5k to tame harshness.
The Neumann U87, considered to be the ultimate Swiss army mics for most studios and an excellent vocal mic, is dead flat in Cardioid mode, and only boosts between 5-12k which gives a vocal presence and air, and potentially, harshness.
So if you are dealing with a particularly strong vocalist, 1-3k is the exact range we should be targeting for control before we hit the record button. Conversely, for vocalists that do not employ a lot of projection, a boost here will provide clarity and detail you may not be able to easily EQ back into the mix.
BEYOND THE MIC
When tracking doubles or harmonies on a section that appears to be overly strong, have the singer change the voice of their double/harmony either on the specific word or note, or on the entire section.
This may mean lowering their vocal volume - reducing the projection - or having them speak, whisper, or compress their vocal so it does not conflict with the primary track.
Reducing projection is not an act of moving away from the mic either. They need to reduce the amount of air flowing over their vocal cords which in-turn reduces the resonance of specific frequencies.
Whispering or speaking the words of their performance may also provide a nice layer effect. It’s not the same as a real double, but it allows for the engineer to layer-in additional nuances of the recording that are normally not recorded. Amazingly Iggy Pop does this on his recordings.
Increasing vocal compression for doubles or harmonies is also a good way to reduce phase cancellation. A singer with good projection is likely using a lot of compression already so taking a second track that is highly compressed should not be a difficult task.
Vocal compression, just like an audio compressor, it tends to limit the transient of words, it levels-out the performance. So conflicts between the double and the primary track on certain syllables and vowels which tend to be the trouble areas can be reduced. This may take some practice for the performer, but it will definitely improve any phase cancellation in the final product.
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