Here are some techniques that I use for synchronizing backup and lead vocals.
These techniques only work in sections of a song where background vocals are accompanying a lead vocal in unison or harmony. In other words, one of the vocal tracks has to be clearly the lead vocal, and the others have to be clearly background vocals. These approaches won’t work for a duet, where both of the vocals are sharing the lead. They all also won’t work for a choir, where none of the vocals is the lead. And, lastly, the vocals have to be some sort of parallel harmony; these techniques won’t work for contrapuntal parts.
Imagine that the lyric is “happy birthday to you,” and imagine that you have a single background vocal track singing a harmony with the lead vocal track. Solo just the vocal tracks and listen carefully. Chances are, the “hard” consonants in the two vocal tracks don’t line up perfectly. In this example, the consonants that potentially cause problems are p, b, d, and t: “haPPy BirthDay To you.” Linguists called these “plosives.” If you listen carefully you may here something like this: “haPPPy BBirthDDay TTo you.” The background vocal may be a little behind or a little ahead of the lead vocal, but in either case you’ll hear the plosives doubled because the lead and background vocals are out of sync.
Here’s my first technique. Just remove the plosives from the background vocal! In whatever digital audio editor you’re using, you probably have a tool for drawing volume curves. Find the offending plosives in the background vocal and draw a steep notch in the volume curve around each of those plosives. Now if you were to listen to the background vocal by itself you’d hear something like “ha-y -irth-ay -o you.” Of course that would be unacceptable for the lead vocal, but the background vocal, since it’s a harmony part, will probably be lower in the mix, and if there are enough other instruments playing at the same time (maybe this is a symphonic metal arrange of “Happy Birthday”) no one will notice the missing plosives. That’s the funny thing about the ear: It’s really good at hearing that something is in the wrong place, but it’s not very good at hearing that something’s missing.
That’s not the only way to solve this problem. The second way is to use time stretching (such as Pro Tools’ “elastic audio”) feature to get the plosives to line up by squeezing and stretching the background vocals (without pitch shifting) between the plosives.
Now, you might be asking “what if the vowels also don’t line up?” You can fade out a background vocal vowel at the end of a word to make it the same length as the corresponding vowel in the lead vocal track. You can time stretch (without pitch shifting) a vowel to make it longer. But these two techniques only work if the vowel is at the end of a word.
What if a vowel in the middle of a word is out of sync, or what if non-plosive consonants aren’t lined up? Well, then there’s the third technique to solve the problem: Re-record the background vocals.
That’s the funny thing about all these cool software tools we have. Sometimes we reach for exotic plugins or complicated sequences of editing commands, when really we should just be pressing the record button.