How to record an unaccompanied choir
'A cappella' means 'in the church style', or unaccompanied. Many popular songs have a cappella sections, particularly in the break downs at the end of the track.
But also, there is a surprising amount of choral singing going on all over the world. It is an incredibly popular thing to do, inside and outside of churches. If there isn't a choir in your home town you must live in a musical desert.
Of course, where music is made there is music to be recorded - for pleasure or profit. Any decent choir would love to have recordings of their work that they could just give to the members, or sell at concerts.
It isn't difficult to get a good recording of a good choir in good acoustics. Generally just a couple of well-placed mics will do the job just fine. You'll make the recording, do some playbacks to the choir master, then take the material home for editing.
Any live musical performance will benefit from editing - this applies to even the best musicians in the world. Any classical music CD will have literally hundreds of edits in the original recording.
So typically you might find that the first part of one take was good, while the second part of another take of the same piece was good. If you edited the two good parts together, you would have a perfect performance.
So you perform the edit. Hardly rocket science with a digital audio workstation. You can crossfade if you wish to conceal the join. Now play it back...
Arghh! The edit sounds dreadful! You can clearly hear where the join between the two takes is.
The problem is endemic to a cappella choirs. They start at the correct pitch, but as the piece progresses, the pitch drops. Without an accompaniment to give the singers the correct pitch all the way through, inevitably the performance gets flatter and flatter. It never gets sharper and sharper for reasons science has yet to understand.
If the descent towards flatness always proceeded at the same rate, there wouldn't be a problem. But what will happen is that one take drops in pitch faster than the other. So where you make the join, there is a sudden change in pitch.
A slow change in pitch is inaudible to most people, but a sudden change in pitch stands out in the most unpleasant way possible.
The best solution to this problem is to make the choir master aware of it beforehand, so that he or she knows that each piece will have to be captured in a single take - as many takes as necessary, but the finished recording will be the whole of one take.
A second best solution is to use an accompanying instrument such as a keyboard with an organ-like sound placed behind the mics so that it isn't picked up to any great extent. If the singers can hear it just enough, they will be able to keep their pitch. But the risk is that it may intrude into the recording.
Of course, pitch correction in software is possible, but it often entails a loss of quality. There would have to be a decision on what is the 'least worse' option.
Perhaps in future, choir members will all have in-ear monitors to give them the correct pitch. Until then, making recordings of a cappella singing remains an art.