[pieterh]

The principle with the 3-1 rule is for miking up a choir or an orchestra for example. In my job (FOH sound engineer) we often get choirs of Middle Eastern origin where the choir leader will insist on 4-5 mics or more for a choir of around 15-20 people in the (mistaken) belief that more is better.

The trouble is that choir mics need to have a relatively wide pickup pattern in order to take in a group of voices. So one mic picking up the left side of the choir will also pick up the centre and right though slightly delayed; similarly a mic picking up the right will also pick up centre and left slightly delayed. Phase differences between these mics will result in a degree of comb filtering where some frequencies are boosted and some are dampened.

The result for our choirs and song groups when using more than two or at the most three front mics is that the sound is worse, not better. Having a couple of mics placed and directed towards the singers at the back instead is one way of compensating for this.

Of course, close miking where there are several more directional mics close to each vocal range (tenor, soprano etc) gives good results but one still should be aware of placement.

When it comes to recording guitars it’s a different matter - the frequencies and sounds from different parts of the guitar are already mixed and interacting with each other by the time they reach our ears. By recording with directional mics that pick up the characteristics of the different parts of the guitar as mentioned above should produce a result close to what we hear when listening un-amplified. Because we normally use directional mics the risk of leakage that can affect the recorded sound is relatively minimal. What’s more, as we almost always listen in stereo the risk for phase cancellation/amplification is also minimal. It can be useful though to check recordings in mono to see what comb filtering there is when summed to mono...