Using two microphones can create a stereo effect that closely copies the effect of our human ears. There are several ways to do this, in terms of ‘stereo set ups’ and can often be set up in mixer controls too, like Mid Side as an example. This is explained in more detail below:
If you imagine that you are listening to a sound and you turn your head, you will notice the sound spatially changing from one side to the other. Our two ears allow us to perceive where sound is coming from and also hear in much more detail compared to one straight mono, single source sound.
Coincident or XY Mode
This means that microphones spaced a way apart can actually have what is known as a cancellation of phase. It’s like as the audio takes longer to reach a farther away mic, the sound waves can be slightly out of phase with each other. In a severe example, this will cause the audio of two sources to cancel out of it was 180 degrees out of phase, but in practice, the difference of phase will be much smaller to a point it is nearly negligible.
Mid side Mode
Mid side means that two microphones are used, as in a stereo set up, but where one mic is primarily the centre section (a Uni Directional Mic), with the other panned to be the sides of the signal by using a Bi Directional mic.
As a result of this, the higher the amplitude or volume of the sides, the more stereo the effect and vice versa. However, as mentioned in the above XY Coincident description, it is important to realise that if the sound is played on a mono reproducer, the effect could be cancelled phases, so care needs to be taken when using thid technique.
Near Coincident Mode
The near coincident stereo microphone technique is one that was designed utilise the advantages of the spaced pair and X/Y techniques, without the disadvantages. Another name for the same technique is ORTF, an abbreviation for Office de Radiodiffusion Télévision Française, who devised the technique.
The idea can be simplified to be as close as possible to our natural ears position, so that the closest sound pickup can be alligned to the natural human hearing position. This technique is used quite often in certain recording situations.
Spaced Pair Mode
When it comes to spaced pair mode which is also known as A/B mode technique, this concept consists of two separate microphones, that are typically using cardioid or omni-directional polar patterns, spaced apart to capture a wider stereo image of an instrument or musical ensemble.
This audio mic technique is susceptible to audio phase issues, that are due to the time differences of the audio sound waves arriving at each mic. To overcome this you can use the rule of 3:1. It is recommended to place the second mic three times as far away from the first microphone, as the first mic is from the sound source.