Well, as a sound wave travels further from its source the energy from the original sound expands into the surrounding space. As the space is filled with sound waves the sound energy is spread out. In mathematical terms doubling the distance means the energy at any point is four times less, hence the term inverse square law. Of course the converse is true also and so sound sources get louder the closer we move our mic to them. The position of the microphone to the sound source creates differences to several physical properties: phase, amplitude and pressure in air at the front and rear of a mic. There is a slight phase difference between the wave meeting the front of the diaphragm and the wave meeting the rear of the diaphragm for low frequency waves but a large one for high frequencies. Microphone electronics will account for this as this will cause phase cancellation in high frequency waves. If the sound has travelled a considerable distance from the source, the additional distance from the front to the rear of the diaphragm is negligible and so they two waves have much the same energy. For example if we place our microphone 1000cm from the source and let's say its diaphragm is around 1cm thick, the wave hitting the front of the diaphragm will have to travel 1000cm, whereas, the wave hitting the rear will travel at least 1001cm. The inverse square law means the difference in energy between the two waves is minimal in comparison to a mic placed closer to the source and this we'll discuss this more a little further on. This difference applies to both low frequencies and high frequencies. So why do we hear an effect at the low frequencies? The reason that the lows are boosted whilst high frequencies emerge from close miking relatively unscathed is because of relative distance and size of the diaphragm. The air striking the front of the diaphragm from a close source has much more energy because of the inverse square law compared to the air striking the back.
If we have our mic placed 5cm away from a source, that extra 1cm of diaphragm means that the rear waves have to travel at least 6cm and loose much more energy proportionally when compared to a mic placed 1000cm away with waves travelling to the back at 1001cm, However the higher frequencies which have small wave lengths suffer phase cancellation at these distances. By comparison lower frequencies with large wave lengths and minimal phase difference suffer no such cancellation .The result is a far more bassier response and the proximity effect is heard! Generally speaking, the proximity effect is undesired but from a more creative standpoint it can be used in a variety of different ways to change the recorded timbre of your sound source. To give you a quick example, you might choose to creatively use proximity effect to expand and add weight to bright or airy vocals. There are limitless ways to use this effect to your advantage but, for the most part, unless you're very sensitive about how you use it, it won't benefit your recordings. Here are four tips to remember that should keep your recordings free from unwanted low frequencies:
1. Four fingers & a thumb from the focal point: A good rule of thumb for close mic recording is to distance the microphone approximately an outstretched hand's length away from the source. This is just a rough measurement but one that usually lends itself well to most close miking situations.
2. Use your eyes AND your ears: A fairly obvious point but one that often gets ignored when the pressure's on! Make sure you listen well to the signal picked up from the mic position, you may have measured it at a fair distance from the source but it still may be too close. Try to make sure your recording has natural clarity and that there are no overpowering frequency regions.