# 5 Things to Know About Proximity Effect

## Article Content

Proximity effect is something that’s fairly complex in principle but pretty simple in practice.

## 1. How It Works

The basic explanation is this — in a directional mic, a signal is generated through the difference in pressure between the front of the diaphragm and the back of the diaphragm. If the pressure on each side is the same, the diaphragm doesn’t move. When is equal pressure exerted on both sides of the capsule? When the sound is coming in completely off-axis. If the pressure on each side is very different, the diaphragm reacts and moves. When is there maximum pressure difference? When the sound is directly in front or directly behind the capsule. There it is, directional microphones.

But does total pressure on the diaphragm stay the same across the frequency spectrum? Unfortunately, no. There is distance between the front of the diaphragm and the back. It’s not a lot of space, but when we’re talking about sound waves — which move very fast — short distances can still account for a lot! Because lower frequencies move through their period at a slower rate, they exhibit less total change in pressure over distance. High frequencies, on the other hand, change in amplitude much faster and exhibit a larger pressure change over short distances.

That means that higher frequencies create a greater pressure differential on a directional microphone. So to compensate for that, mics are built with circuits that roll off top end.

Here’s the trick: at reasonable distances, most of the pressure on the diaphragm comes from the pressure system inside the capsule. However, when the source is very close to the capsule, the actual SPL from the source becomes a greater influence on the total pressure on the diaphragm. Sound pressure does not behave the same way as the differential pressure in the capsule. It’s simple: louder sound equals greater pressure, regardless of frequency. Once that takes effect, the circuit that rolls off top end ends up over-darkening the source, creating the effect of more bass. Proximity effect.

## 2. Omnidirectional = No Proximity Effect

There’s a little debate on this. A true omnidirectional mic does not rely on the pressure system to reject side signal, and so there’s no need for a compensating high-frequency roll-off. Real omnis do not exhibit proximity effect. Switchable pattern mics effectively use two capsules to create different polar patterns. With both capsules engaged — in theory — the directional pressure in one capsule should directly cancel the directional pressure in the other. This switch to omni should also disengage high-frequency roll-off. If there’s no roll-off there’s no proximity effect, but there seems to be some debate on this.

## 3. Proximity Effect Can Be Useful

When the source sound is relatively stable in positioning, proximity effect acts basically like an EQ. In fact, it’s more or less the result of an EQ. By finding the right mic placement within the close proximity field we can roll-off undesirable top end, or essentially, push up a bit of bass. This is great on thin or overly bright sources.

## 4. Proximity Effect Can Be Detrimental

However, the opposite is true as well. Certain sources can get overly boomy very fast if the mic is placed too close. Acoustic Guitar is really the number one offender here. That soundhole kicks out a lot of low energy and will overwhelm a mic placed too close. If you want to put the mic near the soundhole I recommend using an omni mic — no proximity effect, and you capture more of the pick, string, and bridge tone.

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