Some microphones are susceptible to the proximity effect. That means the low-frequency response will be boosted as the microphone gets closer to the sound source.
In this post, I’m going to show you why this happens, how to use it creatively, or how to avoid it.
But if this is our first time meeting, my name is Kyle. Welcome to Audio University!
What Is The Proximity Effect?
The proximity effect occurs when a directional microphone is placed in close proximity to a sound source. It results in an increased low-frequency response in close proximity to a sound source.
Only directional microphones are susceptible to the proximity effect. In a moment, I’ll show you why…
Just know that the closer you get to a directional microphone, the more boosted the low frequencies will seem in comparison to high frequencies. As you’ll see later in the post, this can be a good or bad thing, depending on the application.
First, let’s look at how the proximity effect works…
What Causes The Proximity Effect?
What is it about directional microphones that makes them susceptible to the proximity effect? Why do omnidirectional mics sound consistent regardless of distance to the source?
First, we need to quickly look at how directional microphones work. Don’t worry – we aren’t going to go too in-depth here.
To understand the basic concept, let’s look at the simplest type of directional microphone – a figure-eight mic.
A figure-eight (or bidirectional) microphone capsule has two sides. The output of the microphone will reflect the difference between the two sides.
If there is positive pressure at the front of the capsule, it will create a positive voltage.
If there is positive pressure at the rear of the capsule, it will create a negative voltage.
If a sound approaches the capsule from the side, the equal pressure on each side will cancel out and no voltage will be created.
In reality, a sound approaching the front of the capsule will also reach the rear of the capsule at a slight delay. That presents the possibility of destructive phase interference between the front and rear of the capsule.
If you’re unfamiliar with phase interference, here are the basics:
- If two identical sound waves interact, they will sum together.
- If two opposite sound waves interact, they will cancel each other out.
- If two identical sound waves interact at a slight delay, they will partially cancel, depending on the phase shift between the two signals.
Another important thing to remember is that a given time delay will result in a different phase shift depending on the frequency and wavelength of the signal.
The same time delay leads to a more drastic phase shift at higher frequencies than lower frequencies.
You may be asking, “Why is this important?”.
Let’s imagine these two signals approaching the front of a bidirectional microphone.
For the sake of this demonstration, let’s say that it takes sound .5 milliseconds longer to reach the rear of the capsule than it does to reach the front of the capsule. (Even though it would take far less time in reality)
The 1 kHz wave will create positive pressure when it reaches the front of the capsule and will be 180-degrees out of phase when it reaches the rear of the capsule. This will result in a summation.
Remember – the mic outputs the difference between the two sides.
The 100 Hz wave will create positive pressure on both sides of the capsule because the phase shift is less extreme. This will result in a significant cancellation.
That means directional microphones tend to capture low frequencies less efficiently than high frequencies at a normal distance.
Most directional microphones are designed to compensate for this phenomenon. The capsule is damped either physically or electronically so that low frequencies and high frequencies are captured more evenly.
There’s just one small problem with compensating for the natural frequency imbalance… The amount of damping needed changes depending on the distance between the sound source and the microphone.
You might be familiar with the inverse distance law. The inverse distance law states that every doubling of distance away from the sound source results in a 6dB loss in level.
At a normal distance from the microphone, the level of a sound between the front and rear of the microphone capsule is roughly the same.
However, from right in front of the microphone, the distance from the sound source to the front of the capsule might be close to half the distance of the sound source to the rear of the capsule, which would result in a 6dB difference between the front and rear!
Let’s go back to the example from earlier to tie these principles together…
At a normal distance from the microphone, sound waves will wrap around the capsule, resulting in some cancellation. The cancellation of higher frequencies is less drastic because there is a greater phase shift at higher frequencies.
The extra distance travelled is so small in comparison to the total distance that the level difference between front and rear is very small. Therefore, the low frequencies would be almost completely cancelled if it weren’t for the damping to compensate for this.
At a small distance from the microphone, sound waves still wrap around the capsule.
However, the relative amplitude of the sound waves at the front compared to the rear is much more extreme at this distance. So, we won’t get the same low-frequency cancellation that we normally would.
But the microphone design is still compensating by boosting the lows in comparison to the highs, which will result in a boosted low-frequency response at short distances – the proximity effect in action.
Is The Proximity Effect Good Or Bad?
So, is the proximity effect a good thing or a bad thing? Like many things in audio, it depends on the situation.
Radio DJs and voice over artists have used the proximity effect to their advantage for decades. Getting close to the microphone allows them to achieve a larger-than-life, “announcer voice” tone.
By placing microphones very close to kick drums or bass amplifiers, you can also play upon the proximity effect. You’ll get that deep, round tone that has become an essential element of many genres.
On the other hand, it can be a big problem when an inexperienced singer or speaker is constantly changing their microphone technique. Holding the microphone close, they sound boomy – holding the microphone far away, they sound thin.
Another instance where the proximity effect is often a problem is when recording acoustic guitar. It might seem counterintuitive, but placing the microphone too close to the guitar might result in a boomy or muddy tone. I recommend placing microphones at least 1 to 1 and ½ feet from an acoustic guitar as a starting point.
Many recording engineers prefer to use omnidirectional microphones when they will be placing the microphones a good distance away from the sound source, such as when recording choirs or classical ensembles in a concert hall.
Omni microphones aren’t susceptible to the proximity effect and the frequency balance will remain somewhat even no matter how far away the sound source is.
How To Reduce The Proximity Effect
If you want to avoid the proximity effect, you’ve got a few options.
The simplest option is to use an omnidirectional mic. That’s not possible in some situations though, because omnidirectional mics don’t isolate the signal from surrounding noise whatsoever.
As I said before, bidirectional microphones are the most susceptible to the proximity effect. Cardioids are a mixture of an omnidirectional capsule and a bidirectional capsule, so the proximity effect is less extreme when using a cardioid mic.
If you are using a directional microphone, keep distance from the source in mind when finding the right placement.
Also, try to keep a consistent distance between the sound source and the microphone. This will allow you to use EQ to shape the sound how you want it.