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 video, 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.
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.
Only directional microphones are susceptible to the proximity effect and later in the video
you'll see why. Right here, I've got a Shure SM58. Now, this is a cardioid microphone meaning that it
is susceptible to the proximity effect. You'll notice that as I bring the microphone closer,
although the level changes overall, it also increases the low-frequency response. So
my voice sounds deeper when the microphone is closer to me and it sounds a little bit
thinner when the microphone is further away. Right here, I've got a Shure KSM141. Now,
this microphone has a variable polar pattern which means that it can switch between omnidirectional
and cardioid. I'm going to attempt to smooth out the level differences in post-production so that
we can focus on the frequency response changes instead of focusing on the level differences.
Currently it's in cardioid mode, but let's switch it to omni. In omni, about six inches away.
The microphone in omni, about one inch away. This is a test with the microphone in cardioid,
about six inches away. In cardioid, about one inch away. Do you hear the difference?
The closer you get to a directional microphone, the more extreme this effect becomes. As you'll
see later in the video, this can be a good thing or a bad thing depending on the application.
First let's look at how the proximity effect works. What is it about directional microphones
that make them susceptible to the proximity effect? First we need to quickly look at how
directional microphones work. Don't worry - we're not 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 bi-directional microphone capsule has two sides.
The output of the microphone will reflect the difference between those two sides.
If there's positive pressure at the front of the capsule, it will result in a positive voltage.
If there's positive pressure at the rear of the capsule, it will result in a negative voltage.
However, if 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. If you're unfamiliar with phase
interference, here are the basics. If two identical sound waves interact, they'll sum
together. If two opposite sound waves interact, they'll 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. We can
demonstrate that in a DAW. I've got two copies of a 1 kHz signal and two copies of a 100 Hz signal.
Watch what happens when I delay one copy of each signal just .5 milliseconds. The 1 kHz signal
is completely out of phase with the first copy. Meanwhile, the 100 Hz signals are still mostly in
phase with one another. So the same time delay leads to a more drastic phase shift at higher
frequencies than it does at lower frequencies. You may be asking, "Why is this important?". Well,
let's imagine these two signals approaching the front of a bi-directional microphone.
For the sake of this demonstration, let's say it takes sound 0.5 milliseconds longer
to reach the rear of the capsule than it does to reach the front of the capsule. 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 and this will result in summation.
Remember - the mic outputs the difference between the two sides. The 100 Hz wave will create a
positive pressure on both sides of the capsule because the phase shift is less extreme. This will
result in a significant cancellation. This means that directional microphones tend to capture low
frequencies less efficiently than high frequencies at a normal distance and most microphones are
designed to compensate for this phenomenon 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. The inverse distance law states that every doubling of distance away from the sound
source results in a 6 dB loss in level. At a normal distance from the microphone,
the level of a sound between the front and the 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 6 dB difference between the front and the 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's a
greater phase shift at higher frequencies. The extra distance traveled is so small in comparison
to the total distance that the level difference between the front and rear is very small.
Therefore, the lower frequencies which didn't experience much of a phase shift
would almost completely cancel if it weren't for the damping to compensate.
At a small distance from the microphone, sound waves still wrap around the capsule.
However, the relative amplitude of the sound wave 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. This
will result in a boosted low frequency response at short distances - the proximity effect in action.
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. By placing microphones very close to kick drums and bass amplifiers,
you can also play upon the proximity effect. On the other hand, it can be a big problem
when an inexperienced speaker or singer 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 close
to the guitar might result in a boomy or muddy tone. I recommend placing microphones at least
one to one and a half feet from acoustic guitars just as a starting point. Many recording engineers
prefer to use omni-directional microphones when they'll be placing the microphones a good distance
away from the sound source, such as when recording choirs or classical ensembles in a concert hall.
Omnidirectional mics aren't susceptible to the proximity effect
and the frequency balance will remain somewhat even no matter how far away the sound source is.
If you want to avoid the characteristics of the proximity effect, you've got a few options. The
first option is to use an omnidirectional microphone. Now that's not possible in some
situations because omnidirectional mics don't isolate signal from the surrounding noise
whatsoever. As I said before, bi-directional microphones are the most susceptible to the
proximity effect. Cardioids are a mixture of an omnidirectional capsule and a bi-directional
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.
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