The microphone proximity effect is sometimes used to great effect and other times it’s an unwanted side effect of mic placement. It can accentuate the deep voice of an announcer; the low thud of a kick drum; the boominess of a bass amplifier. It can also cause muddiness and an overrepresentation of low-end frequencies in a mic signal that doesn’t necessarily need it.
What is microphone proximity effect? Microphone proximity effect is the increase in bass frequency responsiveness in a directional microphone as the microphone moves closer to a sound source. The proximity effect happens due to a combination of phase and amplitude differences of sound waves between both sides of the diaphragm.
What does this mean and how do we exploit the microphone proximity effect to our advantage as performers and audio technicians? These questions will be answered in this article!
Table Of Contents
- What Is Microphone Proximity Effect And What Causes It?
- Phase Difference.
- Amplitude Difference.
- Recapping How The Proximity Effect Works.
- The Proximity Effect Of Various Microphone Polar Patterns.
- Practical Uses Of The Proximity Effect.
- Related Questions.
What Is Microphone Proximity Effect And What Causes It?
The proximity effect is the increase in a microphone’s low-end frequency response as the microphone gets closer to a given sound source. The closer the sound source is to the microphone, the greater the bass boost!
Because this effect is predicated on the varying pressure differences between both the front and back of the microphone diaphragm, the effect only takes place in directional microphones.
The rear sides of omnidirectional microphone diaphragms are closed to external sound pressure and sit within small fixed-pressure chambers. They do not exhibit proximity effect.
In other terms, the proximity effect affects pressure-gradient microphones and does not affect pressure microphones.
- Pressure-gradient microphones have both sides of their diaphragms open to external sound pressure and are inherently directional.
- Pressure microphones have only the front side of their diaphragms open to external sound pressure and are inherently omnidirectional.
All microphone diaphragms move according to the pressure difference between their front and back sides.
Directional Microphones And The Proximity Effect
So both sides of a directional microphone’s diaphragm are exposed to external sound waves. The simplest visual example of this is the bidirectional ribbon microphone.
The Bidirectional Ribbon Microphone
A ribbon mic, by default, has a bi-directional pickup pattern (figure-8). It’s most sensitive to sounds directly in front and directly to the back of its ribbon diaphragm.
The figure-8 pattern also means there is a “ring” of silence around the sides of the diaphragm where the microphone does not pick up any sound. This is shown at 90° and 270° in the 2D polar pattern graph (pictured).
Understanding this simple polar pattern will allow us to better understand the proximity effect.
The directionality has to do with the time it takes for sound to travel from one side of the diaphragm to the other. Let’s refer to the difference in time between a sound wave hitting the front and back of a diaphragm as the phase difference.
It’s critical to note that the directionality also has to do with the amplitude difference of a sound wave at the front and back of the diaphragm. The amplitude of the sound wave decreases as it travels through the air. We’ll call this the amplitude difference.
The proximity effect is caused by the increased amplitude difference compared to the phase difference of low frequencies at close distances.
The path of sound travelling on-axis from the front of the figure-8 ribbon microphone:
- The sound wave hits the front of the diaphragm.
- The sound wave travels a distance D around the diaphragm housing (losing strength/amplitude) to the back of the diaphragm.
- The sound wave hits the back of the diaphragm with less amplitude after travelling distance D in a total time T.
So any given sound wave on-axis in the front of the mic hits the front and the back of the diaphragm out-of-phase. This phase difference causes contrasting sound pressure levels between the two sides of the diaphragm at any moment in time, causing the diaphragm to move. This is how the microphone works!
Similarly, the path of sound travelling on-axis from the back of the figure-8 ribbon microphone:
- The sound wave hits the back of the diaphragm.
- The sound wave travels a distance D around the diaphragm housing (losing strength/amplitude) to the front of the diaphragm.
- The sound wave hits the front of the diaphragm with less amplitude after travelling distance D in a total time T.
Conversely, sound travelling from the direct top, bottom, sides, or anywhere around the “ring of silence” has the following path:
- The sound wave hits the side of the diaphragm housing.
- The sound wave travels a distance ½D around the housing (losing strength/amplitude) to reach both the front and the back of the diaphragm.
- The sound wave hits the front and back of the diaphragm simultaneously after travelling distance ½D in a total time ½T. It does so with equal amplitude (though less amplitude than when it first hit the side of the diaphragm housing).
Since the sound arrives at the same time, it’s perfectly in-phase and applies the same amount of pressure to both sides of the diaphragm. This keeps the diaphragm in equilibrium and it doesn’t move. Therefore the microphone doesn’t output a signal in its ring of silence.
The figure-8 polar pattern is sensitive to sounds on-axis from the front and the back while it rejects sound from the sides.
The figure-8 pattern also portrays the most proximity effect of all the polar patterns, which we’ll touch on later.
More more details on ribbon microphones, please consider reading my article Dynamic Ribbon Microphones: The In-Depth Guide.
With this primer, let’s continue our discussion on the proximity effect.
To simplify our discussion on the proximity effect, we’ll consider the sound source to be pointed on-axis in front of a bi-directional microphone.
This allows us to focus simply on the main determining factor of the proximity effect: distance of the sound source to the microphone.
The phase difference between the front and back of a pressure-gradient mic diaphragm is the difference in phase of the sound wave that passes by and “hits” both sides of the diaphragm.
The phase of a sound wave refers to the point in the sound wave’s cycle. Phase difference simply means that we’re at a different point in a sound wave’s cycle.
Natural sound waves are complex and it’s difficult to understand where a cycle may start and end.
Try thinking of a sound wave as a combination of single-frequency sine waves across the frequency spectrum that are all added together to create a complex sound.
With that thought, we can better understand that phase difference is frequency-dependent.
Frequency is inversely proportional to wavelength. The higher the frequency, the shorter the wavelength.
f = ν/λ or λ = ν/f
• f = frequency
• λ = wavelength
• ν = velocity of sound (assumed constant)
For a deeper discussion on sound waves and microphone signals, check out my article What Is A Microphone Audio Signal, Electrically Speaking?
The shorter the wavelength, the less distance between the peak and trough of the waveform.
- The peak of a sound wave is where maximum compression happens. In other words, where the sound pressure pushes the hardest.
- The trough of a sound wave is where maximum rarefaction happens. In other words, where the sound pressure pulls the hardest.
The distance D around the diaphragm housing is constant. As the sound frequency increases, the wavelength decreases, offering a greater potential for the phase difference between the front and back of the diaphragm.
- Higher frequencies yield potentially larger phase difference, larger pressure difference, and more output.
- Lower frequencies yield a smaller phase difference, smaller pressure difference, and less output.
As we can imagine, the maximum displacement of the diaphragm would happen when:
- The peak of the sound wave is pushing at one side of the diaphragm.
- The trough of the sound wave is pulling at the other side of the diaphragm.
This max displacement happens when the wavelength of the sound wave is 2D (twice the distance the sound wave must travel to get from the front to the back of the diaphragm).
Considering how small microphone capsules are, this wavelength is short, and therefore, the frequency is high.
For reference, here’s a short table relating frequency and wavelength (speed of sound (ν)= 343 m/s):
- 20,000 Hz = 17 mm
- 10,000 Hz = 34 mm
- 5,000 Hz = 69 mm
- 1,000 Hz = 3.4 cm
- 500 Hz = 6.9 cm
- 100 Hz = 3.43 m
- 50 Hz = 6.86 m
- 20 Hz = 17 m
The formulas, once again are: f = ν/λ and λ = ν/f
Recall our distance D in which a sound wave must travel between the front of the mic diaphragm and the back of the diaphragm.
The phase difference slowly diminishes as frequencies increase above a wavelength of 2D. At a wavelength of 2D, the peak of the wave will hit one side of the diaphragm while the trough will hit the other side.
Phase difference becomes less and less of a factor as the frequency moves toward a wavelength of D. A wavelength of D means the peak would happen at both the front and back and the diaphragm. In theory, the diaphragm wouldn’t move.
The phase difference gets increasingly complicated yet negligible beyond this point.
To learn more about microphone phase, check out my article Microphone Polarity & Phase: How They Affect Mic Signals.
A Note On Damping The Diaphragm
Generally speaking, a higher frequency means a larger phase difference, pressure difference, and ultimately more output from the microphone.
In fact, the natural sensitivity due to phase difference rises by 6 dB per octave! However, a frequency response like that would not sound good at all.
To compensate for this, the diaphragm is damped to decrease its frequency response by 6 dB per octave. The damping allows for a flatter frequency response, meaning the bass frequencies are better represented and the high frequencies are not over-represented.
For a detailed discussion of microphone frequency response, please check out my article Complete Guide To Microphone Frequency Response (With Mic Examples).
There is also an amplitude difference in sound waves between the front and back of the diaphragm. Unlike the phase difference, the amplitude difference is not frequency-dependent.
A variation in sound wave amplitude means a contrast in sound pressure. As discussed, having different pressure levels at the front and back of the diaphragm causes it to move!
But how does a sound wave’s amplitude change as it moves through the air? A simple answer is found in the inverse square law.
The Inverse-Square Law
What is the inverse-square law? The inverse square law states that sound pressure level decreases by 6 dB for every doubling of distance from the sound source. A 6 dB drop means a quartering of the sound level intensity.
Let’s recall the distance D that sound must travel to get from the front of the diaphragm to the back of the diaphragm. This distance is typically very short in directional microphones.
Let’s say the sound source is far from the microphone. We’ll say that D is infinitesimal relative to the distance between the sound source and the front of the diaphragm. Therefore, the amplitude difference is negligible and the pressure difference between the front and back of the diaphragm is solely dependent on the phase relationship.
What happens as the sound source gets closer to the microphone?
The amplitude difference becomes a greater factor in the diaphragm movement and the proximity effect starts taking place.
For simplicity’s sake, let’s say the sound source is very close to the front of the diaphragm at a distance D (the same distance the sound wave must travel between the front and back of the diaphragm).
- Sound source to the front of the diaphragm equals distance D.
- Sound source to the back of the diaphragm equals distance 2D.
According to the inverse square law, there would be a 6 dB difference in sound pressure level between the front and back of the diaphragm. That’s a serious difference!
Remember how diaphragms are damped at 6 dB per octave in order to deal with the frequency-specific phase relationship? Well, since the amplitude relationship is not frequency-specific, this damping actually leads to a disproportionate increase in bass frequencies.
This “disproportionate increase in bass frequencies” is a major factor of the proximity effect.
Recapping How The Proximity Effect Works
The proximity effect is only present in directional microphones. Diaphragms of directional mics are exposed to sound pressure on both sides (front and back).
SPL differences between sides cause the diaphragm to move and are caused mainly by phase and amplitude differences.
The sound wave phase difference is frequency-dependent (sensitivity increase of 6 dB per octave). Diaphragms are damped to “flatten” the microphone frequency response (sensitivity decrease of 6 dB per octave).
Sound wave amplitude difference is not frequency-dependent. Sound wave amplitude is quartered for every doubling of distance it travels (inverse square law).
As the distance between the sound source and the microphone diaphragm gets smaller, the distance between the front and back of the diaphragm gets relatively larger. Therefore the amplitude difference becomes greater.
As the sound source gets closer to the microphone, the SPL difference that causes the diaphragm to moves becomes more dependent on amplitude difference and less dependent on the phase difference.
So the microphone is damped, and amplitude difference is not frequency-dependent. This means the bass frequencies are boosted much more than the high frequencies as the sound source is moved closer to the microphone.
This proximity-dependent bass boost is known as the proximity effect!
A Real-World Example
The featured image of this post shows the Shure Beta 57A frequency response graph with proximity effect variations. Here it is once again:
The Shure Beta 57A is a directional microphone with a supercardioid polar pattern. Because it works on the pressure-gradient acoustic principle (both sides of its diaphragm are open to external sound pressure), it exhibits the proximity effect.
As we can see above in the Beta 57A frequency response graph, the closer the microphone is to a sound source, the greater the accentuation of the bass frequencies.
Notice that the proximity effect boosts the most around 200 Hz. This is due to the physical build of the microphone, its natural frequency response, and resonant frequencies.
The Proximity Effect Of Various Microphone Polar Patterns
The proximity effect only applies to directional (pressure-gradient) microphones. This is because both sides of their diaphragms are exposed to sound pressure.
However, some directional microphones exhibit more proximity effect than others. Let’s discuss some common polar patterns and their proximity effects.
A general way of thinking about directional patterns and proximity effect is by the extremes:
A pressure microphone is made of one omnidirectional capsule and exhibits no proximity effect. Conversely, the truest form of a pressure gradient microphone has a bi-directional polar pattern and exhibits the most proximity effect.
Many polar patterns are thought of as a combination of pressure and pressure-gradient. Thinking in terms of “percent pressure” versus “percent pressure-gradient” is a solid starting point for determining which microphones have more proximity effect than others.
Bidirectional Polar Pattern Proximity Effect
Bidirectional (figure-8) polar patterns exhibit the most proximity effect.
For a detailed read on the bidirectional microphone polar pattern, check out my article What Is A Bidirectional/Figure-8 Microphone? (With Mic Examples).
Hypercardioid Polar Pattern Proximity Effect
Hypercardioid polar patterns are a 3:1 ratio between figure-8 and omnidirectional. They are quite prone to the proximity effect but less so than bi-directional microphones.
For an in-depth description of the hypercardioid microphone polar pattern, check out my article What Is A Hypercardioid Microphone? (Polar Pattern + Mic Examples).
Supercardioid Polar Pattern Proximity Effect
Supercardioid polar patterns are a 5:3 ratio between figure-8 and omnidirectional. They exhibit strong proximity effect, but less than their hyper-cardioid counterparts.
For more information on the supercardioid microphone polar pattern, check out my article What Is A Supercardioid Microphone? (Polar Pattern + Mic Examples).
Cardioid Polar Pattern Proximity Effect
Cardioid polar patterns are a 1:1 ratio between figure-8 and omnidirectional. Cardioid mics are still very sensitive to the proximity effect, but not as much as the super and hyper-cardioid variants.
For a full explanation of the cardioid microphone polar pattern, check out my article What Is A Cardioid Microphone? (Polar Pattern + Mic Examples).
Omnidirectional Polar Pattern Proximity Effect
Omnidirectional polar patterns do not exhibit proximity effect.
To read a detailed post on the omnidirectional microphone polar pattern, check out my article What Is An Omnidirectional Microphone? (Polar Pattern + Mic Examples).
Practical Uses Of The Proximity Effect
The most common use of the proximity effect is on voice. The proximity effect helps add gravitas and strength to the voice. This can be heard on many radio shows.
The proximity effect is also used effectively on drums, bass cabinets, and other bass instruments to boost an already strong bottom end.
Proximity effect will also come in handy when miking instruments in a thin mix.
For example, say we have an acoustic guitar and voice. There’s not a whole lot of low-end there. By close-miking the guitar and voice, low-end can be added to an otherwise thin mix. Caution should be taken and always try to do what sounds best!
The cons of the proximity effect appear when too many elements are close-miked. This will cause a muddy low-end in the mix, requiring post-equalization.
A popular philosophy states that it is best to capture the ideal sound straight into the microphone and do minimal treatment to the audio signal. However, distant-miking is not always practical in loud environments if we want separation in the mix. Once again, use your best judgment!
What causes microphone plosives and “p-popping?” Microphone plosives are caused by gusts of air hitting the diaphragm of a directional mic. Gusts create “greater than expected” pressure differences between the front and back of the diaphragm. The diaphragm overloads and touches the backplate or magnetic housing, causing a “mechanical clip.”
To learn more about plosives and how to reduce their effects on your microphones, check out my article Top 10 Tips For Eliminating Microphone Pops And Plosives.
Do multi-pattern microphones set in omnidirectional mode exhibit proximity effect? True omnidirectional microphones do not exhibit proximity effect. However, multi-pattern mics commonly utilize multiple cardioid diaphragms in different configurations to achieve various polar patterns. Therefore multi-pattern mics set to omnidirectional mode could exhibit proximity effect!