Hello there! Coming back from the previous Microphone Characteristics article where we discussed about Polar Patterns, we had a few mentions of the Proximity Effect there, and this is what we are going to be discussing in this article. Get set, ready, go!
What is the Proximity Effect then?
The proximity effect occurs under certain circumstances when the audio source is or moves close enough to a microphone, leading to a boost in the low frequencies of the audio signal that is being captured at the time. The closer the source is to the microphone, the more intense the boost gets.
Talking about “certain circumstances” I mean the directionality of the microphone we are using. The polar pattern, the microphone uses, determines if the Proximity Effect is going to be present or not, and how strong it will be.
More specifically, Omnidirectional microphones are not susceptible to the proximity effect while Cardioid and Birectional microphones are, with the bidirectional ones demonstrating almost double the amount of proximity effect when compared to the Cardioids.
This has to do with the transducers that these microphones use, typically distinguished between the pressure and pressure gradient types. Pressure transducers demonstrate no proximity effect at all and are Omnidirectional, while the pressure gradient transducers are bidirectional and will demonstrate the highest possible signs of proximity effect. We’ll discuss about the transducers further down the road, but let’s keep this note in mind for now.
The Proximity Effect can be super helpful on frequencies at or below 200Hz where it is more noticeable. We can use it to our preference to shape the bottom end of the sound we are looking after for kick drums, bass guitars, double basses, or even baritone vocals and acoustic guitars. On the other hand, it needs to be applied with caution as it can make the low end sound less clear than intended and cause more problems than we would wanted.
How is the Proximity Effect caused?
In order to understand how the Proximity Effect is cause, we need to understand first, the basic principle of the Inverse Square Law.
The Inverse Square Law, explains that any specified physical quantity or intensity (in our case soundwaves and their amplitude) is halved every time the traveling distance is doubled.
This means that when we are having an audio source in front of the microphone, when the sounds travels towards the diaphragm, it will have to first hit the front and then the back plate of the microphone, resulting in a delay between hitting the two plates, thus meaning the intensity of the sound will be less when hitting the back plate.
Due to the nature of the soundwaves (which we will discuss on a future article), the low frequencies will exhibit a smaller change in pressure in comparison to the high frequencies of the same sound, that could lead to an increase of up to 6db per octave. This is the reason the pressure gradient microphones have a dampening filter within their circuit that flattens that increase in order to output an as flat as possible frequency response.
However, something very interesting happens when we move the source closer to the diaphragm. The low frequencies, again due to their soundwave nature will exhibit a small pressure change between the front and the back plates. However, the mid and high frequencies, will apply drastically more pressure to the microphone’s plates being closer (and hence louder) and this way the dampening filter, in its attempt to compensate that pressure, will result in doing to such a degree giving the impression the low frequencies are boosted when in reality the higher ones are dampened due to their increased pressure.
I hope my attempt to make this sound as simple as possible did not confuse you! As we get deeper into the microphone characteristics, I hope this will make even more sense to you! Till the next article, take care!
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