Ryan on the Proximity Effect

Ryan on the Proximity Effect


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The “proximity effect” or “bass tip-up” is a common recording phenomena that can dramatically change the sound of a recording and is often overlooked or not realised. So what exactly is proximity effect? Well, it's all in the title. When the distance between a sound source and a directional (pressure gradient) microphone is too small there will be an increase in low-frequency energy. It's important to note that only directional mics tend to be susceptible to proximity effect (omnidirectional mic's aren't really affected). This is because open ports (apertures) are built into the casing of the mic and placed strategically to create its omnidirectional polar pattern. For more information on polar patterns, check out a useful guide here.

These ports provide routes for sound waves to enter the mic and reach the rear of the diaphragm. Air routes from the ports to the rear of the capsule vary in length and design, depending on the desired polar pattern. As it takes longer to reach the rear of the diaphragm compared to the front there are time and pressure differences between air at the front and rear of the diaphragm. It's the pressure difference between these two waves acting on the diaphragm which causes the mechanical motion that allows us to record sound. Holes in a mic casing can channel air through paths that intentionally delay sound waves from reaching the rear capsule and so is capable of cancelling noise in a specific region around the mic. How does this relate back to proximity effect? A low frequency wave hitting the front of a mic diaphragm will be followed by another low frequency wave hitting the rear of the diaphragm shortly afterwards with depleted amplitude. If the source is too close this will result in large amounts of bass energy! Let's have a closer look into this phenomena. First off all sound waves radiate in a spherical manner by inverse square law. What does this mean?
Well, as a sound wave travels further from its source the energy from the original sound expands into the surrounding space. As the space is filled with sound waves the sound energy is spread out. In mathematical terms doubling the distance means the energy at any point is four times less, hence the term inverse square law. Of course the converse is true also and so sound sources get louder the closer we move our mic to them. The position of the microphone to the sound source creates differences to several physical properties: phase, amplitude and pressure in air at the front and rear of a mic. There is a slight phase difference between the wave meeting the front of the diaphragm and the wave meeting the rear of the diaphragm for low frequency waves but a large one for high frequencies. Microphone electronics will account for this as this will cause phase cancellation in high frequency waves. If the sound has travelled a considerable distance from the source, the additional distance from the front to the rear of the diaphragm is negligible and so they two waves have much the same energy. For example if we place our microphone 1000cm from the source and let's say its diaphragm is around 1cm thick, the wave hitting the front of the diaphragm will have to travel 1000cm, whereas, the wave hitting the rear will travel at least 1001cm. The inverse square law means the difference in energy between the two waves is minimal in comparison to a mic placed closer to the source and this we'll discuss this more a little further on. This difference applies to both low frequencies and high frequencies. So why do we hear an effect at the low frequencies? The reason that the lows are boosted whilst high frequencies emerge from close miking relatively unscathed is because of relative distance and size of the diaphragm. The air striking the front of the diaphragm from a close source has much more energy because of the inverse square law compared to the air striking the back.
If we have our mic placed 5cm away from a source, that extra 1cm of diaphragm means that the rear waves have to travel at least 6cm and loose much more energy proportionally when compared to a mic placed 1000cm away with waves travelling to the back at 1001cm, However the higher frequencies which have small wave lengths suffer phase cancellation at these distances. By comparison lower frequencies with large wave lengths and minimal phase difference suffer no such cancellation .The result is a far more bassier response and the proximity effect is heard! Generally speaking, the proximity effect is undesired but from a more creative standpoint it can be used in a variety of different ways to change the recorded timbre of your sound source. To give you a quick example, you might choose to creatively use proximity effect to expand and add weight to bright or airy vocals. There are limitless ways to use this effect to your advantage but, for the most part, unless you're very sensitive about how you use it, it won't benefit your recordings. Here are four tips to remember that should keep your recordings free from unwanted low frequencies:

1. Four fingers & a thumb from the focal point: A good rule of thumb for close mic recording is to distance the microphone approximately an outstretched hand's length away from the source. This is just a rough measurement but one that usually lends itself well to most close miking situations.

2. Use your eyes AND your ears: A fairly obvious point but one that often gets ignored when the pressure's on! Make sure you listen well to the signal picked up from the mic position, you may have measured it at a fair distance from the source but it still may be too close. Try to make sure your recording has natural clarity and that there are no overpowering frequency regions.
3. Roll-off: Luckily, a lot of mics are manufactured with in-built high-pass filters that can roll-off the lower frequencies with different degrees of attenuation. This really helps in live situations where time and capacity to repair the EQ is limited but also for recordings too. Many instruments don't produce low frequencies so recording that “empty space” of bass would be pointless and could be hiding environmental noises (computer fan, air conditioning unit etc) which may colour your recording. The high-pass filter will cut-off frequencies below a threshold. i.e. most mic's with roll-off will have filter around 100Hz which, when recording, will ignore all frequencies below 100Hz and pass all those above. This is great for taking out any sub-bass which may be lurking around the lower frequencies, amongst other things.

4. The power of plug-ins: One of my favourite features about working with DAWs are the libraries of weird and wonderful plug-ins you interact with to shape your mixes. With regards to proximity effect, a simple EQ should be able to fix or at least help the problem. Essentially, using a high-pass filter as discussed in the point above will remove any low frequencies up to a certain point. That's great for any instruments not using frequencies below that mark but for those that do, this method can be quite destructive. Instead, frequencies can be attenuated and not completely removed. Another plug-in that could be used to battle the proximity effect is a multi-band dynamics processor. This handy plug-in can compress frequency regions if they exceed a threshold, perfect for trying to manage sounds with varying dynamics.

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