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Proximity effect
Old 26th July 2005
  #1
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Proximity effect

Hi there, doing a microphone essay and need a bit of help. I understand what the proximity effect is and how it is caused, but can someone tell me what technically causes it? i.e. what happens to the acoustics as we get closer to the microphone and why does the mic emphasize the lower frequencies?

Thanks
Old 26th July 2005
  #2
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Check this out from Klaus Heyne's forum at PSW:

http://recforums.prosoundweb.com/ind...f73862ebe88164

Other than that, you may want to sift through some patent databases regarding technical descriptions of this..

Jakob E.
Old 26th July 2005
  #3
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Proximity effect has to do with the pressure gradient (velocity, or rate of pressure change) changing at a different rate vs distance than the absolute pressure itself.

Omni mics measure pressure, whereas figure-8 mics measure velocity. All other patterns are some combination of figure-8 and omni. Basically, as you switch patterns from omni towards figure 8 you increase the reliance on the velocity measurement, which brings in more and more proximity effect. In order of increasing proximity effect you get:
1. Omni...no proximity effect,
2. Cardioids...moderate proximity effect
3. Figure-8...pronounced proximity effect

Hope this helps you get started

Cheers,

Kris
Old 26th July 2005
  #4
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yeh thats cool,

Thanks people
Old 29th July 2005
  #5
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It has mostly to do with the size of your head. For example, when you design a freestanding loudspeaker you have to include a -6dB high shelf filter in order to compensate for the "baffle step". Since the high frequency wavelengths are small compared to the width of the speaker cabinet, they are forced to radiate only in the forward hemisphere. Lower frequencies with longer wavelengths can wrap around the cabinet and radiate omni-directionally. Half the energy escapes towards the rear causing a -6dB low shelf response. In a speaker the -6dB high shelf filter flattens out the on axis response. Our heads (mouth cabinets) don't have built in shelf filters, so our voices naturally have a -6dB step in the low end response.

Now, due to principles of physics which I won't go into, this baffle step effect only occurs in the far field (microphone distance much larger than head width). In the mid field (mic distance similar to head width) and the near field (mic distance much smaller than the head width) the response flattens out. Thus we get a rising bass response as the mic gets closer.

The mouth and throat also act as a horn. Similarly a horn's bass response rises as you get closer to the near field, enhancing the proximity effect even further. Mic patterns can also enhance the effect, but all pressure gradient microphones will sense a proximity effect. I'm not certain about ribbons since they are velocity transducers.

Thomas

**Note: Far, Mid and Near field have specific meanings in physics relating the distance of the receiver to the size of the source (much greater than, approximately equally to, much smaller than). This is not the same as the loose terms used in studio monitoring where most every case represents the far field.
Old 29th July 2005
  #6


The Physics that you didn't go into (2π space and all that):

Is proximity effect more related to the source, or the receiver? I've never seen or heard a quick, intuitive explanation - the kind where you can get a feel and a couple of "rules of thumb".

I'm interested, because I want to build a kick drum transducer with a 6-8inch woofer. I have no idea how to aproach tuning the thing (distance around an open back cabinet, a series of large ports, sealed enclosure, etc).

A breif decription or any pointers to other material would be apreciated.




-tINY

Old 29th July 2005
  #7
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It's related to both. In the near and mid fields a large receiver will affect the acoustic loading of the source and vice versa. Typically a microphone is much smaller than the source, so the source dimensions primarily determine the proximity response.

If you mount a 6" or 8" speaker an inch or so away from the drum head you probably won't need any cabinet at all. If you do want a cabinet, I would suggest a sealed box on the order of 0.5 to 1 cubic foot in volume loosely stuffed with fiberglass insulation.
Old 29th July 2005
  #8


So, the Vas of the driver won't matter much for a "macrophone" application if I use a sealed box?



-tINY

Old 30th July 2005
  #9
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Just thought I would mention that off-axis coloration and proximity effect are directly related. I think that alone explains a lot.
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