importance of pin hole pressure equalization and how to make one safely - diyAudio
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Old 14th December 2005, 03:37 PM   #1
mazurek is offline mazurek  United States
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Default importance of pin hole pressure equalization and how to make one safely

I have now two nice sealed subwoofers, sounding good with the aid of this forum. Since I got the boxes made by someone that knows what they are doing, they are completely airtight. Is it that important to make a small hole in them so that they don't act as barometers?

My view was that I don't like intentionally making holes in things, and the temperature in my house should be between 65-75 degrees fahrenheit. My naive calculations say if volume is proportional to temperature, air volume in the box should only vary by 2% over the temperature range. The change in air volume equals 1.5% of the driver compliance, I'm not sure how that translates to cone displacement and loss of stroke, and moving out of the distortion free range.

If you do recommend a pin hole, how should I make one that is small enough? My smallest drill bit is 1/16".
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Old 14th December 2005, 06:36 PM   #2
SY is offline SY  United States
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You probably don't need a pinhole- pressure equalization through slow leaks and diffusion will take care of most normal pressure changes, which occur gradually.

There might be a problem if you air-freighted the unit...
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Old 14th December 2005, 06:40 PM   #3
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Yes, no box is ever airtight, there are always tiny leaks around the gasket, or even through the threads of the driver mounting bolts.
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Old 14th December 2005, 06:42 PM   #4
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Read this page:

http://www.linkwitzlab.com/thor-design.htm

Specifically, he says: "The box is completely sealed except for a small pin hole (<1 mm diameter) to equalize internal with external static pressure and to prevent displacing the cone from its normal resting position."
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Old 14th December 2005, 07:48 PM   #5
simon5 is offline simon5  Canada
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It may sound stupid, but a pinhole could be noisy IMO.
I'm right or wrong on this?

Box was mounted in a single environment. So when built, the air outside had the same pressure as air inside.

Even if shipped with air-freight, after an hour or so, the wood will heat back to the new environment and will heat the inside air at the same time, so the pressure is back to normal both inside and outside of the enclosure.

For the pressure changes induced by barometric pressure, oh well, I think we can discard that one.
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Old 14th December 2005, 08:21 PM   #6
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How do you know they're completely airtight?

Push and hold the driver in for several seconds and release it. If it immediately returns to its original rest position, it means no air was forced out and it's pretty darn good.

If it passes that, it would be interesting to see how long it would take.

I think anything close to 1 mm is *way* too big.

Much smaller leaks can make ticking noises, as I once discovered with leaky screw holes.
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Old 14th December 2005, 08:24 PM   #7
dscline is offline dscline  United States
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I can't say whether or not changes from barometric or temperature would cause enough of a pressure differential to be of concern, nor can I say if even a well sealed cabinet would offset them quickly enough. But IF this is a valid concern, and IF you are concerned about a small pin hole causing noise, I think the answer would be to make the hole small AND long. The resistance of a long tiny hole in a small tube would be enough to cause it to be a non-issue during rapid pressure changes when the sub is in operation, but still allow slow bleeding during constant pressure differentials. You could go to a refrigeration supply house and get a very small capillary tube, which is used to restrict refrigerant flow from the high pressure side to the low pressure side. All this is is a long (probably 18-36") piece of soft copper tubing of very small diameter, and it's typically coiled up just to make it's size more convenient. Get the smallest diameter you can get, like something that would be used in a refrigerator or window a/c, as opposed to something that would be used in a central air conditioner. The inside diameter is probably well under 1/8", and the length of the tube would prevent any measureable flow during sub operation. The outside diameter should be large enough that it will fit fairly snugly in a reasonable size hole you could drill into the cabinet, and just caulk it.

Again, I'm not saying that's necessary, but it's certainly something you could do if you're worried about it. Capillary tube isn't very expensive.
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Old 14th December 2005, 08:31 PM   #8
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Or stuff the small hole with open-celled foam.

This has essentially the same effect as using a long tube, which adds viscous friction.
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Old 14th December 2005, 08:49 PM   #9
Keld is offline Keld  Sweden
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How hot will it get inside the box when playing? and what will that do to the pressure (in a theoretical airtight box)?
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Old 14th December 2005, 08:57 PM   #10
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I think the purpose of the pinhole in Linkwitz' design is to maintain constant internal pressure relative to ambient. As the driver moves in it compresses the air in the enclosure, and as it moves out it it decompresses it. Thus the air acts as a "spring" against the driver. With the pinhole the positive and negative pressure on the driver is eliminated or greatly reduced.

I don't know what this does measurably to the performance of the driver, but it perhaps extends the frequency response at the low end and/or increases its efficiency. These are all Linkwitz' words, and they seem to make pretty good sense.
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