importance of pin hole pressure equalization and how to make one safely

[noah katz]

Good question!

I'm amazed that more boxes don't burst into flames, especially the car guys', what with hundreds/thousands of W supposedly being applied to an airtight, insulated box.

I've made paint drying ovens from leaky cardboard boxes and a 100 W light bulb.

I can only conclude that real average power is at most tens of W.

But back to the question, a too well sealed box could have a problem with driver offset when run hard.

[noah katz]

"With the pinhole the positive and negative pressure on the driver is eliminated or greatly reduced."

If that were the case, you could effectively make an IB w/a finite box, or the bass will be reduced because of the rear pressure wave cancelling the front.

Neither of these is true. I'm certain you've misunderstood Linkwitz.

What a small hole can do is add resistance and lower Qts, and is the principle of so-called aperiodic designs.

I say so-called because if they succeeded in being aperiodic=nonresonant, there would be a severe loss in efficiency.

[dscline]

Originally posted by Keld
How hot will it get inside the box when playing? and what will that do to the pressure (in a theoretical airtight box)?

Theoretically, it could get fairly warm, but to know HOW warm, you'd have to know the rate that heat is being added, and the thermal resistance of the sub walls and driver cone (and any batting or fill that may be added to the walls/inside). If you have a plate amp, the amp itself will add some heat. Essentially, half of the acoustic energy that is produced by the driver ends up inside the cabinet and turned into heat, and any inefficiency of the driver is turned into heat in the voice coil (though aluminum drivers help expell some of that outside the cabinet. So there are a LOT of variables, before you even start taking in to account differences in source material (movies that can have long passages of little bass, or bass heavy music with a constant beat).

But I'd say it'd certainly get warm enough to cause a noticeable pressure differential in a perfectly air tight box. If I take a gallon of milk out of the fridge and set it on the counter, it only takes a few minutes for it to warm up enough that there is noticeable pressure when I open the cap. A large driver would have a lot of surface area to be acted upon. By my calculations, roughly .001 psi on a 12" driver would be the same thing as applying 1 pound of force against it.

[adason]

hi guyes
i don't have any theory to offer, what the small pinhope does, but it is quite normal, that woofer does not have a dust cap for it's normal function, sometimes the dustcap is just leaky cloth or something, sometimes there is a hole in the middle of the dustcap, so there is no problem with pressure difference in and out the enclosure, even it is fully sealed enclosure

mazurek, just punch a small hole in the middle of the dustcap, that's it!

[noah katz]

"Essentially, half of the acoustic energy that is produced by the driver ends up inside the cabinet and turned into heat, and any inefficiency of the driver is turned into heat in the voice coil "

Given that sealed systems are typically about 1% efficient at best, the acoustic energy is negligible.

The rest of your comments are on the mark.

[mazurek]

Thanks everyone, this seems to be a debateable issue. The capillary tube sounds like a good way to make a really small hole. My woofer does pass the push in test, it always restores position real quick, unless I loosen a bolt.

I'm going to have to reread the responses and think a while, it is certainly hard to tell if a potential problem could potentially affect me.

Woah, more posts while writing this, lots of info to consider...

[dscline]

Ok, I did some quick googling to find the relationship between temperature and pressure: http://146.139.100.40/webpages/askas.../chem99406.htm

Based on absolutely zero hard data, I don't think it's unreasonable to think that the interior of a sub could EASILY reach 10 degrees higher than ambient. Assuming a 70° F room, that 10° difference equates to 0.2835 psi. For a typical 12" driver, that's 25lbs of force. So obviously, it's a real concern IF the box were perfectly air tight. Obviously, no box would be perfectly hermetically sealed. So the real question is, is a well sealed box still leaky enough to counter the rate of temperature change we could expect to see in actual operation.

[sdclc126]

Originally posted by noah katz
"With the pinhole the positive and negative pressure on the driver is eliminated or greatly reduced."

If that were the case, you could effectively make an IB w/a finite box, or the bass will be reduced because of the rear pressure wave cancelling the front.

Neither of these is true. I'm certain you've misunderstood Linkwitz.

What a small hole can do is add resistance and lower Qts, and is the principle of so-called aperiodic designs.

I say so-called because if they succeeded in being aperiodic=nonresonant, there would be a severe loss in efficiency.

It's quite possible I've misunderstood Linkwitz' reason for using a pinhole, but my main point is that he does use one, and much of the discussion here has been whether to use one or not. If I were to build Linkwitz' Thor I would build it to his exact design, including pinhole.

[poobah]

Wow,

A lot of complexity here... and all of it avoidable.

By all means... all comments regarding the effect that temperature rise, and the ensuing rise in internal STATIC pressure, inside an enclosure could cause adverse effects ARE absolutely correct... I suppose it depends on the driver and exactly how it is built. If you have a paper cone... forget about it... paper has a billion pinholes


OTOH, if you have all poly construction; consider what happens to a bag of Doritos when you drive up to mountains.

Put pinhole (equalizing) in your enclosure, 1/32 -1/16", preferably where you won't hear any tiny hissing it might make. If is does hiss, glue a small piece of thick felt over it (from the inside).

This is about common sense... don't ask an audiophile.

I know this because a friend built a rowboat entirely of wood and exoxy... and he also did a very good job. Under the seats were floatation compartments. He took the boat to the mountains, 9000 ft and literally blew the seats apart. We subsequently calculated that there was about 2000 lbs acting on the seats due to pressure. if you have gooped you drivers etc... a little hole is smart and won't hurt a thing.

Cheers,

[dscline]

Originally posted by noah katz
Given that sealed systems are typically about 1% efficient at best, the acoustic energy is negligible.

However, ALL the energy sent to the sub has to go somewhere. The low efficiency of the speaker at producing sound only makes matters worse. That means that the rest is dissipated as heat, probably mostly through the voice coil. When you consider that 100 watts can heat up a cubic foot of air 10° F in less than two seconds, we can see that it doesn't take a whole lot of power to heat up a small enclosed space like a speaker box.