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".
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".
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."
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."
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
"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.
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.
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).Keld said:
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.
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!
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!
"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.
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.
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...
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...
Ok, I did some quick googling to find the relationship between temperature and pressure: http://146.139.100.40/webpages/askasci/chem99/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.
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.
noah katz said:
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.
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,
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,
noah katz said:
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.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.