If a closed box enclosure is perfectly airtight, the system becomes a barometer of sorts. The pressure of the air inside interacts with the atmospheric pressure to influence the mean position of the diaphragm/cone/coil. If the pressure inside is higher (seasonal changes, heating due to coil dissipation, moving to a higher altitude, etc.) the cone will bulge outwards (and vice versa).
Paper cones and MDF or particle board enclosures are 'leaky' enough, but I'm concerned about other materials which might be too airtight. Linkwitz somewhere recommends drilling a pinhole. Someone else recommends drilling a hole plugged with porous material.
What is the consensus here? What is the best way to do this? The 'leak' has to be slow enough not to affect the response, but fast enough to allow ambient pressure variations to even out. How small does a vent have to be to not make a difference to the desired response of a closed box system?
Paper cones and MDF or particle board enclosures are 'leaky' enough, but I'm concerned about other materials which might be too airtight. Linkwitz somewhere recommends drilling a pinhole. Someone else recommends drilling a hole plugged with porous material.
What is the consensus here? What is the best way to do this? The 'leak' has to be slow enough not to affect the response, but fast enough to allow ambient pressure variations to even out. How small does a vent have to be to not make a difference to the desired response of a closed box system?
many woofers comes with small hole in the dust cap (for completely different reason, to eliminate pressure behind the dust cap, many have hole all the way back through the magnet), and many have phase plug, which has plenty of space around...
I do not see what you are saying as an issue
I do not see what you are saying as an issue
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It might not be an issue for paper cones and MDF enclosures, but what about exotic materials like Mylar and Titanium, and enclosures made from plastic or nylon?
Air lock is a problem indeed, even with fibrous yet resin-glued materials. With two drivers in the same enclose, it is easy to detect air lock, alternatively drill a small hole fitting a syringe.
Even small hole can cause noise, so it should be located at the back of the box or filled with wool, which reduces speed.
Even small hole can cause noise, so it should be located at the back of the box or filled with wool, which reduces speed.
Any rule-of-thumb advice? Equations? If the volume of the closed box is known, what should be the diameter of the hole? Would several tiny holes work better than one small hole?
This is actually for the back chamber in a horn loaded design. Should I have posted this in a different section of the forum?
This is actually for the back chamber in a horn loaded design. Should I have posted this in a different section of the forum?
This type of enclosure needs a good seal. It is also usually small so the displacement you speak of is less of an issue.
You're correct, but the driver is smaller too, and exceeding the excursion limits is usually catastrophic. Titanium doesn't bump well. Also, the small size allows for use of better construction materials - a hermetic seal is not unachievable.
What I'm looking for the equivalent to a DC blocking capacitor in the output of an amplifier. Something that will neutralise the effects of environmental changes, but will have no effect at the frequencies of interest. I can limit the lowest frequency the driver will be required to handle with a suitable low pass filter, but the excursion is limited, and a positional offset due to environmental effects will eat into this already scarce resource.
If I have a driver that has maximum excursion of, say, +/- 3 mm, a 1 mm offset in either direction reduces the excursion limit on that side to just 2 mm.
The mechanical equivalent to a capacitor in this case would be a displacement limited diaphragm. If it were to displace the same volume as the cone to 1mm, you may not have an effective sealed enclosure anymore.
The seal is more critical with some horns than others. Anyway, inside temperatures don't vary too much. I can't say I've found this to be a problem.
The seal is more critical with some horns than others. Anyway, inside temperatures don't vary too much. I can't say I've found this to be a problem.
To prevent it having a significant influence on the response, treat it as a port and check whether it is tuned much lower (at least a factor of 10) lower than the lower cutoff. Performance is not relevant so using just one hole is fine.
You can drill a small hole and glue a piece of felt over it on the inner side of the cabinet.
Regards
Charles
Regards
Charles
The atmospheric pressure is in general changing so slowly that you need only a very very small hole on the loudspeaker enclosure to make it equalize/balance out any pressure difference between inside of the box and outside.
Perhaps as small hole as only 0,5 to 1,0 mm is enough, I would drill it on the backside of the enclosure at the corner, you could further on cover the whole with a thick felt fabric and on top of that screw a piece of wood plate, the air will seep through the thick felt fabric and make it nearly completely acoustically tight.
Perhaps as small hole as only 0,5 to 1,0 mm is enough, I would drill it on the backside of the enclosure at the corner, you could further on cover the whole with a thick felt fabric and on top of that screw a piece of wood plate, the air will seep through the thick felt fabric and make it nearly completely acoustically tight.
Thanks guys. Lots of good advice here.
On a lighter note, if a REALLY rich guy got into this field, this is what he might use:
Industrial Flow Restrictors | Flow Limiters & Flow Setters - Mott
On a lighter note, if a REALLY rich guy got into this field, this is what he might use:
Industrial Flow Restrictors | Flow Limiters & Flow Setters - Mott
Use a pressure equalisation valve of the kind you get on Pelican brand hard equipment cases or some of their copied cheaper equivalents.. It probably won't be any better than a small hole and probably is just some kind of water-proof hole system thing anyway. But you can at least point to it and say it's there and that's what it does.
Hard drives use some sort of fiberous or woven material in a small plug to accomplish this. I'd think a simple 1/16" drilled hole would do the trick. Or do a ported system!
No need to be a rich guy inventing the wheel again, in the pneumatics world you find widley available so called Sintered metal filters and mufflers for small money[FONT=Arial, Helvetica, sans-serif].[/FONT][FONT=Arial, Helvetica, sans-serif]
[/FONT]My pleasure to help you. [FONT=Arial, Helvetica, sans-serif][FONT=Arial, Helvetica, sans-serif]🙂[/FONT]
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Just what I was thinking. 🙂 I'll be covering the inside of the box with felt (or similar material) anyway, so this doesn't seem like too much extra work.
I was considering lining the entire back panel with felt and then screwing it into place, leaving a layer of felt all around the edges (essentially using felt instead of glue), but I think that might be too much leakage. 😉
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