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Home » Sponsored » Pi Speakers » 4 PI by Andy
| Re: 4 PI by Andy [message #75290 is a reply to message #75289] |
Sun, 20 January 2013 13:01   |
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Wayne Parham
Messages: 18787
Registered: January 2001
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Illuminati (33rd Degree) |
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Damping material is not needed in traditional subs run outdoors, distributed subs in a multisub arrangement, or pretty much any subwoofer that is acoustically small. But that's a key: The subwoofer cabinet must be acoustically small, which means the low-pass frequency must be low enough that internal standing waves do not develop in the passband.
Otherwise the rules change.
If you run a big box like the one you posted as a flanking sub, then it will receive some lower midrange frequencies, and at certain frequencies, standing waves will develop inside. So I would put insulation in a big box like that, if I were running it as a flanking sub. And I wouldn't just line the walls, I'd also span the cross-section. In fact, I'd measure the finished product and make sure it was clean in the lower midrange, to even determine whether or not it was suitable as a flanking sub.
That's the whole reason we take such care with our mains, the ones that are physically large. The reason is standing waves develop in the lower midrange, and at those frequencies, the insulation inside is unable to effectively damp the standing wave nodes. So we do careful analysis to make sure the driver and port don't fall on a pressure node and we also put insulation inside that spans the cross-section. That's a more effective location for midrange attenuation - Bass goes right through but lower midrange is damped more effectively.
A physiclaly small sub, something like a 20" square box, cannot develop standing waves below 300Hz. So it's completely safe to run as a flanking sub. You can easily run it up to the Schroeder frequency without having any internal standing waves.
But even if we just increase a single cabinet dimension to three feet, now we have standing wave modes below 200Hz. This is getting down into the lower midrange, and it can adversely affect response if the pressure node lies where the driver and/or port are.
Lots of DIYers build cabinets using T/S box modelers and forget about internal standing waves. The Helmholtz frequency may be right and response below 100Hz is perfect, but the lower midrange can develop a 3dB to 6dB peak somewhere. They won't even know it unless they measure, and even then, sometimes they mis-identify the cause, thinking it's an outside reflection or even baffle diffraction.
Of course, reflection anomalies from nearest boundaries do fall in this frequency band too. But they tend to be most noticeable as notches, not peaks. And you won't see them in an outdoor measurement, in any case.
So anyway, all that to say be careful of internal standing waves in a physically large cabinet. Large cabinets are preferable, in my opinion, because they improve bass response (Hoffman's Iron Law) and because they give ample baffle area, keeping baffle diffraction at a low enough frequency it is masked by the room (and mitigated with flanking subs). But physically large boxes take more dilligence to optimize, because the designer must always be mindful of internal standing waves.
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| Re: 4 PI by Andy [message #75291 is a reply to message #75290] |
Sun, 20 January 2013 13:41 |
zheka
Messages: 80
Registered: June 2012
Location: Chicago burbs
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Viscount |
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Thank you very much.
The box I am working on is also 10ft^3. The longest side is about 36" internal. It would not be used as true flanking sub but it may be low passed as high as 120Hz. If standing waves indeed form in this case, they would come form the harmonics, not main content
Would you recommend using damping material for such a box? If so, what material is effective at such low frequencies? Is there a downside to damping subwoofers if it's not really needed?
Thank you.
| Wayne Parham wrote on Sun, 20 January 2013 13:01 |
Damping material is not needed in traditional subs run outdoors, distributed subs in a multisub arrangement, or pretty much any subwoofer that is acoustically small. But that's a key: The subwoofer cabinet must be acoustically small, which means the low-pass frequency must be low enough that internal standing waves do not develop in the passband.
Otherwise the rules change.
If you run a big box like the one you posted as a flanking sub, then it will receive some lower midrange frequencies, and at certain frequencies, standing waves will develop inside. So I would put insulation in a big box like that, if I were running it as a flanking sub. And I wouldn't just line the walls, I'd also span the cross-section. In fact, I'd measure the finished product and make sure it was clean in the lower midrange, to even determine whether or not it was suitable as a flanking sub.
That's the whole reason we take such care with our mains, the ones that are physically large. The reason is standing waves develop in the lower midrange, and at those frequencies, the insulation inside is unable to effectively damp the standing wave nodes. So we do careful analysis to make sure the driver and port don't fall on a pressure node and we also put insulation inside that spans the cross-section. That's a more effective location for midrange attenuation - Bass goes right through but lower midrange is damped more effectively.
A physiclaly small sub, something like a 20" square box, cannot develop standing waves below 300Hz. So it's completely safe to run as a flanking sub. You can easily run it up to the Schroeder frequency without having any internal standing waves.
But even if we just increase a single cabinet dimension to three feet, now we have standing wave modes below 200Hz. This is getting down into the lower midrange, and it can adversely affect response if the pressure node lies where the driver and/or port are.
Lots of DIYers build cabinets using T/S box modelers and forget about internal standing waves. The Helmholtz frequency may be right and response below 100Hz is perfect, but the lower midrange can develop a 3dB to 6dB peak somewhere. They won't even know it unless they measure, and even then, sometimes they mis-identify the cause, thinking it's an outside reflection or even baffle diffraction.
Of course, reflection anomalies from nearest boundaries do fall in this frequency band too. But they tend to be most noticeable as notches, not peaks. And you won't see them in an outdoor measurement, in any case.
So anyway, all that to say be careful of internal standing waves in a physically large cabinet. Large cabinets are preferable, in my opinion, because they improve bass response (Hoffman's Iron Law) and because they give ample baffle area, keeping baffle difrraction at a low enough frequency it is masked by the room (and mitigated with flanking subs). But physically large boxes take more dilligence to optimize, because the designer must always be mindful of internal standing waves.
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| Re: 4 PI by Andy [message #75293 is a reply to message #75291] |
Sun, 20 January 2013 17:11 |
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Wayne Parham
Messages: 18787
Registered: January 2001
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Illuminati (33rd Degree) |
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There is no downside to lining the cabinet walls with acoustic damping material. I've had best luck with R13 insulation. So feel free to add it.
If you're concerned with midrange, you can also install a sheet that spans the cross-section. It seems to work best at midrange frequencies when the cross-section sheet is between woofer and port.
But if you're going to use it as a prosound sub, I would probably discourage the cross-section sheet because there's a lot of air moving at full tilt. A prosound sub should be purpose oriented, and big boxes with big ports should be used without anything impeding airflow between them.
A box with 36" internal dimension will have a standing wave at 180Hz. It will have others too, but the longest dimension sets the lowest axial mode. So that's the one to watch.
I wouldn't be comfortable using that as a flanking sub unless I measured it and knew it wasn't making any artifacts all the way up to 200Hz.
And that's really the issue here - I'm not saying standing waves will create response anomalies, I'm saying they might. If signals are presented to a woofer in a cabinet with at least one 36" dimension, then standing waves will exist inside the cabinet. If the woofer or port sits on a pressure node, then those standing waves will cause a response peak. So you would probably want to make sure the port and woofer are positioned in such a way that response is good. That's what we do when designing the mains too, or at least, that's what I do. It's why I tell people to be careful when contemplating baffle changes and mods.
This is basically a measurement thing. Measure the box and look for ripple above 150Hz. If you don't see any, then there's nothing to worry about. If you do see ripple, then you can either change the baffle and put the port and/or woofer in a different location, or you can limit the frequency range, and don't send signals at or above the anomalous range. Low-pass it below the frequency where it gets wonky.
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