As I continue to think through my sealed midbass section design I'm wondering about the importance of back wave reflections (and mitigation) if the section is to play within 80 to 500Hz.
The driver is 10" and a sealed enclosure yielding Qtc ot 0.71 would be 10 liters, which is pretty small.
To simplify the logic: a cubic 10-liter box would be 21cm per side. If the back panel is 21cm, then the first standing wave back-to-front would be at 800Hz, while the first top-to-bottom and left-to-right standing waves would be at 1600Hz.
If the driver is to play 80 to 500Hz...should I care about the internal shape? Will frequencies below 800Hz be reflected back through the driver?
I was initially thinking a long, 10 liter cone (60cm long) would be great to mitigate back wave reflections by placing absorvent material at the cone end, but upon looking at the issue from the above point of view I am confused as to whether I should care at all.
The driver is 10" and a sealed enclosure yielding Qtc ot 0.71 would be 10 liters, which is pretty small.
To simplify the logic: a cubic 10-liter box would be 21cm per side. If the back panel is 21cm, then the first standing wave back-to-front would be at 800Hz, while the first top-to-bottom and left-to-right standing waves would be at 1600Hz.
If the driver is to play 80 to 500Hz...should I care about the internal shape? Will frequencies below 800Hz be reflected back through the driver?
I was initially thinking a long, 10 liter cone (60cm long) would be great to mitigate back wave reflections by placing absorvent material at the cone end, but upon looking at the issue from the above point of view I am confused as to whether I should care at all.
Hi LewinskiH01,
my basic réflexions :
The tube shape could be worst for the front wave polar response according V Dickason speaker cook book although S. Linkwitz uses it with a 8" in the same frequency range but firering towards the ceilling.
The more you stuffs the cabinet the more you increase the volume (100% stuffing= +20% volume). But at those frequencies, the internal damping with classical glass-fibers are certainly not enough helpfull to flat the response curve ?
If you find too much damping gives you a flater curve but a too damped sound, an empty cabinet with a diy variovent could help.
At those frequencies the wave length is high so powerfull, the cabinet can play with the driver, not only by the rear wave. You may try mass spring effect with a two layer cabinet : e.g. : internal chipboard + external ply wood... as a base and after several soft damping.It seems the internal brassing is very important : why not make it with a 1 cm or more foam carpet for horizontal and vertical layer Inside the middle of the cabinet ? Not so expensive to try and reversible without remaking a cabinet ! Of course you have to calculate the volume of those layers with big empty holes (of several sizes) to "glue" to your Qts/volume ratio.
I read also than a non cubic with non parrallel walls helps with sealed load... as I had some delirious ideas not checked with experience as well, like to tie from the top Inside wall some squatsh rubber balls at different heigths ! Lol ! Certainly homeopatic in relation to a true thinked design with already approved solutions knowed to work already !
Just some thoughts I had myself on the same project than you I gived up in the reflexion phase (I go for a synergy design now but assume I will have exactly the same problems as you at those frequencies...for even a littlier sealed load).
Read it as simple réflexions before experienced specialists here input real advices...
my basic réflexions :
The tube shape could be worst for the front wave polar response according V Dickason speaker cook book although S. Linkwitz uses it with a 8" in the same frequency range but firering towards the ceilling.
The more you stuffs the cabinet the more you increase the volume (100% stuffing= +20% volume). But at those frequencies, the internal damping with classical glass-fibers are certainly not enough helpfull to flat the response curve ?
If you find too much damping gives you a flater curve but a too damped sound, an empty cabinet with a diy variovent could help.
At those frequencies the wave length is high so powerfull, the cabinet can play with the driver, not only by the rear wave. You may try mass spring effect with a two layer cabinet : e.g. : internal chipboard + external ply wood... as a base and after several soft damping.It seems the internal brassing is very important : why not make it with a 1 cm or more foam carpet for horizontal and vertical layer Inside the middle of the cabinet ? Not so expensive to try and reversible without remaking a cabinet ! Of course you have to calculate the volume of those layers with big empty holes (of several sizes) to "glue" to your Qts/volume ratio.
I read also than a non cubic with non parrallel walls helps with sealed load... as I had some delirious ideas not checked with experience as well, like to tie from the top Inside wall some squatsh rubber balls at different heigths ! Lol ! Certainly homeopatic in relation to a true thinked design with already approved solutions knowed to work already !
Just some thoughts I had myself on the same project than you I gived up in the reflexion phase (I go for a synergy design now but assume I will have exactly the same problems as you at those frequencies...for even a littlier sealed load).
Read it as simple réflexions before experienced specialists here input real advices...
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The good thing about a closed box is it is not that difficult to find a level of internal damping that has a reasonable effect on standing waves without too great an effect on the fundamental. If in doubt, moderately stuff.
Some actual science on damping internal waves:
http://www.diyaudio.com/forums/multi-way/215133-box-colourations-really-22.html#post3086482
http://www.diyaudio.com/forums/multi-way/215133-box-colourations-really-22.html#post3086482
This is why B&W Nautilus works well. I did similar thing with Nautaloss and Dagger enclosures. Dagger is a tall 3 sided pyramid. No parallel walls and stuff progressively dense.
Yeap. This is part of what got me thinking and doing the math. I saw another thread from 2012 where Speaker Dave stated the same point of view, and I remember seeing you in that thread as well. In light of this a cubic enclosure (behind a wide panel) seems a good idea. Easy to build and line too. A cylinder would also be easy to build.
But then why do people make a big deal out of subwoofer back wave? At those wavelengths the sound would never bounce back and out through the driver.
Have you tried different boxes and confirmed what Speaker Dave says?
That's good. But would you agree with the underlying assumption that only frequencies above that of the first standing wave get reflected back and out through the driver?
By fundamental, I meant to say Fc. This of course is not necessarily related to the cabinet dimensions nor does it need to be absorbed.
With regard to standing waves where gradients have dimensions that can develop within the cabinet space, the regions of higher velocity can be accessed with damping material that occupies a fraction of the cabinet within. It has been suggested (could have been speaker dave) that 40% of each dimension makes for sufficient coverage if placed reasonably.
With regard to standing waves where gradients have dimensions that can develop within the cabinet space, the regions of higher velocity can be accessed with damping material that occupies a fraction of the cabinet within. It has been suggested (could have been speaker dave) that 40% of each dimension makes for sufficient coverage if placed reasonably.
For frequencies outside the passband of the woofer, I wouldn't worry about it too much. The cabinet is just like a room, if you don't excite the lowest room modes with a subwoofer, you aren't going have to deal with the room modes. If there is no sound from your woofer at 800 Hz, you don't have to worry about the standing waves affecting the woofer output.
IMO, stuffing provides air resistance and makes the cabinet appear larger than it is. This is why it is used with subs, not to absorb standing waves.
This is my opinion, I may be wrong of course. Where is Speaker Dave when you need him? 🙂
IMO, stuffing provides air resistance and makes the cabinet appear larger than it is. This is why it is used with subs, not to absorb standing waves.
This is my opinion, I may be wrong of course. Where is Speaker Dave when you need him? 🙂
I don't want to make out that I've done studies on the subject but yes, stuffing increases box losses and the efficiency/bandwidth tradeoff is universal in many aspects of physics.
Yes, I picked up on the dagger too. In fact a triangular cross-section version of the cone I mentioned above. But the same question applies to that shape: if only frequencies above the first standing wave are subject to back wave radiation, and if I'm not going to run up to those frequencies in this section...then why bother with making things complicated when a simple solution would work as well?
BUT all this is hanging from the one big assumtion highlighted in blue above. Do you agree with it?
Box colourations, really ?
Some how, I missed this thread. Either that, or I don't remember it 🙂
In any event, thanks for the link. I started from the beginning and it's the type of thread I find MOST interesting !!
Oh, by the way, frequencies out side of any given band pass can, and often times, still do influence the sound quality. You be surprised what you can
hear, that is 30-40 db down.
http://www.diyaudio.com/forums/multi-way/215133-box-colourations-really.html
Some how, I missed this thread. Either that, or I don't remember it 🙂
In any event, thanks for the link. I started from the beginning and it's the type of thread I find MOST interesting !!
Oh, by the way, frequencies out side of any given band pass can, and often times, still do influence the sound quality. You be surprised what you can
hear, that is 30-40 db down.
http://www.diyaudio.com/forums/multi-way/215133-box-colourations-really.html
I don't understand where you got the blue highlighted portion. Back wave radiation happens at all frequencies that the cone emits within its pass band set by the driver and XO. For a 2 way the woofer will be expected to go up to 2kHz or more. For a 3 way probably 1kHz. It's the full spectrum within the pass band that needs to be absorbed as much as possible.
I'd wager a guess he is trying to establish a correlation between what might be considered the first back wave reflection vs the pass band. There was a white paper written by an elder statesmen friend of mine, suggesting to keep the operating frequencies below where this occurs.
I really wonder, after decades of addressing the issue, why we don't have a standardized method of absorbing internal waves.
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