Damping is most effective where there is velocity maxima (antinode) of a standing wave. Walls have nodes, no velocity but pressure, and damping is not as effective. So, you might have similar results with less damping material midway of the cabinet than on the walls. In reality, measure, listen and adjust 
I respectfully believe most suggestions here are more work than they are worth. I would suggest using CLD. If you want to go all the way you can use a double layer of 3/4" BB on all sides w/ damping material between (sikaflex or similar) Then use one or more CLD cross braces and fill with cheapest r12 fiberglass insulation.
aiphex
aiphex
That is true, my suggestion was to build it 30cm high though (or add another 10cm layer), so not just the wall, it is absolutely true it's not as effective as the same ammount in the middle, it's still working though. However, placing it in the middle, such a dense damping material literally suffocates the sound, a too dense packed sealed enclosure sounds 'dead'. The 2nd woofer is exactly at the half height. So it would not only mean to dampen the low mids very much, it would also mean both drivers are differently damped in that range - which is quite counter productive. But there is another good reason I suggested to put it at the bottom (read below), if you are chosing to place it in/near the middle, let there be at least 5cm distance below the bass driver.
Yes, it got other disadvantages. Mineral wool contains micro fibres wich are respirable dust. While that's already quite bad, they are carcinogenic, with glass wool being the worst at that. The manufacturers claim they've changed the production to not contain these micro fibres anymore but they aren't free of it, it's just reduced in the ammount and you are the one who has to prove you've got the cancer from that. Rock wool is much better at that (far less carciogenic/causing cancer) and similar to the effectivity of glass wool (a few %, I believe it was ~3% less). It's also much less itchy than gw, you should avoid contact with it anyway.
If you decide to use mineral wool, use a respitory filter and disposable gloves, put clothes directly into the washing machine afterwards. To pack it to the middle, especally close to the driver makes it extremely difficult to contain it (fine woven cloth, open pore foam/filter foam etc) because it has to be tight at a lot bigger surface and you'll get more or less every time in contact with it you change something at the drivers, cables etc.
Oh, wile we are talking about the cables: I'd suggest you'd implement a small cable channel to be able to run the cables non-visible outside and just use small boards screwed on to cover them. Not much work but a great aesthetical improvement anyway.
Uhm, back to the wool: I would absolutely avoid that! I've had a moth infestation once and I can tell you, NEVER AGAIN! It took YEARS to get them finally out of my apartment! Yes, it was a vented enclosure and not a sealed but I've seen moths in supposedly sealed enclosure aswell, they've entered through the foam seal and bores of the terminals, it didn't lead to a huge infestation though. Aside that, it doesn't dampen well below 800Hz without tight press, it's also very expensive compared to others and doesn't hold the form well, unlike polyester wool or polyurethane foam, it saggs pretty much over time.
Okay, maybe then I should use rock wool. There's a brand called that as well. It's available everywhere. If it's practically as effective (I mean what even is 3%?) I would always like to use less poisonous materials, even in a sealed enclosure. I've actually got glass fiber blocks in every corner of my room as bass traps. No cancer yet though . I'll replace them with rock wool as well, this isolation stuff comes in ridiculous amounts anyway. I should make some kind of nice packaging for them anyway.
About the wires, I already got that figured out very needly. Since the amplifiers are inside of the bass enclosure any way, the midrange and tweeter wires will go thru the inside of the bass enclosure to the top where they exit via two pairs of binding posts. They will then be connected to the binding posts of the midrange and tweeter enclosure with some short cables.
. I'll replace them with rock wool as well, this isolation stuff comes in ridiculous amounts anyway. I should make some kind of nice packaging for them anyway.About the wires, I already got that figured out very needly. Since the amplifiers are inside of the bass enclosure any way, the midrange and tweeter wires will go thru the inside of the bass enclosure to the top where they exit via two pairs of binding posts. They will then be connected to the binding posts of the midrange and tweeter enclosure with some short cables.
I would second the CLD recommendation by aiphex. To improve on the CLD technique, decouple the baffle from the enclosure. The ultimate would be to also decouple the drivers from the baffle. The aim of decoupling is to prevent or sharply reduce resonances at the source rather than just trying to kill them later.
Well there's also something called bass detail which is also pretty important. This is characteristic is defined by for example the ability to separate the individual sweeps of bass notes. In music, bass isn't just a sine wave of a low frequency, it's constantly sweeping between two frequencies (though not too far apart) at a certain frequency. A speaker with good bass detail is very capable of really separating these sweeps instead of washing them into something that's more like a sine wave again. So bass detail is definitely a thing and it's very important.
And I'm happily reading all your posts and suggestions about different kinds of damping material but what do "BB" and "CLD" stand for?
R.I.P. Niki Lauda
And I'm happily reading all your posts and suggestions about different kinds of damping material but what do "BB" and "CLD" stand for?
R.I.P. Niki Lauda
I figured I should also line the walls of the bass cabinet with butyl damping. It's often referred to as the extra mile if you really want to get the best damping and least vibrations in your cabinet. I was thinking of using the Gladen AERO-Flex stuff. Gladen make several different types of butyl damping each with their own characteristics. The AERO-Flex should offer the very best vibration damping and also pretty good stabilization and decouple/contact damping (whatever that's supposed to mean). If it's really as big as an improvement as I've been told and it really dampens pretty much all of the vibration out of the cabinet, considering these will already be extremely stiff and heavy of themselves, resulting in near-zero cabinet coloration in the bass, I think €74,- for the four sheets I will need is a fair price to pay.
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Ok, so do you have any thoughts on this?
Just so you know, this does a different job to the stuffing?
I think you need to stop what you are doing on this project and learn about CLD - constrained layer damping. There's a plethora of information about it on this site. Trust things by said by Gedlee, Adhoc1, they know CLD. Also AllenB, Norm Bates, Bill Waslo, Patrick Bateman are humble gurus I trust.
BB stands for Baltic birch. it's just another dimensionally stable plywood. I prefer it to MDF because it doesn't produce the same horrible dust, it doesn't dent as easily, it takes screws better, and it off-gasses a little less.
Remember you are going to want to CLD the front and rear baffle, possibly the sides. As well as using a CLD cross brace - think of a piece of lumber going from front to back (and another side to side), cut it in the center, put one under the other and extend them out so they overlap, then 'glue' them together with a viscoelastic polymer. There are more elegant ways but that's the basic idea.
Also a bit of fiberglass insulation or similar in the center of the box is all you need, no lining the walls with felt or butyl or anything like that.
aiphex
BB stands for Baltic birch. it's just another dimensionally stable plywood. I prefer it to MDF because it doesn't produce the same horrible dust, it doesn't dent as easily, it takes screws better, and it off-gasses a little less.
Remember you are going to want to CLD the front and rear baffle, possibly the sides. As well as using a CLD cross brace - think of a piece of lumber going from front to back (and another side to side), cut it in the center, put one under the other and extend them out so they overlap, then 'glue' them together with a viscoelastic polymer. There are more elegant ways but that's the basic idea.
Also a bit of fiberglass insulation or similar in the center of the box is all you need, no lining the walls with felt or butyl or anything like that.
aiphex
An important thing to remember is that this is a 3-way speaker. Bass and mid set different challenges and different means must be used - wavelength is the key.
Bass-mid xo is usually somewhere 250-400Hz. Wavelength of 300Hz is 1 meter, so bass box should not have such long dimension internally. Likewise wall contour/tilting has practically zero effect to standing waves. CLD and damping on walls has minimal effect - as long as the box has maximal rigidity.
Midrange 300-3000Hz is the most important driver and it presents the real challenge to both external and internal dimensions and shape, and all details. Very small dimensions and differences will have an effect. Driver hole's inner edges must be chamfered. Non-parallel walls and asymmetric shape help a lot. Rigid walls, damping or wedges on walls and wool stuffing can have great effect, but overfilling should be avoided.
Great advice here by Zaph and Troels
Zaph|Audio
DIY-Loudspeakers.
Bass-mid xo is usually somewhere 250-400Hz. Wavelength of 300Hz is 1 meter, so bass box should not have such long dimension internally. Likewise wall contour/tilting has practically zero effect to standing waves. CLD and damping on walls has minimal effect - as long as the box has maximal rigidity.
Midrange 300-3000Hz is the most important driver and it presents the real challenge to both external and internal dimensions and shape, and all details. Very small dimensions and differences will have an effect. Driver hole's inner edges must be chamfered. Non-parallel walls and asymmetric shape help a lot. Rigid walls, damping or wedges on walls and wool stuffing can have great effect, but overfilling should be avoided.
Great advice here by Zaph and Troels
Zaph|Audio
DIY-Loudspeakers.
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Not something i like to do. It adds mass without adding stiffness, so you lower the frequency of anf the broaden the Q of any potential resonances, first making them more likely to get excited and more likely to interfere with the music.
CLD is indeed a very good method to get cabinet sthat make little noise*, but it makes for a considerably harder build (not that this one is an easy build), and i have developed our cabinet sto the point where CLD offers no advantages, but i am mostly designing full range or WAWs. Use a good FR for midTweeter instead of the mid+tweeter and you could have a WAW. The advantages of only 1 XO down low, trumps the shortcomings of a FR (which is going to work way better with no need to do bass. Biamping simplifies the XO, if properly designed can get away with a PLLXO, or the diyAudioStore will see B4 kits soon, or you can go with a digital XO and be able to add in significant EQ capability.
*(in the bass, use of push-push woofers can dramatically reduce the load on the box (re resonance excitation) — to prove the point we built a test cabinet out of 15mm BB for a pair of push-push SDX10. No cabinet intrusion until levels some 10dB louder than it would ever be played — some of that could have come from the amp clipping).
dave
I know all about the effects of the design and damping in the midrange chamber, which is why I spent so much attention to it. I agree it's the most important aspect of the cabinet design. As you have seen I pay a lot of attention to designing the shape and the diffuser in the midrange chamber. Optimizing the self-damping of the chamber, minimizing reflections and standing waves and the need for damping material. I suspect I will only cover the rear of the chamber with a thin layer of polypill, just to really make sure no reflections come back, but it will be as little as possible. Overly damping the midrange chamber I know has catastrophic consequences. In my experience, it removes all weight, snap, musicality and fullness. Leaving a lifeless overly bright and thin sound and it also ruins the soundstage and imaging.
This is why I also still wonder why and how many manufacturers can completely stuff their midrange chambers and it still sounding like it seems it should.
I know about vibration cancelling and always incorporate it in my designs when possible (only my ultimate concepts really). Though, if you ask me, when using opposite mounted woofer, they have to be mounted in the width (on the sides). Mounting one on the front and one on the back causes a time difference between the two woofers. Though, opposite side mounting the woofers (an the sides but also on front and back) I think limits the crossover frequency. And though many speakers cross way lower than 300 Hz, I would like to keep it around that to give me that weight, scale and fullness in the mid bass. Most side firing woofers I see are crossed below 200 Hz. Not only would that remove the sound of the big woofers, the MR16P-4 also starts rising in distortion linearly below 300 Hz.
So for this design and the sound I'm going for, side-firing woofers aren't that good of an option.
But about the butyl damping. I know it doesn't make the enclosure stiffer but I thought it is supposed to absorb vibrations in the cabinet walls. But you're saying it only lowers the resonant frequency? I f this is true, I might as well not use it but is this really true?
This is why I also still wonder why and how many manufacturers can completely stuff their midrange chambers and it still sounding like it seems it should.
I know about vibration cancelling and always incorporate it in my designs when possible (only my ultimate concepts really). Though, if you ask me, when using opposite mounted woofer, they have to be mounted in the width (on the sides). Mounting one on the front and one on the back causes a time difference between the two woofers. Though, opposite side mounting the woofers (an the sides but also on front and back) I think limits the crossover frequency. And though many speakers cross way lower than 300 Hz, I would like to keep it around that to give me that weight, scale and fullness in the mid bass. Most side firing woofers I see are crossed below 200 Hz. Not only would that remove the sound of the big woofers, the MR16P-4 also starts rising in distortion linearly below 300 Hz.
So for this design and the sound I'm going for, side-firing woofers aren't that good of an option.
But about the butyl damping. I know it doesn't make the enclosure stiffer but I thought it is supposed to absorb vibrations in the cabinet walls. But you're saying it only lowers the resonant frequency? I f this is true, I might as well not use it but is this really true?
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