How important is the thickness of cabinet wall?

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Hi, I’m building a pair of floor-standing speakers. The height is about 120 cm. Width 29 cm. And depth 36 cm. At first, I planned to use the 20 mm. MDF as the cabinet walls and 1-inch thick MDF for bracings inside the cabinet, as seen in some commercial speakers. I went to my local carpenter shop, he usually build car speaker enclosures, and had him build my speaker cabinets. Unfortunately, he used wrong thickness on my pair. He used 12 mm. MDF totally, the bracing construction and dimension are correct every point though. He offered me discount for these cabinets due to his mistake. I’m now hesitate whether I should accept his offer or not. Could anyone please tell me what is the result of using thin cabinet wall? What way will the sound go and affect? I’d like to know if the sound is distorted, how? Bass boom, Midrange echo, etc. Please explain me. Thank you in advance.
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12mm is pretty thin for a cabinet that tall but it depends on how well braced it is and what the drivers are doing. Did he use the 12mm for the baffle also? Is this your design or someone elses? Pictures? Personally I would not accept the work from the carpenter if he made a mistake that basic (if you were clear with him about the plan).
Presumably the thinner you go, the more you are listening to the box resonances and less the driver output.

Play something with content <300hz (or feed it specific frequencies from a tone generator) and touch the box to feel where it vibrates or not. We'd expect the most vibraton to be somewhere on the largest 2 faces of the box.
You should find some vibration 'hot spots' somewhere.

Of course, increasing the volume (signal) should reveal more than listening to quiet tones.

If it is braced well enough, then you may not have a problem, and it may just be fine.

If it is resonant, you could try to mitigate it by post-tensioning the interior (chain/cables, turnbuckles), i.e. trying to make the cabinet implode by tightening the cables... until it pushes the resonances up out of the offending range.

...of course you can't do tensioning on any removable panels (if there are any), without some trickery, but handling 2 of 3 vertices should still be an improvement.

And ultimately, if implemented less than optimally, post-tensioning could also rip apart the inside of the cabinet. YMMV.

* I've seen automotive stethoscopes suggested for locating resonances as well, but the ones I deal with are much larger than to require such a device. Hands on the cabinet suffice for me so far.
Imho thin walls will mainly have effect on the mid frequencies (say 250 to 1500Hz). Lows are invariably ruled by the room response. Effective decoupling of the mid driver can be very helpful (BBC did just that way back in the seventies).
Your carpenter could double the walls for sure. But then I would opt for a Constrained Layer Damping version. You might want to check that out
In MDF minimum 18mm, preferable for big power speakers 21mm or more. In plywood you can do it with 9mm if you damp them wit bitumen for lower power speakers, but i would use at least 12mm or 18mm for big power or subwoofers.

I did use MDF in the past, but i went to birch plywood, as those speakers are better. They are more solid, and lighter and in my opinion (opinions differ, i know) it also sounds better. I have the speaker that i use as top now, also made in mdf before, and the 12mm plywood version (with bitumen damping) sounds a lot better than the 18mm MDF version (without bitumen damping). Both were 11.5L sealed cabinets with a Mark Audio Alpair 10M gen 3 in it, and braced the same way and filled with polyfill.

But when you choose for MDF, don't do the 12mm version, it will be not good. MDF is less strong than plywood, and 12mm is not strong enough. 18mm is the minimum there.
Could anyone please tell me what is the result of using thin cabinet wall? What way will the sound go and affect? I’d like to know if the sound is distorted, how? Bass boom, Midrange echo, etc. Please explain me. Thank you in advance.

The baffle needs to as stiff as possible to reduce the amount of vibration entering the cabinet from the drivers. The stiffer the better. This doesn't want to be made of 12 mm MDF with a large speaker.

The stiffness of the remaining cabinet walls is less important. The thicker the wall, the higher the resonant frequencies but generally the SPL from the cabinet remains roughly the same. What these walls usually need is damping to reduce the size of the resonances (usually because there is more than one design approach for a quiet cabinet). If the internal bracing is reasonably extensive 12mm MDF might be just about OK but it wouldn't be ideal.

Are you adding damping to the walls? How extensive is the bracing? How thick is the baffle? How is the midrange enclosure mounted to the main enclosure?
If mine, I will just double it with 10 mm plywood with something in between like acoustic black sheets they use in cars (inside the doors, roof...) or the one to proof the roofs (though toxic inside home ?)

Mass-spring-mass helps a lot. And plywood is cool enough not to need veenering.

Arbeth speakers just use 19 mm mdf and both side 1 mm veenering and no one seems complain of the M40 coffin with its 15" bass unit (and it has no brassing inside but a rod and acoustic foam)
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Weight does not matter… unless you have issues with the reactive energy of the driver cones starts rocking the cabinet — all from the woofer.

But if you are aiming at elegance, using a pair of the size smaller woofers push-push to eliminate any chance of that, and reduce the load on the box dramatically, i estimate 90% (depends on how tightly the woofers are coupled.

We did a purposeful experiment with push-push woofers to check the magnitude of the benefit. We built a box from 15mm BB (15mm is a lot easier to work with then 12mm) with 2 x 10” SDX10 — about the same size, with what i’d call minima bracing. You could pick the box up with a couple fingers in the driver cutout (with the drivers i’d want 2 people to move it far). It worked REALLY well.

From this, and what w learned, came my guess that good 12mm multiply could be made to do the job.

Funny thing is that the differences between -say- 18mm and 12mm alone aren’t that big. Think of throughput on some resonant frequencies of about 3-5 dB. Lower frequencies will be dampened more, but higher ones could turn out worse with thick materials. The real nuisance is the resonance itself; you will notice that ‘boxy’ sound. Thicker panels do NOT resonate less long. Lighter or heavier material only shifts the resonant frequency. Pushing them outside the band pass of a driver can and has been done (Celestion among others), but not with wood-like materials.
Even bracing doesn’t solve all trouble, but it has its merits. Increasing internal damping of the panel material is the best way to reduce panel resonances (and sound throughput of the speaker box). Hence I referred to CLD.
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