Thick walls or thin walls with damping?

sbronf

Member
2017-11-20 8:07 pm
Italia
Hi guys,
I'm building two speakers of about 32L, two-way, in bass reflex with seas speakers.
according to you a box with 12mm birch ply walls (18mm frontal one), with 2 or more layers of bituminous damping material, can be ok?
That to follow more or less the thought of the bbc (I already have a pair of Harbeth c7es3).

Or better to follow the road of thick and rigid walls?
I am very undecided.
I have already built some boxes following the road of rigidity, (21mm birch ply everly wall) and the bass is more controlled than the harbeths, but on the voices there is no comparison ... but maybe it is also thanks to the crossover and the speakers ...
thank you all if you would like to share your experiences in this regard.
 
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This is an area of some "lively debate". When well done both have merits (and some limitations) and they are not your only options. Matrix enclosures are also v good. The case for high stiffness, high damping is not to be dismissed lightly and there are CRO results to prove it. A paper generated here in Oz came to the conclusion that a short reverberation time was a good indication of subjective cabinet contributions to colouration.
Btw there are other threads running recently that explore this topic
might be productive to chase those up and save 'reinventing the wheel'...
...Cheers Jonathan
 
Whatever sounds deadest when tapped with a soft hammer is probably best - this is fairly easy to experiment with. But of course that's not the whole story - a soft pliant cabinet will sound dead, but will let low frequencies straight through.

Composite designs combining stiff materials and damping materials provide more flexibility to play with. With larger cabinets bracing becomes more important as smaller amounts of flex correspond to large amounts of air moving.

If you have to give in and allow the cabinet to flex appreaciably you want its response to be as untuned as possible, so different height/width/depth helps, as does placing point masses to break up vibration modes - and its going to get complex directionally unless its all low frequencies.

Another issue is absorption v. reflection interally - energy you can absorb cannot then reflect or go through the walls - but its hard to absorb lower frequencies without a lot of heavy wadding.

A ported enclosure has less pressure internally to cause flexing at low frequencies, so if the cabinet is heavy enough to resonate lower than the port frequency the port will reduce flexing for you. But just increasing the wall thickness increases stiffness much faster than it increases mass.

My suspicion is that the ideal speaker walls would have a varying composition, starting as acoustic wadding to heavier wadding/carpet, to heavy pliant damping layers and finally a stiff layer designed to resonate at a fairly low frequency with the mass of the damping layers, and with low Q of course. Sand filled pockets or double walls are one way to do damping with extra mass.


Concrete and stone make great materials for speaker walls as they are very heavy and have low-Q for their resonant frequencies, they naturally damp vibrations. Practicalities may prevent using these materials!
 

sbronf

Member
2017-11-20 8:07 pm
Italia
Whatever sounds deadest when tapped with a soft hammer is probably best - this is fairly easy to experiment with. But of course that's not the whole story - a soft pliant cabinet will sound dead, but will let low frequencies straight through.

Composite designs combining stiff materials and damping materials provide more flexibility to play with. With larger cabinets bracing becomes more important as smaller amounts of flex correspond to large amounts of air moving.

If you have to give in and allow the cabinet to flex appreaciably you want its response to be as untuned as possible, so different height/width/depth helps, as does placing point masses to break up vibration modes - and its going to get complex directionally unless its all low frequencies.

Another issue is absorption v. reflection interally - energy you can absorb cannot then reflect or go through the walls - but its hard to absorb lower frequencies without a lot of heavy wadding.

A ported enclosure has less pressure internally to cause flexing at low frequencies, so if the cabinet is heavy enough to resonate lower than the port frequency the port will reduce flexing for you. But just increasing the wall thickness increases stiffness much faster than it increases mass.

My suspicion is that the ideal speaker walls would have a varying composition, starting as acoustic wadding to heavier wadding/carpet, to heavy pliant damping layers and finally a stiff layer designed to resonate at a fairly low frequency with the mass of the damping layers, and with low Q of course. Sand filled pockets or double walls are one way to do damping with extra mass.


Concrete and stone make great materials for speaker walls as they are very heavy and have low-Q for their resonant frequencies, they naturally damp vibrations. Practicalities may prevent using these materials!

thank you all for the answers. but what is a material q and which does it depend on?
So,the best material is thin and heavy?
 

PRR

Member
Paid Member
2003-06-12 7:04 pm
Maine USA
www.diyaudio.com
> what is a material q and which does it depend on?

"Q" = Quality of resonance

A bell made of special brass rings very well. A bell made of wax has almost no ring.

If you make bells, you want a high-Q material.

If you make speakers you want low-Q. Sheet metal rings, not like a good bell, but pretty good. Wet cardboard has about no ring at all (but is sloppy and weak). Some spruce wood is pretty ringy, high Q for wood. I have seen oak and pine with much less ringiness.

But you already know the basics even if you didn't have the letter "Q". Tar-board is a standard way to damp (reduce the Q) of panels. Personally _I_ do not think making the box 10 times thicker and heavier is really 10 times better. But yes too-flimsy does add a "thin box" sound. I have never been 100% happy.
 

sbronf

Member
2017-11-20 8:07 pm
Italia
> what is a material q and which does it depend on?

"Q" = Quality of resonance

A bell made of special brass rings very well. A bell made of wax has almost no ring.

If you make bells, you want a high-Q material.

If you make speakers you want low-Q. Sheet metal rings, not like a good bell, but pretty good. Wet cardboard has about no ring at all (but is sloppy and weak). Some spruce wood is pretty ringy, high Q for wood. I have seen oak and pine with much less ringiness.

But you already know the basics even if you didn't have the letter "Q". Tar-board is a standard way to damp (reduce the Q) of panels. Personally _I_ do not think making the box 10 times thicker and heavier is really 10 times better. But yes too-flimsy does add a "thin box" sound. I have never been 100% happy.

since I have already made some speakers that point to stiffness with thick walls and actually sound a little cold, now I'm thinking of making two boxes of these internal measures h49cm w22cm d31cm. the front wall with a thickness of 24mm, the rear wall of 15mm and all the others 12mm, with bituminous damping 4-6mm thick. something like bbc school. what do you think?
 
since I have already made some speakers that point to stiffness with thick walls and actually sound a little cold, now I'm thinking of making two boxes of these internal measures h49cm w22cm d31cm. the front wall with a thickness of 24mm, the rear wall of 15mm and all the others 12mm, with bituminous damping 4-6mm thick. something like bbc school. what do you think?
Sounding a little cold probably means you have lost that lower mid/upper bass "bloom" that most of us are used to. It adds an apparent warmth, so losing it makes the speaker sound cold. I hate cabinets, much preferring none, so panel speakers ('statics, etc) would be my preferred option, which have their own problems*. The last pair of box speakers I built used curved sand filled side panels, and had that "coldness" you refer to.
* I have just bought a pair of Quad 2805s and one has a faulty bass panel! But really I meant room placement and cancellation issues.