Alternative Speaker Box Materials

[mondogenerator]

This sort of thread always ends in a debate about who's study to believe, and who paper to read. Suffice it to say there are benefits in both strategies. Stiffness alone without damping is not advisable any more than low stiffness panels and no damping is a good idea. I can recommend both Ply and MDF. MDF has flaws...it swells, stinks, and you generally need a thicker panel. I have had good results on a small 7 litre monitor using 8mm MDF and 12mm Plasterboard lath on the inside. Walls lined with offcuts of thick wool carpet. Worked very well, the only thing I would mention about the plasterboard is that it reflects alot better than MDF or wood......I ended up with Carpet plus maybe 1.5" of foam/cotton wool/felt and a bit of BAF. I just about tamed it.

Now the Birch ply is also a good idea. I have some steel box I might use in conjunction with ply and some resin and glass fiber. Before that I have another with granite for the panels, only 8mm I think. This will be epoxied to a steel section 'cleat' structure. It would be really cool to GRP that!

[speaker dave]

Originally Posted by mondogenerator

This sort of thread always ends in a debate about who's study to believe, and who paper to read. Suffice it to say there are benefits in both strategies. Stiffness alone without damping is not advisable any more than low stiffness panels and no damping is a good idea.

No one is advocating low stiffness and no damping. I am advocating high mass with high damping and relatively low stiffness. Harwood and other studies support that. Most audiophiles strongly advocate high mass and high stiffness, while ignoring damping, yet I don't know of any studies or papers that confirm that as a proper approach.

David S

[Moondog55]

A layer of plasterboard glued to the inside panel works, but it is heavy and needed full glue coverage, I used standard PVA after the box was bulit.
A friend uses standard Plaster of Paris wet down with full strength PVA glue and paints it over the inside

[DrDyna]

Originally Posted by speaker dave

Most audiophiles strongly advocate high mass and high stiffness, while ignoring damping, yet I don't know of any studies or papers that confirm that as a proper approach.

I saw a project with a small 2 way monitor a while back that was made out of granite. I'd imagine that would be as good as it gets if you're shooting for mass and stiffness. The folks who made it seemed to like it, but I've never listened to such a thing.

[rob g]

People focus a lot on materials. Shape also has a significant effect on stiffness.

Linkwitz Pluto uses rigid tubes, these are very, very heavily stuffed/damped to reduce the strong single peak from the length of a tube, then finally the speaker is actively equalized, reducing this further.
Cabinet colouration simply doesn't rear it's ugly head, together with it's small curved form making for very low diffraction issues the speaker disappears from the ear like a pair of ESL's (which need a greater listening distance to pull this off).

[speaker dave]

Originally Posted by DrDyna

I saw a project with a small 2 way monitor a while back that was made out of granite. I'd imagine that would be as good as it gets if you're shooting for mass and stiffness. The folks who made it seemed to like it, but I've never listened to such a thing.

Granite is a great example. Lots of mass and stiffness but rings like a bell. That is not what you want.

For work I have been looking into the possibility of a transportable temporary theater with a tent-like or bubble construction. The question is what the acoustics of a heavy walled bubble or tent would be. It turns out that flexible walls almost perfectly follow what acousticians call "mass law" characteristics. Their sound issolation is a pure 6dB per Octave curve. Every Octave you go up they give 6dB better transmission loss or issolation. Start to add stiffness and you will incur resonances. At resonances you have a narrow band of total transmission. Once you have resonances your only hope is that damping can be added to reduce resonance level and thereby transmission.

So the ideal box would be a massive, soft and lossy, dense rubber-like material. Rapped with your knuckles it should give a dull pitch-free sound.

Regards,
David S.

[planet10]

I can see Dave's speaker building philosophy is way at the other end of the spectrum thsn mine.

The short version being -- everything resonates, but if they are never excited it as if they don't exist.

Then think about the fact that the energy available to excite a resonance is inversly proportional to the frequency -- althou it can be argued, that above a certain frequency, it is actually proportional to the 4th power of the frequency.

Where this has led me, is to practical boxes that are very quiet.

dave

[vinyl dragon]

Originally Posted by planet10

I can see Dave's speaker building philosophy is way at the other end of the spectrum thsn mine.

The short version being -- everything resonates, but if they are never excited it as if they don't exist.

Then think about the fact that the energy available to excite a resonance is inversly proportional to the frequency -- althou it can be argued, that above a certain frequency, it is actually proportional to the 4th power of the frequency.

Where this has led me, is to practical boxes that are very quiet.

dave

That's fascinating, never looked at it that way and it would surely explain why I have always preferred the sound of the ultra rigid approach (though I like the overall BBC type balance, just not the bass :-)

So what would be an ideal speaker cabinet construction/material?

[mondogenerator]

Originally Posted by speaker dave

Granite is a great example. Lots of mass and stiffness but rings like a bell. That is not what you want.

...snip....

So the ideal box would be a massive, soft and lossy, dense rubber-like material. Rapped with your knuckles it should give a dull pitch-free sound.

Regards,
David S.

ABout the Granite...well it does ring like a bell, when undamped. From what I know about control theory, a system is at its most responsive when it is a fine balance between a heavily resonant system that is critically damped to tame it. This was the thought that inspired me to consider granite as a material, in the same way as concrete is appealing. The resonance is very narrow bandwidth , and high in pitch, and easy to damp as a result.

I also have some fairly dense 1" cork, and 1" rubber. Both I considered a good option for a extra lossy material. When I tried it initially, the result was less inspiring than I first thought.

Polyester casting resin would be an interesting material to try. I spotted some aluminium loaded resin last week and now im just waiting for the Easter eggs to come, and their vacuum moulded plastic packaging(which I plan to use to make a mould). An easter egg shapped pod computer speaker system is exactly what I need

[fatmarley]

I wonder if a different box for each driver would be the ideal approach. I know a woofer/subwoofer works best in a solid, well braced cabinet because i've tried the with/without bracing experiment and the bass was much tighter and better in the well braced cabinet. Also, you'd never buy a good quality subwoofer that wasn't in a well braced cabinet.
The resonant frequency of a well braced cabinet would be much higher than that of an unbraced cabinet, So the low frequencies that the woofer produce, would never exite the box resonant frequency.
For the tweeter perhaps the soft rubber material that Dave suggests would be best?
I don't know how you'd decouple one box from the other though.

EDIT: If you remove all of the resonance out of the pass band of each driver, how much difference would it make in reality? I mean, would it be worth the effort?