Equivalent to/improvement on 19mm MDF

[VantheMan]

Regarding my Mk 2 boxes. I´d like to make them a bit less behemoth-like (after Totem arros, anything is too big) and I have spoken to the designer and he has given me some new INTERNAL dimensions to work on so long as I respect the previous baffle step/port dimensions. With a view to reducing them a bit more, say, in the width could I use some other material for the sides (Marine Ply springs to mind or anything else for that matter) which, for a given thickness might be roughly equivalent in whatever parameters are considered important (rigidity etc.) to the 19mm MDF I used originally. Also would a sandwich of 2 thicknesses of MDF or whatever be equally as suitable/stiff as just one solid piece for a given thickness. I am thinking of sandwiching the front baffle and cutting different diameter concentric holes with a tank cutter thingy to avoid having to use a router (which I haven´t got).

Grateful for any help,
Chris

[Scottmoose]

As no-one else has offered anything yet...

Ply is much stiffer than MDF. It's resonant behaviour is also superior IMO, as it's milder & spread over a wider frequency range, unlike MDF, where you get a narrower, and sharper notch as it lets go. Both need bracing if used for large panels, though the MDF will need more.

Yes, you can double up thicknesses of MDF, which will theoretically push panel resonance down below the cabinet passband. 1 1/4in - 1 1/2in should be sufficient for most applications, although that means you're storing even more energy. One solid piece or several? I'd go with several as you get a laminated medium. The energy going into it has to meet several layers / interfaces, and the more of these it meets, the greater the losses / damping. Which automatically applies to plywood of course.

[John T]

Originally Posted by Scottmoose

The energy going into it has to meet several layers / interfaces, and the more of these it meets, the greater the losses / damping. Which automatically applies to plywood of course.

Reading this, I seem to remember Dave Berriman extolling the virtues of making up panels with a number (4-6?) of layers of hardboard glued together with impact adhesive. I think this type of adhesive was recommended because of its "elastic" nature which enabled energy to be dissipated by the layers moving relative to each other.

[Scottmoose]

Interesting. Multiple layers of material certainly work better than a single layer in this application anyway. The more layers you have, the more interfaces you get, and thus, the more energy is lost. Well, not lost -you never loose energy, it's just converted into something else, but you know what I mean.

Incidentally, chipboard (particleboard) is not to be sniffed at. I'd rather use that than MDF.

[Cobblers]

Originally Posted by Scottmoose

The more layers you have, the more interfaces you get, and thus, the more energy is lost. Well, not lost -you never loose energy, it's just converted into something else, but you know what I mean.

Are you trying to say the resonances are potentially spread evenly in a more benign way across more than one layer, hence less noticeable?

[Scottmoose]

Nah. Form of layer damping. To paraphrase my friend Ron, who works in this field, when energy encounters an interface of a different value to the one it's traveling in, it suffers more losses than it would do if simply travelling through a singular medium. So there more interfaces (in this case, layers) energy has to encounter the greater the loss, and thus the more it will be damped. They found something similar with the wooden backing (usually teak) given to early iron armour plating in naval vessels during the 1860s - 70s. Many thin layers (with the grain travelling in the same direction if you want to get really precise) worked better than a lesser number of thicker layers.

[Cobblers]

It's a thorny one.

Small amounts of damping will supposedly just move the resonances to a different frequency and slow energy release.

Hence perhaps the best results are achieved by 2 different extremes.

Massive damping via massive build such as in large high end designs such as Mission Pilastro or high end Focal speakers (too expensive, costly and complex for DIY).
Or lossy panels with light damping such as Harbeth.

The LV Auditoriums sound fantastic with their chipboard cab's.

Cabinet material would probably be less important in the aperiodic design
WD25's (compared to closed box and especially reflex) since the aperiodic vent is releasing much of the energy within the cab's.
Certainly cab colouration is not noticeable in isolation, despite lowly MDF .
I found adding the damping compound to be a subtle difference in that design, which became more obvious when playing loudly.

[Scottmoose]

There's basically two options when dealing with panel resonances (not automatically the same as cabinet colouration -the two are linked of course, but the latter is more general & encompasses reflections back through the cone, vent noise & lots of other things). You can take the high-mass approach, & push panel resonance down, below the passband of cabinet operation, or you can attempt to push it up out of the critical zone. The latter usually causes less problems as the stiffer, lower mass panels aren't storing energy -higher the mass, the more is stored. Bracing helps a lot of course. It's really all about controlling resonance, because it's impossible to completely erradicate, and if you could, there'd almost certainly be a price to pay elsewhere.

[john & Jake the dog]

Hi Scott,
As I am still awaiting the Hemp drivers I have read this thread with interest.
Could you please give me your ultimate cabinet material (for the box you modelled) as I am only going to build these once and for keeps.
cheers

[pre65]

Hi-from what Peter was saying on this months newsletter it would seem chipboard is the preferred material performance wise (and cost !) with an outer layer of MDF to give you a barrier layer and make them easier to finish (ie look good).

I remember in my youth,making some chipboard cabinets for the Kef Kit3 speakers and they were a bugger to finish off.Still got the cabinets,they now support my worktop in the conservatory !


Philip