Back to topic - why to use a few layers of thin MDF?
Coincidence frequency goes down when thickness of a material rises - 19mm MDF is significant worse as 10mm at 1,6kHz (but better in most other areas of course). At 18-25mm thickness this effect is very strong - and these are the usual enclosures.
A better solution would be to use 8-10mm layers and put e.g. greenglue in between for dampening. Just useing "hard" glue will not help tooo much cause you create a stiff pannel again.
Coincidence frequency goes down when thickness of a material rises - 19mm MDF is significant worse as 10mm at 1,6kHz (but better in most other areas of course). At 18-25mm thickness this effect is very strong - and these are the usual enclosures.
A better solution would be to use 8-10mm layers and put e.g. greenglue in between for dampening. Just useing "hard" glue will not help tooo much cause you create a stiff pannel again.
Agreed 100%, yet people still mostly build flexy cuboid boxes then try to stiffen the hell out of them to mask their huge weaknesses - it's an entirely inelegant engineering solution, but I confess that I used to do it too! Never again - there are considerably better (stiffer) shapes available!
I'm not disagreeing from that particular perspective, but it seems to be based on the assumption that a panel with high rigidity is a bad thing.
Getting into mastik territories now. Some have been developed for the motorhome/caravan industry that always stay flexible. I've always fancied building a laminate of several thin ply sheets with thin cork sheets bounded in between them.
I would be so grateful if I could hear someone say the difference in sound between a box made of MDF and birch plywood? What do you get for the price difference? 4 times more cost.
Soft polyurethane is good. It feels like marshmallow when set and is effective at damping.
its not clear to me what to kill the acoustics means
The idea is that the only surfaces who makes the sound should be the drivers diaphragms
The cabinet faces if they do not move they not generate sounds If a speker cabinet resonates adds something to the sound And this is not good in general
Lets take Audio Note famous for using resonating cabinets I would like to try the same drivers and crossover in a non resonating cabinet
Why they should sound worse ? a loudspeaker is not a musical instrument It is reproducing a sound not making a sound
almost all very extreme subwoofers have metal cabinets And often also metal drivers Just to say
interesting but extremely complex to make Would not be better using just two materials
one that provides stiffness and the other that provides dampening ?
steel for instance is extremely stiff and MDF has very good dampening properties To name two materials that can bolted or glued together
A great material for dampening could be the plastic one used in layered bullet proof glasses And i think only a very thin layer can be very effective
https://matterchatter.files.wordpress.com/2012/04/normalvsbulletproofglass1.jpg
instead of glass a metal would give the needed stiffness
The best way to use it is to sandwich it between two equal pieces of something, maybe MDF. If you drop a hammer on MDF it rings, but if you do this on a polyurethane constrained layer damped panel you only get a soft dull thud that goes quiet straight away.
Now that, in my 40+ years in electroacoustics, I have never seen!
There isn't a single answer to that as it depends on the box design, the panel dimensions & thicknesses & how it's used. Speaking broadly however, for a given set of dimensions a material with a lower MOE is likely to have an Fs (& therefore its main harmonic modes) at a lower frequency -which in many cases, especially with bass enclosures, means in a region where more energy is available to excite those resonances. Result is a time-delayed panel resonance at that frequency which as a sub-emitter lobes with the direct radiation from the driver & can cause partial cancellation at that point / those points. Insert your favourite subjective audiophile verbiage here about the resulting characteristics. 😉 Suffice it to say -it's not ideal. You can compensate to a degree by taking the BBC type approach of thin walls, partly or entirely damped with bitumen / car body damping panels (and preferably discontinuities from screwed-on front & rear baffles [think 'cracked bell'] ) and this works to a point but it can take some effort with the measurement and material selections to optimise. The LS3/5a for example requires (as in requires) beech internal bats for mounting the front baffle. Other materials they tried altered the resonant frequency of the panels to a point where they became audible, without excessive fooling around with more & more damping & other materials, which in turn altered the rigidity once again, which in turn... you get the idea. No doubt there are alternatives -but how long do you want to spend trying to find them?
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As far as subwoofers go, metal cabinets = 'I rest my case'. 😉 Ideal for subwoofers. Alloy cones also = 'I rest my case'. Equally ideal for subwoofers. 😉
In fact, I rest my case (m'lud)
When I was at school, I got hit on the head with a softball, I've been a soft dull thud ever since.
If we take the example of a flexible but damped enclosure of something like the BBC LS35a, is it possible that the flex could work like an aperiodic port, the movement may be tiny in comparison to the cone movement, but the surface area is so much larger, surely the tiny movement would be enough.