Speaker box damping and composites

MDF is a cheap and easy to use box material. It works quite well for sub boxes when adequately braced, but it’s far from ideal for full range speakers due to the resonances around 300 Hz. Plywood is stronger and lighter and is considered by many to have better acoustic properties, but it can tend to ring.

The ideal enclosure for a full range speaker box is first of all very strong and stiff. This pushes resonances up higher in frequency where they can then be damped and more easily reduced.

Here is a quick ‘n dirty test of some different composite options, each evaluated by the sound of a small panel when knocked. This isn’t the ultimate test, but it does give some indication of performance.

The MDF and ply samples both had an obvious ring in their raw state, the ply actually being slightly worse in that regard.

Best option
4 layers of 3mm MDF were curved and laminated together with liquid nails to a total thickness of 15mm. Cornice adhesive reinforced with perforated metal was added to half of the sample.

Knock test:

MDF and ply were sandwiched together. In one part of the sample, cornice adhesive and pegboard were added instead of the MDF. Performance was about the same.

Pegboard and cornice adhesive was added to ply.

A composite was made with a polystyrene core connected to an outer layer of pegboard with cornice adhesive and perforated metal reinforcement.

 MDF and ply made a good combination, a dramatic improvement over each on their own and also better than a double thickness of each. Their different properties appeared to combine well. A small amount of damping is offered by cornice adhesive applied to ply sufficient the damp the ringing. The polystyrene core composite didn't perform well with pegboard - it lacked inherent rigidity. Curve laminated MDF with cornice adhesive was the best option. 


Update


The next day I discovered the cornice adhesive bonds poorly in all cases. I may have been a little impatient since this stuff needs time to fully harden when thick, but I found that in all cases it didn't hold very well. So in the end the most effective turned out to be the ply/MDF combination. I will be investigating other damping methods.