Hi,
la toilette said:
Next up, what materials shall I use? I'm doing this on a relatively tight budget of maybe £100. Is ply best, if so what type (B&Q cheapo/marine/baltic birch etc), or is MDF perfectly acceptable?
Also, looking at various diy cabs I've seen wall thicknesses that vary from 15mm to 50mm, does thicker really = better or is there an optimum thickness?
A German magazine made an exhaustive study of this ages ago:
http://www.exdreamaudio.de/Boxen-Baumaterial/boxen-baumaterial.html
A brief summary:
1) 19mm MDF (7.5kg):
The upper response trace is the signal before inserting the test wall into a speaker (so the actual stimulus) the lower is the output after inserting the testwall. Observable is a main resonance around 170Hz which takes long to decay, as the "lossy" mechanism in MDF includes energy storage.
2) 18mm Plywood (6kg):
Same as above, we note that despite having less mass (which would suggest a high resonance) the resonance with plywood has shifted down to 150Hz suggetsing less stiffness, but more crucially, the decay of the resonance is much quicker and very rapid initially and thus suggest a loss mechnism without much energy storage.
3) 44mm coarse fibre board (a little coarser than MDF) 14kG:
The much thicker and heavier material isolates a little better and again due to the coarse and heterogenous structure decay is rapid.
4) For reference - 18mm plywood, 12mm sandfilling, 16mm plywood sandwich (18.5kg):
Only marginally better than cheap, generic thick woodfibre board for a lot of effort.
5) For reference - 20mm Marble (26kg):
Overall, as we can see, attenuation and suppression of the resonance somwhere between 100Hz and 300Hz (paradoxically the resonance seems to go up with added weight, but instead it is actually the increased stiffness from increasing the acthive thickness that does it and outweights the added weight) seems mainly related to weight and thickness (few surprises).
Equally though, going from 19mm MDF to 44mm coarse fibre board does not seem to bring much improvement.
The only real surprise is that Plywood (good grade) decays much quicker than more amorphous and homogenus materials, meaning despite a strong initial resonance peak when excited the audibility of this will be much less as (unlike MDF) Plywood seems to "drone on" much less long.
Okay, what if we apply simple re-inforcements (10cm 18mm board glued on plate edge on):
No material improvement.
How about bitumen Damping Mats (quite expensive in Car Audio Shops):
No material improvement. You need to go to the length the BBC did where they used equal thickness of Bitumen and Plywood to see real improvements.
What if we brace the plate against the opposite board like this?
This is a starting point BTW to the matrix bracing B&W use and which I also recommend.
Wow, now we are cooking with GAS! This is WAY better than 44mm of the same stuff!
What is the upshot?
At similar thickness all materials (except stone and metal - metal was not tested, I used this myself before) resonante in a similar fashion. Plywood will have a less audible and objectionable resonance due to it's lossy nature, if you use a really good grade, (building grade plywood is bad though - see figure 7).
Damping mats and the like improve little. Bracing to opposite walls majorly improves things. If you tie the bracing together in the center of the box so all walls connect together (matrix style) thing get even better.
So, use sensible thickness of Plywood (18 - 22mm) of good grade and brace the heck out of the box will result in the most economical solution.
"Excotic" enclosure materials such as stone and metal are vastly superior but usually not applicable in DIY.
I hope this helps.
Ciao T
):-
When you going to make a dedicated website for those Shin? They deserve one.
