more bracing needed???

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wintermute

Just another Moderator
Paid Member
2003-08-03 11:43 am
Sydney
Hi everyone,

I put together the first of my new mtm boxes today, each driver has a separate compartment and the midbass enclosures are 5.5L (without bracing) for a nominally 5L box. I have 1" on a side (actually looking at my pic I think it is 19mm.... hmm) quarter round in the corners top and bottom as glue blocks, and a 1" square piece of MDF in each midbass enclosure to act as an extra surface for the Baffle to glue onto.... the MDF is all 1" (25mm) except the partitions in the middle of the box which are made up of a couple of laminated pieces of 12mm MDF (reason being I had to make the pieces on the tweeter side shorter to allow me to mount the tweeter).

Anyway I was thinking about using some 1/2" dowel cross ways and perhaps top to bottom, but after looking at the boxes I'm thinking maybe no additional bracing is really necessary....

The internal dimentions (in mm) are 151 W X 211 D X 173 H (for each mid encolosure) the overall side length is 443mm not counting the top and bottom MDF pieces, but it effectively has 2 braces at 173mm in the form of the partitions. Oh and there will be a brace between the two partitions in the tweeter compartment as well... you can see one half shoved in in the pic (it is a bit tight and needs to be sanded a little).

So.... In your opinion do I need to add any more bracing, or just run with it the way it is in the pic :)

Tony.
 

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wintermute

Just another Moderator
Paid Member
2003-08-03 11:43 am
Sydney
My Old speakers have the worst cabinets imaginable (I didn't make them) they are 70L boxes made out of 18mm chipboard and only have glue blocks in the corners, no other form of bracing... so I have gone overkill the other way ;) I could have got away with 18mm MDF but decided to go the whole hog and use 25mm :)

I think ( based on a couple of accoustic tests) I have some gremlins to work out, but hopefully nothing that adequate damping wont fix. I'm getting a deep dip at 2300Hz,, which wasn't present when testing the drivers on the baffle alone, so I need to do a bit of investigating!

Tony.

edit: a quick check with Vikash's standing wave calculator and I get

Mode Height Width Depth

1 991 Hz 1.14 KHz 813 Hz
2 1.98 KHz 2.29 KHz 1.63 KHz
3 2.97 KHz 3.43 KHz 2.44 KHz
4 3.97 KHz 4.57 KHz 3.25 KHz
5 4.96 KHz 5.72 KHz 4.06 KHz
6 5.95 KHz 6.86 KHz 4.88 KHz
7 6.94 KHz 8.00 KHz 5.69 KHz
8 7.93 KHz 9.15 KHz 6.50 KHz

so it looks like maybe I should be looking at damping the side walls :) since the second mode corresponds pretty much with my problem freqency :) I'd only been trying damping material on the top bottom and back before.... edit note that this calc is based on 150mm width instead of the deign of 151mm due to me not checking my MDF thickness which was actually 25.5mm instead of 25.....
 
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