Is my math off?

MrDHEJ

I got the wood today to start building my sub box "3/4 cabinet grade birch plywood" for my Polk MM124 sub, and I’m sitting here calculating the dimensions to fit in my truck to get 0.88, so the measurements I get are 7.5"x16.5"x18.5" outside deminsion, which gives me 17"x15"x6" internal, and that comes to 0.885 cubic feet.

So to make sure 6" depth was going to be ok I pulled up the specs again for the MM124 on Polk, and ran into their sub box plans for this sub http://www.polkaudio.com/car/toolbox/subboxplans/subbox_plan.php?id=61 . They are saying the overall demensions should be 17"x15"x10", with internal demensions at 15.5"x13.5"x8.5" my calculations put this box at 1.029 cubic feet.

Now they say use minimal polyfill, is that suppose to get it back down to 0.88? Or am I calculating the volume wrong. Or did they misprint it.

RuSsMaN

Fill will typically make the speaker think its in a *bigger* box than it actually is. I think Polk put 'minimal' fill to A. avoid that, and B. to help absorb/cancel the back wave (sound) from the driver inside the box.

Perhaps the plans take into account the displacement of the driver *in* the box, so after the driver is installed, it is .88ft?

Cheers,
Russ

MrDHEJ

Could be russman, wonder if a Polkmeister could step in and clarify this.

Another user stated they take the sub volume already into account, so when they say 0.88 they mean 0.88 total box volume.

RuSsMaN

Originally posted by MrDHEJ
Another user stated they take the sub volume already into account, so when they say 0.88 they mean 0.88 total box volume.

That makes perfect sense to me, as far as what they *should* do. Is there internal bracing that takes up additional space perhaps?

I did as what you stated intially for my EX10 (purchased a box, didn't build it - but followed the same rule).

I'm sure an official repsonse will appear monday.

Cheers,
Russ

sntnsupermen131

ok, youre external dimensions are 7.5"x16.5"x18.5"
the wood is 3/4 in thick
so that leaves you with 6.75x15.75x17.75
which is .5625'x1.3125x1.4791666=1.0920408ft^3
-Cody

sntnsupermen131

to further explain
take your measurements...subtract each one by .75 to make up for the thickness of the wood
then divide each indvidual number by 12
that converts your measurements from inches to feet
multiply those 3 numbers together, that gives you your total volume
you want to make the box to .88, displacement has already been accounted for
its better to have a smaller box than a larger one
poly fil is used to make your box seem bigger and to keep unwanted resonance from happening
for poly fill, go to hobby lobby, walmart etc, its a lot cheaper there than at speaker places, and its the same thing
-Cody

MrDHEJ

You have to take 1.5" off of the measurement. Remember there are 2 peices of 3/4" wood for each measurement.

sntnsupermen131

ouch, horrible mistake on my part, yes, .885 is correct
-Cody

CPHILHOWER

Silly question here. Why are you using Plywood? It would be stronger to use MDF or ParticleBoard.
Chris

sntnsupermen131

that type of plywood is sposed to be some of the best speaker box building material out there...so soes Jstas and PBD, i wouldnt use paritcle board for components, much less subs...
-Cody

MrDHEJ

The 3/4 cabinet grade birch plywood is in the sence a solid peice of wood only stronger, and stiffer. There are absolutly no knots in this sheet. You pay for this perfection in cost, at $58.00 a sheet it'll sting some. I got lucky and got the last peice they had, which was on the bottom and one corner had been chewed up by a forklift. So they gave it to me for $15.00

PoweredByDodge

cabinet grade birch ply is an amazing box building material... as sonically "good" as MDF, but about a zillion times more rigid and durable... and it doesn't split when u screw into it.

you can throw it off a buidling and it'll be fine.

Thom said he had to smash apart his polk test boxes with a sledge hammer and still the joints wouldn't come apart (when liquid-nailed together)... can't say that abotu MDF.