Could someone please check my enclosure math?

Hey all, I am having a hard time understanding why my box calculations come out so wide, and might need some help. In comparison to the way Mr. Marv's box for his JL 13TW5 looks from pictures, did he shrink the box volume, while changing the speaker performance as a tradeoff?

Constraints - Not removing child anchors, and not removing the bin mounting tabs. I want to replace the bins eventually if I ever sell or trade the truck. Maybe this is why my box needs to be wider? I also need to leave room for two Alpine PDX amps to be mounted side by side. Also, I don't want penthouse-volume hanging beyond the normal speaker box...the amps would then require mounting to the box, which I don't want to do (vibes prematurely kill electronics).

Box Calculator I'm using (trapezoidal): http://www.hometheatershack.com/forums/enclosure-volume-calculator/

Height = 18.5"
Top Depth = 3.5"
Bottom Depth = 7.5"
Width = 28"
Thickness = 0.75"

JL 13TW5V2 Driver Volume = 0.052 cubic ft

My Notch (internal) measurements: 2.125" x 5.375" x 28" = 319.8125 cubic inches. I then input this into the calculator as either a bracing or a port volume (as the straight up trapezoidal box calculation already has the material thickness removed without this notch. The notch is like a big brick consuming volume, just like bracing or a port). For any variation in width, I change the width variable and manually recalculate, then re-enter into the volume calculator as either bracing or a port.

The result Volume is 0.806 cubic ft, which is right on for this sub. (JL's volume recommendation is 0.8 cubic ft)

Are my calculations and assumptions on the notch correct? If so then 28" wide it is I guess..

 

As far as the math goes...
Your calculation for the first formula is correct. 319.8125 cubic inches.
319.8125 cubic inches equals .18508 cubic ft.

Where do you get the result 0.806 cubic ft from?

 

I received the result straight off the Calculator:


(meant to attach this earlier, but the website wasn't 'seeing' the file I wanted to select). I resorted to using one of these calculators because I got tired of switching between the computer's calculator, forum thread's, etc, then having to convert cubic inch to cubic feet and back. I was also doing lots of manual calculation as well, by separating by determining the 3 different side areas, adding those together, then multiplying by the width. Calculator makes it much easier to make minor adjustments, and it takes care of material thickness for you.

 

Ok now I understand what you are looking for.
The math looks good.

 

Thanks!

At 28" wide, I'm left with a little more than 1" width clearance if I were to mount two PDX's side by side.

 

I built up the box above on Saturday. I still haven't test fit it, but I'm pretty confident it fits since my final box measurements are spot on to my drawing.

Once I confirm fitment, I plan on sharing my drawing and cut plans.

 

looks to be accurate,
Try using .5 birch instead of MDF. Its just as strong as .75 mdf , lighter and easier to work with plus the finish is gorgeous. You will also gain some additional volume by doing so. You won't need any additional bracing either

 

Mr Marv's box is built in the sealed box configuration... You are doing the ported version which requires more airspace. Unless I misread your post. I know you are taking in account for the restraints and stuff.

 

I built up a sealed box - the calculator I found directly subtracts both bracing and port volume from the internal volume. Its somewhat confusing because the calculator site shows a stock picture with a port.

 

I was waiting for some one to bring this up hahaha. Its almost a "Gotcha"

 

actually.... thats what i get for answering a post while trying to go to bed. the brain had already commited to sleeping versus trying to process the post correctly. lol

JL even specs this out for only being in a sealed enclosure.

my brain got hung up on the "notch" calculation. i was wondering why is was so big.... then i realized when i was taking my measurements i had removed all the problem areas from my truck. he is leaving them in.

 

I think you are actually an 1 inch short on the width mesaurement.

JL Recommended box spec
0.800 Cubic feet
Driver displacement
0.052 cubic feet
Notch displacement
0.185 Cubic feet
Total volume needed
1.037 Cubic feet

 

I substituted the internal notch volume, which is 2.125" x 5.375" x 28" = 319.8125 cubic inches, in for bracing volume. My straight up trapezoid box, without the notch, is 1.043 cubic feet.

1.043 - .052 (driver) - .1851(notch) = 0.8059 cubic feet .

 

Editted this post:

Nevermind - i rechecked and corrected myself. I was slightly off though, luckily its not detrimental.

2.125 x 5.375 x 26.5 (not 28"!!) = 302.67968 . (EDITED)--Putting this into the calculator yields 0.816 cubic feet (with the same external 28" length).

This leaves me a tiny bit of volume for dowel rod braces, running front to back.

 

The recommended internal box volume (inculding driver displacement) from JL
0.854 cubic feet
adding the notch displacement
0.175 cubic feet
Total internal volume needed
1.029 cubic feet

1.029 - 0.175 - 0.052 = 0.802 final volume

by no means am i challenging you on this. i am making sure my math is correct too... lol

 

that 1 inch difference means roughly 0.03 less cubic feet.

Even i wouldn't bother redoing it. i would run what you made already.

ive been running one of Mr Marv's 10" sealed boxes for years now. was going to make a ported box but just haven't gotten around to doing it.