Description

This article is from the Car Audio FAQ, by Ian D. Bjorhovde (ianbjor@mobileaudio.com) with numerous contributions by others.

4.13 I know the box volume required for my subwoofer, but what are the best dimensions for my enclosure? [IDB]

The specific dimensions of a subwoofer enclosure aren't really
important. Once you know the appropriate volume of the box, and you
know where in your car you want to install it, you will have some idea
of the restrictions in the dimensions. For example, if the distance
between the floor of your trunk and the bottom of the rear deck is 16",
then you probably shouldn't make your box any taller than 16".
Likewise, if width of the trunk (between the wheel wells is 38", then
you've got that much space to work with.

You can also infer some other information about your box, from the
speaker specifications (for a PPI PC10):

          Mounting Depth:         4.5625"
          Speaker Displacement:   .032 ft^3

Here is an ASCII drawing of a subwoofer enclosure to help illustrate a
few things:

           |--- L ---|
           __________     ___
          /         /|     |
         /     +   / |     |         + = Center of speaker
        / x       /  |     D         x = center of port
       /_________/   |     |
       |         |   |    _|_        W = Width of box
       |         |  /     /          L = Length of box
       |         | /     W           D = Depth of box
       |_________|/    _/_

Because of the speaker's mounting depth, you know that the box MUST be
at 5.5" deep (it's always good to leave at least 1" of space behind the
speaker, but leave more if you can). If you use a straight port (3"
diameter PVC) then the box will need to be at least 13" deep (leaving 2
inches between the end of the port and the back of other side of the
enclosure), assuming that your port will reside completely within the
enclosure. If you use Flex-Port or choose to have a portion of the
port extending outside of the enclosure, you can make the box less deep.

We also know that with a 10" woofer, the length and width should both
be a minimum of 12" (leave 1" on either side of the woofer). Obviously
both dimensions can't be 12" otherwise we can't put the port on the
same face as the speaker.

With volume, remember a couple of things. Recommended volume is the Net
Internal volume. Both the speaker and the port will displace volume
from the box - this means that you MUST account for these (i.e., make
the interior volume of the box larger than the recommended value). In
this case, you know the following:

          Required Volume:       1.25 ft^3
          Speaker Displacement:  0.032 ft^3
          Port Displacement:     ??

3" (interior diameter) PVC will usually have an exterior diameter of
3.5" (0.25" wall thickness). Volume of a cylinder = pi*r^2*h, where r
= 1.75" and h=12.25" (13" port length - 0.75" for the thickness of the
MDF). Therefore the port displacement is: 117.9 in^3 or 0.068 ft^3
(1728 in^3 = 1 ft^3).

          Required Volume:       1.25 ft^3
          Speaker Displacement:  0.032 ft^3
          Port Displacement:     0.068 ft^3
 
          TOTAL INTERIOR VOLUME: 1.35 ft^3 or 2332.8 in^3

Now, to calculate the dimensions of the box: For a rectangular box, L
x W x D = Volume.

If we know that the minimum interior depth is 15" (because the port is
13" and you should leave 2" between the port and the wall of the
enclosure), we can say,

          L x W x 15"  = 2332.8 in^3   or
          L x W        = 155.52 in^2

Since we know that L must be at least 12" (since the speaker is 10" in
diameter), we can continue:

          12" x W      = 155.52 in^2
          W            = 12.96"

So, we have determined that the interior dimensions of the box must be
12" x 12.96" x 15". Obviously this can't work, since there is nowhere
to put the port! You will either have to have the port extend outside
of the enclosure, or use flex-port. Keep in mind that if you have part
of the port "sticking out" of the enclosure, the Port Displacement will
also change!!.

For simplicity, (in this example) let's use flex-port. This will let
you decrease the thickness of your box to about 6". So,

          L x W x 6"  = 2332.8 in^3
          L x W       = 388.8 in^2
          12" x W     = 388.8 in^2
          W           = 32.4"

This is a little more reasonable. This will be a flat, wide box.
Again, keep in mind that these are internal dimensions. If you use
3/4" MDF (and you should) to build the enclosure, this will add 1.5" to
each of these figures for the external dimensions. e.g.:

         13.5" x 33.9" x 7.5"

Remember, the 6" depth and 12" length are just the minimum values, you
can alter them if, for example, you don't have enough space between your
wheel wells for a 34" box. If you only have, say, 28", then:

          maximum external width = 28"
          maximum internal width = 26.5"
 
          L x W x D        = 2332.8 in^3
          L x 26.5" x 6"   = 2332.8 in^3
          L                = 14.7"  (internal)

Got it? It can be a pain, but that's part of the fun in building a
speaker enclosure.

 

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