Current car subwoofer theory suggests that the best type of design for in-car use is a small sealed box, based on the observation that the 12dB/octave rolloff of a sealed box is a perfect match for the car's "cabin-gain", which is said to provide a free 12dB/octave to any subwoofer design, starting from a corner frequency that is dependent on the size of the cabin (the larger the cabin, the lower the corner frequency). Typically this corner frequency lies between 60 Hz and 80 Hz.

There are some problems with this "theory" though.  First of all, cabin gain isn't an exact 12dB/octave - there is some loss at lower frequencies as the cabin isn't perfectly sealed.  Secondly, there is the problem of the masking effect of road noise (the main reason why a car audio system that's tweaked to have a perfectly flat response sounds thin on the road).  

According to my personal observations, ideally you want to achieve an in-car bass response that displays a gain of  around 6dB/octave as the frequency drops below the corner frequency (or slightly above).  To my ears, this produces the most natural sound in your car.  To achieve this particular response though, you are left with two options:

1. Use a sealed box design and EQ
2. Design vented (or other) box using the final in-car response as the target response.

"Maximally-Flat" designs
The problem with option (2) above is that many box design programs will provide you with only a "maximally-flat" vented alignment for any given driver.  Very few programs will allow you to predict what the in-car performance of the system will be (and I guarantee it will be VERY DIFFERENT to what such programs tell you), and even less can generate an alignment based on a target response curve.

Designing with Cabin Gain in mind
Recently, I redesigned my ported.xls spreadsheet so that it can be used to predict in-car frequency response.  I then used the "Comparisons" section to observe the effects that different box sizes and tuning had on subwoofer alignments using my two JBL 1200GTi subs (retrieved from my last car audio subwoofer design).  A couple of things became readily apparent:

1. To achieve a flat in-car response down to frequency F, Fb for the alignment must be approximately For lower.  In other words, if I wanted to achieve response down to 20 Hz, I had to start with Fb=20 Hz.
2. Box size is the primary determinant for the minimum value for Fb.   For example, my spreadsheet told me that I could achieve almost ruler-flat response from 12 Hz to 70 Hz if I use a 1.06 cu.ft. box tuned to 27 Hz.  However, tuning a box that small to that particular Fb would involve the use of a port that's too small in diameter.  The only alternatives are to settle for a larger box size, or a higher Fb.

The approach to designing the box was then as follows:

1. Select an initial Fb, based on performance requirements. For my project,  I selected the intial Fb to be 20 Hz.
2. Select the minimum minimum port diameter and maximum port length that you're prepared to use to achieve Fb.  In my case, that worked out to a 3" port with an effective length of 18" (basically one of the 3" flared ports from www.madisound.com ).
3. Determine the minimum box size, based on the port specifications.  In my case, that worked out to to be 2.35 cu.ft. - a rather large box.
4. Using the "frequency response with cabin gain" graph in my ported.xls spreadsheet, adjust box size and Fb until an acceptable response was displayed. The alignment I arrived at was a 1.86 cu.ft. box tuned to 22.5 Hz.  For this smaller box, I sacrificed about 1.5 dB of output at 20 Hz , which I'm sure wouldn't bother me much.

Construction
The next step was then to build the box.  I decided to go for a design that would maximize the use of the trunk's floor space (which would in turn produce a box with low height).  Below are some pictures of the enclosure that I put together:

Basically, I measured the available floor area, then used this and some algebra to arrive at the correct height for the box to achieve Vb=1.89 cu.ft.  (the additional 0.03 cu.ft. will be occupied by braces, ports and the drivers themselves). 

Impedance Response
The graph below shows the impedance response of the woofers in the enclosure.   If you ignore the small peak below 20 Hz, it looks like an impedance response for a sealed system.  For in-house use, this would probably be described as a badly-tuned vented system, for for car use it seems just fine.

Below is a graph depicting the system's predicted frequency response.   The light-colored curves depict the system's in-car response, assuming no losses.

Results
I was quite happy with the sound of this subwoofer.  It sounded like a well-constructed sealed box, with excellent low end extension. It certainly did not sound like a vented box.

Comments or suggestions?
If you've got any comments or suggestions, please e-mail them to me, thanks!

Links

• Measuring a car's transfer function - Jason Cuadra