W-Body Car Audio

Aperiodic Enclosure Subwoofer

This isn't really an enclosure per se. It's an enclosure with a resistive port. Basically you are using a controlled choke on the woofer to yield a flat impedance curve. The same advantages apply to infinite baffle subwoofers in that you will have lots of trunk space and you need to seal the listening area from the rearward wave.

You must have the following to make this work:

• a woofer with Qts less than .45
• a digital multimeter (it's easier than using an analog meter)
• a 10k resistor
• a tone generator or test tone CD

Your enclosure can work one of two ways:

1. You make the resistive port cover the whole woofer cone, with the back of the woofer playing into the listening space
2. You make the resistive port cover the rear side of the woofer cone, letting the front play into the cabin as nature intended. This will allow you to design a smaller port with a less resistive material.

Resistive material is easy to come by. Any hardware store should carry wall insulation, just be sure to get the yellow stuff, since it comes apart in layers easier than the pink stuff. Get some window screen to sandwich the insulation with, as well as some MDF rings in the needed diameters

How much insulation should you use? Depends. If yoiu're going to cover the front of the cone, you should start with 2" uncompressed in the same diameter as the woofer. If you're going to allow the cone to play into the listening area and vent through the rear, then 1" thick uncompressed in half the diameter of the woofer cone should suffice.

Building the structure should be easy, make it as small as possible, while leaving room for cone movement (if you're front loading it) or pole piece venting (if you're rear loading it). Make the resistive port by sandwiching the insulation between the screens and MDF. (You should consider gluing the screen to the MDF).

Just like a ported enclosure, best results will come from tuning. Here's where the 10k resistor and multimeter (set to AC Voltage) come into play. Connect the 10K resistor between the amp and the + terminal on the speaker. Then connect the voltmeter at the amp terminals. Use the tone generator (or test tracks) and make a note of the reading. Thanks to the 10K resistor, the voltmeter will be reading ohms. It should read flat through the speaker's range. If the resistance is low, add more layers of insulation. If the resistance is reading high, remove some insulation. It will take time, but this subwoofer alignment will add to your listening pleasure. (Note: before you play music, be sure to remove the 10k resistor.)

Pros:

• Near-perfect sonic reproduction
• Won't use as much trunk space as "box" designs will

Cons:

• Time to tune the system.
• The trunk must be sealed from the passenger area
• Eats up lots of power
• Reduced SPL in the name of musical accuracy.

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