Calculating Volume of an Irregular Shape
When designing and constructing an enclosure, it is not always possible to build an enclosure that resembles a regular shape. At times, the vehicle requires you to build a shape that forms to the interior, like the spare tire well. Calculating the volume of this location can be difficult.
In the past, installers would use several methods to calculate the shape of enclosures such as these. One method would be to divide the enclosure into several basic shapes. In irregular types of enclosures, this will only give you an approximation of the actual size. Not very accurate. Another method would be to fill the enclosure with some material (popcorn, sand, water?), empty the contents into a basic shape enclosure and measure the amount. This method gives better results but the accuracy will depend on the compressibility of the material. Not to mention this can be real messy.
Another method, not commonly used can be extremely accurate. It involves using a sine wave generator and a test fixture. Don’t worry, it is not that complicated. The procedure is to build a test plate with a woofer and a port and to install this plate over the woofer hole, sweep the enclosure and find the tuning frequency and look up the tuning frequency on the reference chart and presto the actual enclosure volume.
Step by step instructions
Tools Needed:
-Sine Wave Generator or CD player and CD #101
-Power amplifier · Test Speaker (6”-8” woofer
works well)
-Test Port (Choose diameter and length from chart)
-Test Plate for the speaker and the port.
Step1:
Construct the enclosure. Sounds easy so far.
back 
topStep2:
Choose test port. Refer to charts on following page for test ports. Charts support Port with a diameter of 2” to 4” and a length of 2” to 10”. Choose a port based on what you think the enclosure volume will be. For example: You think the enclosure is a cubic foot. A good rule of thumb is to choose a port length that will give a tuning frequency around 40 Hz. In this case, a 2” port 4” long will work well.
Step 3
Build the test plate for both the woofer and the port. Make sure the test plate you build covers the woofer hole and does not restrict the airflow around the woofer of the port.
Step 4
Place test plate on enclosure. Make sure you have an airtight seal. Any leaks will drastically affect your accuracy.
Using the sine wave generator or CD #101 (Tracks 10 through 98 have sine wave outputs corresponding to the track number. This disc is available from Autosound 2000™), sweep the enclosure and find the frequency where the woofer stops moving and large amounts of air rushes from the port. HINT: Slightly above and below the tuning frequency to woofer will move much more.
Once you have determined the tuning frequency, look up the tuning frequency on the chart corresponding to the test port you have chosen. EX. You used a 3” test port 6” long and determined the tuning frequency to be 37 Hz. Looking on the chart we see that this corresponds to an enclosure volume of 1.70 cubic feet. That’s it. Simple Quick and easy. Good Luck.
Charts coming soon
Volume
of a rectangle
