These traits are very important to the design of an SPL enclosure. However, the key element of this design is the rear sound wave interaction, better yet, the coupling with the front wave of the speaker. These two waveforms are naturally out of phase with each other (figure 1) and can cancel one another out in a free-air setting. The idea is to get the rear sound wave in phase with the front sound wave. This can be accomplished by the design of the enclosure and will yield a significant increase in output that is ideal for the SPL lanes (figure 2).
Unlike a normal ported enclosure that is built for a broad range of frequencies, an SPL enclosure is designed to be more "frequency specific." In other words, sound quality is not a priority in the design of these earth-shaking mutants. Rather, the concentration will be to produce a one-hit (frequency) wonder. This will likely require a substantially sized enclosure and the port or vent to be commonly larger than the Sd (cone area) of the subwoofer.
The Design Process
The first step in designing an SPL enclosure is finding the frequency that will work best in your vehicle. But how do you know what is the correct frequency to use? This is not as complicated as it may seem.
A single frequency sound wave will take the form of a sine wave. This sound wave can be measured or equated by using a simple formula. Sound waves are normally measured in feet, but since we are working in the small confinements of an automobile, it will be more practical to show this in inches. This means you must first start by converting the speed of sound, 1130 ft/sec., into inches. This can be accomplished by simply multiplying 1130 x 12 for a result of 13560 in./sec. (figure 3).
Now let's start the design of the enclosure within the vehicle. Using the rules for dB Drag Racing (Super Street and Extreme competition), we have the following: "All loudspeaker enclosures, and/or baffle boards, with the exception of those mounted in the kick panels or doors, shall be located behind an imaginary plane that stretches from the trailing edge of the driver's door to the trailing edge of the passenger door". With this in mind, placement of the enclosure will likely be positioned slightly behind the door jam, as shown in figure 2.
In figure 2, there are four lines marked A, B, C and D. Since line A is technically in-phase, we will start with this. Line A relates to the measurement from the speaker cone to the point of reference at the dash where the microphone is placed during competition. Let's use a nice round number for this length, say 50 in.. Knowing this we can now calculate the frequency, employing the equation in figure 4a. At 50 in., the equation tells us that the full waveform at the dash will be 271Hz. Unfortunately, this will not fly in dB Drag Racing where the rules state that the frequency used in the competition must be 80Hz or below. So how do we get the frequency from 271Hz below the 80Hz mark? If we break down a sound wave into four sections, 90/180/270/360 degrees, it is possible to lower our peak frequency. In this case using one-quarter of the waveform, 90 degrees, our peak frequency will lower to a very usable 68Hz (see figure 4b). Perfect!