Now that we know that 68Hz is going to be our peak frequency, it is time to hit the computer for our enclosure volume and port dimensions. There are several enclosure programs available that can perform this task. LEAP by LinearX is far superior to most because it takes into consideration the behavior of a loudspeaker at high power levels.. But others such as Termpro and BassBox Pro do an excellent job as well. The goal of designing the enclosure is to create the loudest peak possible near the one-quarter wave frequency (see figure 5). Be prepared, the design of the enclosure may take some time; this is basically trial and error to find the correct volume, and an estimated port area and length.
Once the enclosure volume and port are decided, the actual design must take place, the most integral part of the process; so we are certainly not out of the woods yet. Figure 6 displays both the front and rear wave and how they should form in the vehicle. Reverting back to Figure 1 for a second, the front of speaker is noted as 0 degrees and the rear is 180 degrees. This is the same as in Figure 6 (noted by the yellow and green lines). The idea is to align the rear wave with the front wave. This means that the front wave starts at 0 degrees and reaches the reference point at the dash at 90 degrees. On the flipside, the rear wave of the speaker starts at 180 degrees and reaches the dash in-phase with the front wave at 90 degrees. This means that the rear wave of the speaker must be exactly three times the length of the front. In equation form: 3A = B + C + D (refer to figure 2).
There is a good chance that you may be using more than one subwoofer in your competition vehicle, so there is one other issue you need to consider. By now you understand how to control the sound waves produced by the subwoofer. When multiple subs are used, each needs to be on the same wavelength, so to speak (playing the same frequency). This means that each sub needs to be the same distance from the microphone as the next. This also means that the rear sound wave has to be the same distance. Keep in mind the enclosure volume and port must stay the same per subwoofer, but the dimensions of the enclosure may change so that the rear wave remains in-phase with the front. Now you are set to start building the enclosure-but be sure to leave yourself room for adjustments, mainly with the port length. Computer programs will give you a good base to work off of, but it's not always exact.
There are several other aspects to the design of an SPL competition vehicle, such as altering the interior volume and strengthening the enclosure. The interior volume should generally be more than or equal to that of the enclosure; otherwise the peak frequency may change and alter the output of your design. The strength of the enclosure matters greatly to reduce harmful resonance. Any structural support should be done on the outside of the enclosure to assure that there are no additional reflective surfaces in the enclosure that may add cancellation.
Hopefully, I have provided you with an understanding of what is happening in some of these competing enclosures on wheels. Controlling the waveform inside a vehicle can have substantial benefits in terms of SPL, and coupling the rear wave of the speaker with that of the front wave substantially increases volume, which is advantageous when you are reaching for high scores.
Temperature vs. Sound
The speed of sound can vary depending on the temperature of the air. As an example, at 60F, the quarter wave of a 50Hz sound wave will form a complete wave in 67 in., whereas the same frequency (50Hz) needs 69.13 in. at 90F. A drastic change in temperature may vary the peak frequency in your vehicle by 1 to 3Hz.