Have you ever wondered what it takes to be a big time competitor in the SPL arena, posting the big numbers -- you know, the ear-shattering 170-plus decibels? If you know something about physics, you may think you have it figured out. You would know that rapidly compressing and rarifying the air can change the pressure within an airtight environment, in this case the cab of the vehicle. And if you were so inclined, you would likely apply this knowledge in SPL competitions by searching for the "golden subwoofer" with high power handling potential and the longest excursion (so that as much air as possible could be moved). Once you found the "golden subwoofer" you would measure the vehicle to see just how many of these puppies you could stuff into the interior. And, since you want to compress and rarify the air, it would make sense to have a sealed chamber, where the front and rear wave of the speaker could not interact. All the while you would want to minimize the volume of the chamber (cab of the vehicle) so there would be less air to move. In the end, you wind up with a sealed enclosure.
Stepping into the SPL lanes will cause most physicists to become confused. Immediately, they'll ask, "Where are all of the sealed enclosures?" You see, the majority of warped and extreme competitors taking top honors in SPL competitions today use ported enclosures, not sealed. Why? I think it is safe to say that many of these competitors don't actually know themselves, they just learn from trial and error. Cut and try.
So where does the answer to high SPL lie? The physicist is actually correct in theory to recommend a sealed enclosure. However, there is one small problem: subwoofers. Current subwoofer designs cannot move enough air due to excursion limitations. On the other hand, ported enclosures are slightly less dependent on excursion. So how are they producing such high output? There are a couple of reasons for this. The first is the design of the enclosure. Ported enclosures can be tailored to have a large gain in response over a small bandwidth by simple alterations of the enclosure volume and port dimensions. These simple alterations can produce a peak from 10 to 15dB, raising the sensitivity of a normal speaker within the small bandwidth over the 100dB mark. The second reason for such high output is the alignment of the front and rear sound waves based on a specific vehicle. This is the portion of the design that eludes most competitors.
In Box Basics, Part 1 (January 2002), I covered the differences amongst a variety of today's popular enclosure styles. One particular enclosure, the ported, is the basis for the SPL waveform theory I am about to go over. But, before I get into thick of things, I would like to recap the design principles of a ported enclosure.
Ported enclosures are distinguished by a vent or duct in the structure. This vent allows the rear sound wave of the woofer to interact with the front sound wave. The coupling of a vent to the air inside the enclosure reinforces the low-frequency response of the subwoofer system and can greatly increase the efficiency. By changing either the length or surface area of the port, the resistance to motion of the column of air within the port changes its resonant characteristics, thus causing the tuning frequency of the enclosure to change.
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 that you must first start by converting the speed of sound, 1130 feet/second, into inches. This can be accomplished by simply multiplying 1130 x 12 for a result of 13560 inches/second (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". Knowing this we can now calculate the frequency, employing the equation in figure 4a. At 50", 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 a 1/4 of the waveform, 90 degrees, our peak frequency will lower to a very usable 68Hz (see figure 4b). Perfect!
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 (see p. 72). 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 1/4 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 that 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. v
The speed of sound can vary depending on the temperature of the air. As an example, at 60 degrees (F), the quarter wave of a 50Hz sound wave will form a complete wave in 67 inches, whereas the same frequency (50Hz) needs 69.13 inches at 90 degrees (F). A drastic change in temperature may vary the peak frequency in your vehicle by 1-3Hz.