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.
SPL WaveformIn 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 hick 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).