As mentioned the goal is to design and build a subwoofer that has a linear sound pressure level throughout its frequency range but also offers accurate reproduction of a recording; from the fast bass drums in rock or techno music to smooth jazz and thunderous explosions when watching movies. This is no easy task as fast bass drums and thunderous explosions are far apart from a subwoofer point of view, the latter demanding lots of sound pressure for a prolonged period of time, whereas accurate reproduction of a bass drum, without it starting to sound sluggish, needs a fast subwoofer, able to follow the recording's every detail. This is where lots of commercial subwoofers fail, as many subwoofers meant for home theater use perform less than ideal when playing back music and vice versa. The first requires lots of air to be moved to accurately reproduce explosions and other low frequency sounds, the second requires control, and lots of it.
So obviously a good subwoofer needs to be able to combine both. The ability to move lots of air is simply governed by effective cone area; the total surface area of the woofer's cone and its excursion, or rather how far the cone moves from its center position. Without having to resort to complicated math it is easy to understand that in a scenario where you have two woofers, of which one has half the effective cone area of the other, that smaller woofer needs to have twice the excursion of the bigger woofer to move the same amount of air. Unfortunately it is simpler to build woofers with a bigger cone diameter than making smaller woofers that have a larger excursion. This is partly because the mechanism that drives the cone, the voice coil suspended in the woofer's magnet magnetic field, only has a small range where it operates linearly.
A look at the Peerless XLS10 10" woofer we will be using in this subwoofer design.
A typical woofer will have an excursion of +/-8mm, push it further and it will still work but introduce non-linear distortion due to the voice coil not being fully inserted in the magnetic field of the magnet. Non-linear distortion obviously is unwanted, as it introduces harmonics in the reproduction which were not in the original recording. But of course you cannot simply keep increasing the cone diameter either, as the cone needs to remain stiff and move as a whole, rather than the center piece being driven and the edges trailing behind it. Obviously larger cones are also heavier requiring more powerful magnets and voice coils to drive them. Realistically 15" is about the limit where conventional materials and engineering can be used to construct a proper functioning woofer. In our case 10" and 12" woofers offer a good balance between accuracy, effective cone area and maximum sound pressure level (spl).
The Peerless XLS10 woofer, note the die-cast frame, ventilated spider and double magnet.
Selecting the right 10" or 12" woofer is not easy though, as there is a plethora of woofers available some more suitable than others. So before we make a list of criteria for the woofer it is important to set a few design requirements for the subwoofer as that will largely determine what specifications the woofer will need to have. These design requirements are listed below and selected from personal and professional experience from the author.
1. Compact design, with about 30-litre effective volume.
2. Closed, vented or passive radiator box.
3. Active system with amplifier equalization and active filters.
4. Usable frequency response down to 20 Hz at max spl above 100 dB.
5. Total price of parts below $800 or 700 euros.
The 10" or 12" woofer we are looking for has a large excursion, allowing it to move lots of air, but is also equipped with a powerful enough voice coil and magnet system so sensitivity is not affected. It also needs to have a low resonance frequency (fs), a low equivalent volume (VAS) and a low Q factor (Qts) so it can be mounted in a small box and still be able to offer accurate and deep bass reproduction. From all of the woofers which we have looked at and that are commonly available Peerless' XLS10 (830452) and XLS12 (830500) are most suitable for our needs. As per their type numbers the XLS10 is a 10" and the XLS12 is a 12" woofer. Lets now evaluate what box type is best suited for the XLS10; we will start off with a closed box design with a 30-liter volume.