The optimum Qtc is usually considered to be about. 7, and Fs just under 40Hz, but not by much. I built my enclosures slightly bigger than 60L, so I will have a Qtc a bit lower than. In the example above, Qtc is Qts * 2, so Fc will be Fs * 2 or 42Hz - which is a pleasantly low value. The ratio Qts : Qtc is the same as the ratio Fs : Fc. So the box volume should be a third of 170L, or 56L The ratio of VAS to box volume = (Qtc/Qts)^2 - 1 : 1 Interaction of enclosure volume, VAS, Qts and Fs Due to space considerations, these are quite seldom used, and acoustic suspension is a much more common alignment. In the opposite case, the box is made so large that this spring effect is inconsequential (or close to). Obviously the air in this small box is too "springy" - it has too much effect on the motion of the cone. The cone is pushed so hard back towards its rest position that it will greatly overshoot, and will move back and forth for several cycles before coming to rest. When this huge speaker cone moves, the air pressure in the box will change by a large amount, and so the restoring force will be similarly large.
Now imagine a huge bass driver in a very small box, just big enough to fit the driver. The smaller the box, the greater this effect. This pressure difference exerts a force on the driver, that pushes it back towards it's resting position. When the box volume is changed, the are inside will be pressurised differently to the air outside the enclosure. When the speaker cone vibrates, it is moving in and out of the box, and this changes the box volume (slightly). How is a volume of air considered to be spring-like?
Since bass drivers have loose suspensions to achieve a low Fs, they also tend to have a big VAS. VAS is the volume of air that has the same compliance as the suspension system of the driver. This is a way of measuring how springy the driver is. The Q of the driver itself is called Qts, that of the driver-enclosure system is Qtc. Big magnets and big enclosures give a low Q, and the lower the Q, the more tightly does the magnet control the movement of the cone, whereas a very high Q speaker is less controlled, and will oscillate wildly at resonance. This is not a scale for determining how good a speaker is, it is simply an indicator of how the speaker behaves at its resonant frequency. When the driver is put into an enclousere, the resonant frequency is raised - the new value is termed Fc. The reaonant frequency of the raw driver is called Fs. Fs below 20 is exceptional, with 30-40 being about average for a woofer, and Fs values around 100 are common for small multimedia / muzac speakers. A heavy cone and loose suspension will give the deepest bass. This resonant frequency is determined by the weight of the cone and the tightness of its suspension. The cone of your driver is similar, having a frequency at which it tends to oscillate most freely.
If you hit a drum, the skin vibrates (creating soundwaves) at a multitude of frequencies, with the stronger vibrations defining the sound of the instrument. base the guesstimate on the values for similar drivers).
(If you don't know these values, and you can't measure them, then guesstimation / trial and error will have to do. It will help if you know certain things about the driver External dimensions don't matter a lot - it's the volume of air in it that matters. Once a driver has been selected, you have to choose the size of the enclosure. If your reference system uses four enormous kevlar-coned woofers per channel, there won't be any cheap shortcut to getting the same sound. The simplest way to make this decision is to clone, or at least imitate, a system that you know and like. If loud and deep bass is important to you, a large and high-powered driver (or even an array), may be required.
If you like bass-light music (folk, chamber music, jazz), then clarity is the key: a single light-coned speaker of modest dimensions should be fine. You have to start with a driver that suits your needs. because there is not much that can go wrong with them, and a lot that they do right. Sealed or acoustic suspension speakers are definitely the ones to build if you