Sealed or
acoustic suspension speakers are definitely the ones to build if you
- have never built a loudspeaker before
- want to get it right, first time
- you simply like the way they sound
...because there is not much that can go wrong with them, and a lot that they do right.
Driver selection
You have to start with a driver that suits your needs. 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. If loud and deep bass is important to you, a large and high-powered driver (or even an array), may be required. The simplest way to make this decision is to clone, or at least imitate, a system that you know and like. If your reference system uses four enormous kevlar-coned woofers per channel, there won't be any cheap shortcut to getting the same sound.
Number crunching
Once a driver has been selected, you have to choose the size of the enclosure. External dimensions don't matter a lot - it's the volume of air in it that matters. It will help if you know certain things about the driver
(If you don't know these values, and you can't measure them, then guesstimation / trial and error will have to do... base the guesstimate on the values for similar drivers).
Fs: resonant frequency.
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. The cone of your driver is similar, having a frequency at which it tends to oscillate most freely. This resonant frequency is determined by the weight of the cone and the tightness of its suspension. A heavy cone and loose suspension will give the deepest bass. 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.
The reaonant frequency of the raw driver is called Fs. When the driver is put into an enclousere, the resonant frequency is raised - the new value is termed Fc.
Q: quality factor.
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. 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.
The Q of the driver itself is called Qts, that of the driver-enclosure system is Qtc.
VAS:
This is a way of measuring how springy the driver is. VAS is the volume of air that has the same compliance as the suspension system of the driver. Since bass drivers have loose suspensions to achieve a low Fs, they also tend to have a big VAS.
How is a volume of air considered to be spring-like?
When the speaker cone vibrates, it is moving in and out of the box, and this changes the box volume (slightly). When the box volume is changed, the are inside will be pressurised differently to the air outside the enclosure. This pressure difference exerts a force on the driver, that pushes it back towards it's resting position. The smaller the box, the greater this effect.
Now imagine a huge bass driver in a very small box, just big enough to fit the driver. 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. 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. Obviously the air in this small box is too "springy" - it has too much effect on the motion of the cone.
In the opposite case, the box is made so large that this spring effect is inconsequential (or close to). This is termed an infinite baffle. Due to space considerations, these are quite seldom used, and acoustic suspension is a much more common alignment.
Interaction of enclosure volume, VAS, Qts and Fs
The ratio of VAS to box volume = (Qtc/Qts)^2 - 1 : 1
Example (my own bass speakers):
Qts = .35 Fs = 21.3 and VAS = 170L
I wanted a box Q (Qtc) of .7, so the equation becomes:
VAS:box volume = (.7/.35)^2 -1 : 1
= 2^2 -1 : 1
= 4 -1 : 1
= 3 : 1
So the box volume should be a third of 170L, or 56L
The ratio Qts : Qtc is the same as the ratio Fs : Fc.
In the example above, Qtc is Qts * 2, so Fc will be Fs * 2 or 42Hz - which is a pleasantly low value.
I built my enclosures slightly bigger than 60L, so I will have a Qtc a bit lower than .7, and Fs just under 40Hz, but not by much.
The optimum Qtc is usually considered to be about .7, as this gives the most extended bass: the -3dB point is the the Fc of the system and the bass roll off is at 10.5 dB per octave. Transient response is good. Power handling is OK.
A Qtc of up to about 1.0 is quite acceptable, and has only half the volume of the .7 Qtc speaker. The power handling will be higher, and the mild peak in response at Fc gives the impression of more powerful bass. This combination makes for a rather marketable speaker. On the downside, the bass does not go as deep, it rolls off more steeply at about 12 dB per octave, and the transient response is degraded.
If the enclosure is made even smaller, the transient response will very poor, and the Fc will be high and over-emphasised. So a Qtc of much over 1 is not recommended.
As the enclosure size is increased, Qtc and Fc drop (but they never go as low as the unboxed values). Low Fs and Qtc values are good to have, so a large box generally gives better sound than a small one - but there is a catch. As the Qtc gets very low, the system gets decidedly un-resonant at Fc, and so bass output is somewhat reduced. Also, the driver can easily move a large distance from its rest position, since the "spring effect" of the enclosed air is very mild... and this reduces the power handling of the driver, and increases the risk of causing damage to the driver when played loud. Still, a Qtc lower than .7 is pretty good - it gives the best transient response, and the most gradual roll-off of the bass, and is worth doing if the driver is up to it.
It's only worth aiming for a really low Qtc of .5, if you are obsessive about sound clarity, and are willing to invest in a very good, large speaker (or an array of big speakers), with good power handling and bass capabilities - otherwise the reduced power handling / lowered output will ruin your fun. This also requires a really big enclosure - my drivers would need a volume of 170L. So a low Qts is only good if you can afford the drivers and the space.