I would not buy a sub that does not list the T-S params.
the performance of a sub is absolutely dependent on how it is spported by its enclosure.
without the T-S, you cannot build the enclosure with the correct volume
From the DIY for audio,
you can rake a peek at what is involved. Its not hard, but you can't ignore it and get decent results:
Lets look at a popular, affordable 8" driver that has a paper cone and foam surround, and lets call this Driver A. The manufacturer recommends that these drivers be used in ported enclosures, and for good reason!
The T/S parameters we are interested in are as follows for Driver A:
Fs (free air resonance) = 43 Hz, Qes (electrical Q/damping of the driver) = 0.352, Qms (mechanical Q of the driver) = 12.11, Qts (combination of Qes and Qts, total Q of the driver) = 0.342, Vas (equivalent air volume of speaker compliance) = 1.02, Xmax (peak linear excursion) = 0.15".
The first thing to look at is the Efficiency Bandwidth Product for this driver. The EBP can aid you in selecting the proper enclosure for any speaker. To get this, we use the following equation:
EBP = Fs / Qes = 43 / 0.352 = 122
An EBP of 122 is indicative of a speaker best used in a ported enclosure. If the EBP had been lower, say between 50 and 90, you would get decent performance in a variety of enclosures - a "Jack-of-all-Trades" driver. EBP below 50 are generally best for sealed boxes. Don't misunderstand, you can put any driver into any enclosure, but you might be disappointed with the results. EBP is simply a guide, however, and good performance has been attained with drivers seemingly unsuited to a particular application.
Using a scientific calculator and the formula's (see Useful Conversions and Formulas) to configure an optimum ported enclosure, the Vb for Driver A comes out to be 0.703 cu. ft., with a cutoff frequency (F3) of 50.21 Hz and tuning frequency (Fb) of 47.43 Hz. Driver A in an optimum sealed enclosure with a Qtc of 0.70 would require a Vb of 0.32 cu. ft., and an Fc and F3 of 88 Hz! Obviously, this driver is optimized for ported enclosures, as evident by the EBP and indicated by the manufacturer. But what else is there that makes this driver a poor choice for a sealed box?
Look at Driver A's Xmax of 0.15". With such a short linear throw, it is not well suited for sealed box duty. Speakers specially designed for sealed boxes will generally have a greater Xmax than others. For comparison, another popular 8" driver which is optimized for very small sealed boxes, Driver B, has an Xmax of 0.343". This is not a law, however, and many very good ported drivers have a fairly high Xmax. Also, Driver A has a free air resonance of 43 Hz, which is rather high for a sealed box, and most drivers that perform best in sealed enclosures will have a low Fs. Remember that a sealed enclosure with a Qtc of 0.70 will generally have a resonance and cutoff of approximately the same value, and these will always be higher than the actual free air resonance frequency. Thus, it makes sense to design a sealed enclosure around a driver with a low Fs to begin with. Drivers designed for sealed box applications will generally have slightly smaller magnet structures, and therefore lower damping than equivalent ported drivers.
Conversely, If you try and use a driver designed for very small sealed enclosures (higher Qts and Xmax, low Fs) in a ported enclosure, you will often find that the port length must be very long to tune the box to Fb with a port diameter large enough to avoid vent noise. A good example of this is the above Driver B, which if put into an optimum ported enclosure of 0.50 cu. ft. would need a 3" diameter port 35.9" long to tune the system to an Fb of 31.47 Hz!
if sonic has no clue, the manufacturer will supply the params
