What do the driver T/S parameters mean and will they help me choose the best driver.

The drivers Thiele/Small parameters were brought about in a bid to standardize and bring meaning to the behavior of a cone loudspeaker. Most were specified in the early sixties and seventies by A.N. Thiele and R. Small, these two also published landmark papers on vented box (ported) low frequency systems in 1961 (Thiele) and later Small in 1973, both papers were published in the JAES. While we always think about driver parameters and vented cabs as being T/S we should not forget the work that went on before these two, as vented box principles were first described and patented by Thuras in 1932, with most of the mathematical models up until Thiele coming from people like Locanthi, Van Leeuwen, de Boer, Beranek, Lyon, W.M. Leach and importantly Novak.

So what does it all mean. First Iíll give a brief description of what most of the useful T/S parameters mean and later how you can use them to evaluate drivers for selection. I will not be going into great mathematical or mechanical detail here as this is aimed at the novice.

fs        Driver free air resonance.

The point at which all the moving parts of the driver sympathize or resonate. Resonance is a hard thing to explain simply, but a rule of thump is that you will find it hard to produce lower frequencies than the driverís fs. So a driver with an fs of 60 Hz will not produce 35 Hz very well. A driver with an fs of 32 Hz will produce 35 Hz, if the box is tuned low enough. These two examples relate to closed, ported and bandpass cabinets, horns are less affected by fs as they use the driver as a piston.

Qts      Driver total Q.

It had to happen at some point, weíve hit the Q word. Q is basically a describing word, it is used to describe a quality or characteristic about an electrical or mechanical part of the driver. So Qts is the overall Q of the driver, both electrical and mechanical. Qts can be thought of as how strong the motor and magnet system are. A driver with a low Qts of around 0.20 would have a large magnet and be able to move the cone with a lot of force. This makes for a tight driver. A driver with a Qts of 0.45 would have a smaller magnet and less control over its cone. So low values of Qts give a tight and punchy sound but with little weight or low bass and high Qts values give a slow and heavy sound that will give you lots of low frequency output. Watch out for drivers with really high Qts values of 0.6 or above, these would require such a big box to work correctly that in normal size boxes you donít get much low end. They are better of being used on the rear parcel shelf of your car, where they can enjoy a massive rear chamber.  

Qms    Driver mechanical Q

Qms is the mechanical Q of the speaker and only takes the speaker's mechanical properties into consideration. It is a measurement of the control coming from the speaker's mechanical suspension, which is made up of the surround and spider. 

The total driver Q is Qts and is derived from the electrical Q (Qes) and the mechanical Q (Qms).

Qts is defined as 1/Qts = 1/Qes + 1/Qms

Qms is calculated

                                   Fs sqrt(Rc) 

                           Qms =   ----------- 

                                         f2 - f1

Drivers with a very high mechanical Q can sound more open, cleaner and have a better dynamic range. This is because they have less loss. The surround is more flexible, the spider is better constructed, they have better air flow and usually have higher sensitivity. So a high mechanical Q is a very good indicator of energy storage behaviour.

So Qts is just a product of Qms and Qes and an understanding of what they are is important when designing a loudspeaker system. Qts, Vas and fs are all that is needed to determine the box size, but when you get to a very advanced stage of designing, its parameters like Qes and Qms which become the foundation of what you do.

BL        Driver motor strength.

The higher the value the stronger the motor. Given in tesla meters. Drivers with high BL values of around 30 or more have the ability to control their cones very accurately. These drivers will almost certainly have very large magnets and will weigh a lot. Note also that drivers with high BL values will normally have a low Qts value. Drivers with a low BL value of 20 or less will be less able to control their cones. These drivers will not feel as tight as those with higher BLís. They will also normally have higher Qts values of over 0.28 and while at home in ported or bandpass cabinets I call these drivers mud motors because of there slow and heavy sound with a less than perfect transient response.

Vas       Volume of air equal to the driver compliance.

This can be thought of as how stiff the movement of the cone is. The value is given in litres or cu inches. There are a lot of variables that determine the Vas, so you canít really say that high values of Vas mean a certain thing or are better. A single or double suspension spider will affect Vas, so does the size of the cone. The temperature of the air and also the humidity will affect Vas and so it is one of the hardest parameters to evaluate.

Mmd      Mass or weight of the speaker cone assembly.

This is how heavy the cone, coil and other moving parts are. An 18Ē driver with a Mmd of around 100 grams will have a light cone and will usually be more efficient than a driver with a heavy cone. A light cone can also move quicker. Light cones are usually found in higher Qts value drivers, but not always. This would appear to give them the advantage of having a quicker transient response as the cone is light, but the weak motors found in higher Qts drivers offsets any advantages of having a lighter cone. Drivers with Mmdís of over 200 grams will have heavy stiff cones. They will usually be less efficient, have double spiders and have lower Qts values. Drivers with heavy cones should have a slower sound, but not if they also have a low Qts and high BL. The strength of the motor system is able to counteract the high cone weight and still give a fast transient response. Do not confuse Mmd with Mms. Mms is the total cone assembly mass including radiation mass. Some loudspeaker design programs will want you to enter the Mmd and will calculate the Mms for you, while others will want the Mms and will calculate the Mmd for you.

Sd         Effective driver radiating area.

Given in sq cm or sq inches. Basically means how much area the cone has to move air with. Larger cones will have bigger Sdís and smaller cones will have smaller Sdís. An average Sd for an 18Ē cone would be 1150 sq cm and a 15Ē driver would have an average Sd of around 890 sq cm. But the depth of the cone also has to be taken into account. A deeper cone will give you a higher Sd for the same diameter. So thatís why you see different Sdís for same size drivers. The ones with the higher Sdís have deeper cones or have less surround material or both.

xmax     The amount of voice coil overhang.

Boring description for the parameter some of us love the most. Usually given in mm it represents the distance over which the coil can travel in one direction and maintain a constant number of turns in the gap. So a driver with an xmax of 10 mm can move is cone twice as far as one with an xmax of only 5 mm. Do not confuse xmax with maximum excursion. Maximum excursion is how far the cone will travel before 1. the coil hits the back plate or 2. the cone moves so far that it is limited by its suspension. xmax is how far the cone can travel with the coil still in the magnetic gap. Thereís no point in driving the coil outside of the gap as it will no longer be under control form the motor system. More xmax means the cone can move in and out further whilst still being under control. Maximum excursion is meaningless as no sane person drives their cones to full extension all the time, although I have seen it done with a recone kit needed soon after. Note that the xmax figure is for one direction only, so an xmax figure of 5 mm means the cone can travel 5 mm outwards and 5 mm inwards past itís resting place whilst still being under the control of the motor system. If the xmax figure is not given but the voice coil length and gap height are given you can work out the xmax from these. Take the voice coil length and subtract the gap height then dived by 2. This is how most manufacturers determine the xmax, some will also add 15% to the figure to allow for 3% third-harmonic distortion. So if you do the calculation and the manufacturers xmax figure is higher than predicted they have added 15%.

Vd         Displacement volume.

Also something you catch from people with an appetite for drivers over 24Ē. Vd is Sd x xmax. Itís often overlooked but if moving lots of air at low frequencies is your game then you should know about it. Iíve put the parameter here after Sd and xmax because it ties in with both of these. Basically to make sound you need to move air, and the lower the frequency you are trying to reproduce the more air you will have to move for a given output.  You can do this with larger cones, which will give you more Sd, or you can do it with smaller cones that move in and out more (have more xmax). So an 18Ē driver with an Sd of 1150 sq cm and an xmax of 5 mm can move 5750 cubic centimeters (cc) of air. Think of it like a big scoop that will hold 5750 cc of air and then get some one to throw that air at you very quickly and repeatedly, thatís a speaker. Now take a driver like the Precision Devices PD 1850, it has 11.25 mm of xmax and an Sd of 1150 sq cm. So its Vd would be 12,975 cc. Throwing 12,975 cc of air at someone is going to hurt a lot more than 5750 cc of air. Some of you will have noticed than 12,975 is over double 5750, now can you start to see why I rant on about drivers like the PD 1850. Comparing Vd figures is very useful for working out how much bass a driver can produce and is something most people donít do.

no          Free air reference efficiency.

This is given as a percentage. I find it more useful to look at the reference efficiency than to look at the manufacturers sensitively figures. A lot of the sensitivity figures quoted are useless and inflated, some manufacturers donít even quote the no, they just give you their sensitivity figures, what does that tell you. The no figure is the efficiency of the driver before the manufacturer has put it into a box and decided the sensitivity figure for it. For bass drivers noís of around 3.8% to 5% are good, the driver would have a sensitivity of around 97.9 to 99.2 dB for the 5% driver. More common are noís around 1.8% to 3.8% and these drivers would not be as efficient. An no of 1.8% would give a sensitivity of 94.7 dB and 3.8 % would be 97.9 dB. The figures quoted here are for 1w/1m. You will find that drivers with high xmax figures do not have high no figures. Because they have longer voice coils which are heavier for the motor to move they are less efficient. So unless you really need that extra output and can justify the extra expense of buying an amplifier that can really move the cone to itís limit you might as well use smaller amps with more efficient drivers. You will never get the same amount of output from a driver with less xmax, but you will get more output for the same input power from a more efficient driver with a smaller xmax. If you never really drive your speakers hard then the use of the more efficient short voice coil, low xmax models would save you cash on the cheaper driver in the first place and the less powerful amp needed to get the most out of these drivers. You would also have the benefit of less weight. If you drive your speakers very hard and need the maximum output for the size enclosure they are in then you will need to use the less efficient longer voice coil models that have big xmaxís. You will also need the budget for the big amps you will need to move these beasts, most need over 1000 watts to get them near their xmax limits and because of their lack of efficiency, itís not until you drive them hard that you will get the benefit of extra output over more efficient drivers. If I only had 500 to 750 watts to give to each driver then I would use the more efficient lower xmax drivers. If you use low efficiency big xmax drivers in this situation you will not get as much output and I could come along and make more noise with the same amp power using more efficient drivers costing half as much. If I drove my speakers with over 1000 watts each then I would use the less efficient bigger xmax drivers. You will get the most output from drivers like these but you really have to push them for that little bit extra. You can explain it like this. If I go to my local club and their small disco system is driving a 100 watt amp into some efficient 15Ē and horn type cabs, I would be amazed at the volume being produced. I would be thinking if I brought in one of my big 18Ē ported cabs that has a driver with an xmax of 10 mm and connected it to there 100 watt amp I would probably not even here the 18Ē driver. The difference is that they have a very efficient 100 watt drivers and they being driven to their maximum, they will never get any more volume out of there system, not even if I bring in a 1500 watt amp. But if I brought in a 1500 watt amp and connected it to my big 18Ē ported cab and turned it up I would probably shake the venue to bits. But I bet I would need around 500 watts just to equal their 100 watts before I really started to make more noise than them.

Power compression

Not a T/S parameter but useful if the manufacturer has given the value. It is given in dB and worries most speaker manufacturers so much that they donít even quote the figures. The figure gives the amount of sensitivity that the driver will loss due to the heating of the voile coil. Bad drivers will lose around 5 to 6 dB at full rated input. Better drivers will quote between 3 to 5 dB of loss at full rated input. There are very few drivers if any that have lower losses than 3 dB. JBL quoted 2.8 dB for one of its 18Ē drivers recently and thought it was a record. Its funny but Precision Devices have an 18Ē driver with a power compression loss of only 1.6 dB for full rated input and they donít go round shouting about it. So if you drive the PD 1850 with 600 watts and put the same amount of power into a driver with a power compression figure of 4.6 dB the PD will be 3 dB louder. Can you still see why I keep on ranting about the things. 3 dB louder and able to move much more air that most other 18Ē drivers. I think the word super driver comes into play.

Thatís taken into account most of the parameters you will need to evaluate a driver for your shortlist. There are loads more parameters I could have mentioned, but I would have to go into the world of mathematics and physics to explain their meanings, and most of them just explain what I have said above anyway. You only really need the fs, Qts and Vas figures to work out the ported box size, the other parameters will give you an indication of how it will work when it is in the box. Itís these three parameters (fs, Qts and Vas) that will be most helpful in determining what the best use for a given driver is. I donít want to go into to much depth about it as I have elsewhere on this site, but EBP is a good way of deciding what to do with a driver. If you need driver for a horn, a proper horn with a horn length over 1.8 meters then picking a driver with as high an EBP as you can find will help. Make sure it has as low a Qts as possible and as powerful a magnet as you can find. The magnet strength is given in BL, so the higher the better. So donít put a driver with a Qts of 0.48 and a BL of 17 in a horn. It will not be able to move the column of air inside the horn very well at all and will break up if you drive it with serious amounts of power for long periods. This high Qts driver is screaming out for a ported box. When I say ported I mean reflex loaded or a vented box. If your driver with a Qts of 0.48 had a Vas of 290 and an fs of 35 Hz then the optimal size ported box would be 400 litres, this is a big box, but I did say earlier that the higher the Qts the bigger the box. If we keep the Vas and fs the same and reduce the Qts to 0.35 then the optimal size box would now be 139 litres, which a lot more manageable. So for reflex enclosures Qtsís of between 0.28 and 0.45 are useable. Drivers with Qts values under 0.28 would work well in horns and for values over 0.45 you are going to have to have a very large enclosure, use the rear of your car or put the driver into a smaller box than it dictates and have less low bass output. If we look at another 18Ē driver that has a Qts of 0.19, an fs of 40 Hz and a Vas of 230 litres and work out the optimal size of reflex enclosure we come up with 22.5 litres. Great you say, a nice small and lightweight box, but not so great is that this combination has an f3 point of 112 Hz. So not even 60 Hz would be reproduced very loudly. This driver is screaming out to be horn loaded, stick it in a really long horn and stand well back. The f3 point is the point or frequency at which the bass has rolled of to -3 dB, it denotes the beginning of low end rolloff. Just to see if you have understood all of the above, guess which driver out of the two examples above would have the lowest BL. Your right if you think that itís the first driver with the Qts of 0.48

Iíll conclude this section with a little look at box parameters. Most of you should understand what they all mean because you will have used them when you design your reflex boxes in programs like WIN ISP or Bass Box Pro etc. Also worth noting is that you must keep all parameter values in the same donation. So donít use Sd in cu inches and then workout Vd in cubic centimeters. The two donít mix. Itís all metric or all imperial Iím afraid.

Vb         Internal volume of a ported enclosure.

Vc         Internal volume of a closed box.

Fb         Tuning frequency of a ported enclosure.

Fc         Resonate frequency of a closed box.