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
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
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.
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
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
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.