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 I’ll 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 driver’s 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, we’ve 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 don’t 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 BL’s.
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 can’t 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
18” 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 Mmd’s 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 Sd’s and
smaller cones will have smaller Sd’s. An average Sd for an
18” cone would be 1150 sq cm and a 15” 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 that’s why you see
different Sd’s for same size drivers. The ones with the
higher Sd’s 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. There’s 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 it’s 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 24”. Vd is Sd x xmax. It’s often overlooked but if
moving lots of air at low frequencies is your game then you
should know about it. I’ve 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 18” 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, that’s 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 don’t 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 don’t 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 no’s
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 no’s 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 it’s 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 xmax’s. 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, it’s 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 15” 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 18” 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 18”
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 18” 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 don’t 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 18”
drivers recently and thought it was a record. Its funny but
Precision Devices have an 18” driver with a power
compression loss of only 1.6 dB for full rated input and they
don’t 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 18” drivers. I think the word super driver comes
into play.
That’s
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.
It’s these three parameters (fs, Qts and Vas) that will be
most helpful in determining what the best use for a given
driver is. I don’t 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 don’t 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 Qts’s 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 18” 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 it’s the first driver with the Qts of 0.48
I’ll
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 don’t use Sd in cu inches and then workout Vd
in cubic centimeters. The two don’t mix. It’s all metric
or all imperial I’m 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.
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