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fiogf49gjkf0d  noviygera wrote: | | I am very curious what "properly made" means when perusing the tonal attributes of LF. What properties or specifications other than driver diameter do you evaluate when perusing a better tone? |
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Herman, you are asking
me where does the tone comes from in context of bass reproduction? Well, this
is a multifaceted subject that takes huge amount of explanations in order to be
understood properly. I will mention the very basic things that affect tone.
What we have in a bass driver? We have a moving cone. So if we want to
have good and complex tone, we have to ensure the cone will be excited in
accordance to the signal of the amplifier’s output terminals. And when we see
all the influencing factors, we realize we are sunk deep in the swamp. The cone
is excited by the voice coil movements. These movements should mimic the
voltage waveform of the signal at the output of the feeding amplifier. Here is
a short list of the factors that distort that accurate mimicking.
First of all, the voice coil should move in an absolutely uniform
magnetic flux during its movement, no matter how loud the driver is being
pushed. It is some kind of ideal theoretical situation which is never realized
in practice. Most of the magnet systems have so unstable magnetic field in
their gaps, that it is very funny to call this field “permanent/constant
magnetic field”. One needs truly heroically constructed magnet system to assure
stable working point of its magnet and to avoid field modulations in order to
ensure a stable platform for achieving accurate VC movements. There is another
problem here. Achieving stable magnetic field does not ensure this field is
evenly distributed through the height of the gap. In most of the cases the
induction varies to a large degree in the different sectors of the gap. The
difference in the field at the bottom and at the top of the gap sometimes
exceeds 40%!! You can easily imagine how much distortion could be produced with
that kind of motor. So one needs smartly designed motor, where the magnetic
field does not collapse when the field of the voice coil pushes against it, and
also it must be the same in all points in the gap. This is not cheap to be
produced. Until now we looked at only half part of the problem with achieving
accuracy for VC movements. Let’s see the other half of the problem. Most of the
bass drivers have their VCs longer (sometimes considerably longer) than the
height of the magnetic gap. It creates a whole new world of problems and
distortion forming mechanisms. One does not need to be very smart to realize
that with that type of motors, the VC doesn’t “see” constant number of magnetic
lines when moving back and forth. This is another process causing distortion.
But wait, there is even more into it! The voice coil has parasitic electrical
parameters. One of them is called inductance. The self inductance of the coil
acts like a build in low-pass filter inside the driver. The problem is that
when the coil moves, during its movements it sees different amount of steel
around it. Steel affects the inductance value of the coil and if the steel is
not of a constant amount around the coil when it is moving, the VC changes its
inductance constantly, depending on its position in the gap. That means there
is constantly changing crossover frequency due to inductance variations. It is
called inductance modulation and affects the upper working range of the driver.
So if one is looking for a bass driver which he intends to use with first order
filter, he has to ensure there are no inductance modulations in order to
prevent this new type of distortion mechanism.
How to eliminate all these variables in order to ensure proper
excitation behavior on the cone? Well, there is only one really efficient way
to do that. The solution is called underhung motor and it is used by some
manufacturers in their drivers. Here we have a VC that is shorter than the
height of the magnetic gap where it is moving. It sees constant amount of steel
during its movement and that kills the inductance modulations. Also it is
driven by a constant flux during its entire movement. It lowers some types of distortion
to nearly zero level and eliminate transient compression. And this is very
important.
At the second stage, we realize that although the coil moves in stable
and constant magnetic field, its behavior is hugely dependent by its electrical
interaction with the driving amplifier (that’s why the tone is dependent from
the amplifier/speaker interaction). We need the current flowing in the voice coil
to change in the same manner as the voltage waveform at the output of the
driving amp. This is very difficult to happen because there are eddy currents
flowing in the iron (this is the former material of the magnetic circuit) and
there are hysteresis loses in that iron and all this distorts the current
flowing in the coil. The human ear is very sensitive to that type of
distortion, so all the good speakers have very serious magnet systems where
some efforts are taken to break up these eddy currents and to lower that nasty
sounding distortion mechanism. Speaking
of speaker-amplifier interface, I have to mention that every amplifier need
exact amount of wire length in the gap in order to drive the VC properly.
That’s the reason why most (practically all) of the bass drivers do not sound
good with solid state amps. It is not because the solid state amps are bad, but
because they need low inductance/resistance voice coils in order to work/sound
properly at LFs. There is also another phenomena: imagine you have two bass
drivers with identical moving parts and with identical magnet systems. The
difference between them is in the type of the voice coil. The first speaker has
2 layers with 30 turns per layer (60turns in total) and the second speaker has
1 layer of 60 turns of exactly the same wire. The DCR of the two coils is the
same, but the 1-layer coil sounds much better compared to the other. But why??
Do not ask me why – I do not know. I only know it affects the tone a lot, and I
simply get use of it, without any explanation why it happens.
When all of these 5-6-7 different types of distortion mechanisms are
eliminated (I mean lowered), one has the opportunity to hear the unmasked/undistorted
tone of his bass. I deliberately used the word “opportunity” because now we
have to convert those accurate VC movements into sound waves and if we fail to
do it, we will not get good tone out of our bass drivers. Good example of this
kind of failure is JBL 1501Al bass driver. It has very very very smartly
designed underhung motor, but its moving system is crap and the result is a
lack of tone in its sound.
OK, now let’s imagine we have kind of ideal situation and our voice coil
follows exactly the signal waveform and amplitude. We have to convert those
movements in air molecules excitation, and this excitation should mimic exactly
the VC movements. We further realize that we are not only deeply sunk in the
swamp, but we are on the bottom of the swamp.
What we need to have uncompromised moving system? We need to attach to
the VC a cone that has density equal to the air density and at the same time to
have as much radiating area as possible in order to achieve good efficiency. That
type of cone does not need any suspensions or electrical damping to reproduce
the signal precisely. But unfortunately we do not live in an ideal world, so we
need to think for a while how could we get as close as possible to the above
mentioned criteria. So the first thing we need to do is to find a material
having mechanical impedance as closer to the air mechanical impedance, while
being rigid enough in order to maintain accurate behavior. We realize the only
suitable material is long-fibered paper. Now we need to “invent” smart cone
geometry to make the cone rigid enough by using smart construction geometry and
not stiff materials. There are a LOT of tricks
here, but I will not mention any of them. All this affects tone a LOT. But we
need our cone to flex very gradually in order to avoid nasty break-up peaks,
because every material on this planet flexes at some point when there is
applied enough mechanical acceleration to it. Just a quick illustration: take a
look at the following video (the part at 4:27-4:35):
http://www.youtube.com/watch?v=QFlEIybC7rU&feature=related
You can clearly see how this obviously rigid baseball bat flexes like an
elephant ass when there is enough force applied to it. The same happens with
speaker cones. The whole trick is to take control over these break-ups. It
influences the tone a lot when the driver is being used over a wide range (this
is the case with 1st order filtration).
Now let’s imagine we have two identical voice coils. The first one
drives 8” cone and the second one drives 18” cone. What happens? Since the
bigger cone has more radiating area, it reproduces the LFs more efficiently and
its tonal balance is centered at lower frequency compared to the small cone.
The small cone has higher amplitude for the upper range of the input signal and
its tonal balance is more upward oriented (Paul already mentioned that, and he
very intentionally used irony when calling a 10 inch driver to be a “bass”
driver) although it is perfectly suited to reproduce low frequencies if having
low Fs. So properly designed big cones have better tone and higher efficiency
at lower frequencies compared to small ones.
The less radiating area of smaller cones, means there is less coupling
to the surrounding air. That means there is less air control to the cone
movements and now these movements must be controlled by different ways. One
needs harder suspended cone in order to control these movements. Of course this
type of cone control has negative consequences to tone, because it acts with no
own intelligence – it just blindly resists to these movements, applying a force
in order to turn back the cone at its rest position. This process inserts its
own tone and adds its taste to the real tone. Air loading does not have this
behavior defect and it controls the cone in a smart way, applying no own
intelligence, thus allowing one to hear more complexity of tone at LFs.
There is one drawback of using too
big cones for bass. This drawback is due to the raised moving mass
compared to a smaller cone. This mass stores kinetic energy and if this energy
is not damped by the air/suspension, it makes the VC to generate more back
voltage and sent it directly to the driving amplifier in different point in time (after the original signal that feeds the VC). The amplifier should be
able to absorb that energy. Sometimes this is a big stress for smaller
amplifiers and they go really crazy while trying to absorb that energy, especially at Fs of the system, where the returned energy is returned in different points in time. The more powerful the amp
is, the better it resist that sent back energy coming from the bass driver’s
VC. By the way, that’s one of the main reasons why low powered amps cannot
drive the big woofers with confidence:
http://www.goodsoundclub.com/Forums/ShowPost.aspx?postID=9390#9390
Now when we have good driver with high tonal potential at LFs, we have
to find a way to use it. We have to prevent acoustic shortening effect. There
are a lot of possibilities how to achieve that. Every solution has its own
character, because its affect the cone behavior. The best way to ensure
undistorted cone movements, is to load the cone symmetrically. The air loading
from behind the cone should be the same as the air loading in front of the
cone. This is satisfied with infinite baffle configuration. In this situation there
is a lot of air behind the cone and it breathes freely in the air, providing
pure tone to the listener. Every closed volume of air behind the cone manifests
itself like a breath-blocking device, no matter how damped it is from the
inside. Make a simple experiment: take huge amount of damping material and
insert it in your wardrobe. Now put your head inside and listen carefully. You
feel your head is inside a small and highly damped chamber. Very unnatural
feeling, isn’t it? Now take your head out of your wardrobe and go outside in
your garden. How do you feel now? Do you feel the freedom and lack of closed and stuffed chambers? Yes? Well, the same
is true in context of your bass driver operation. You can always recognize
there is trapped air volume behind the cone. This affects tone too, although
there are no air-born resonances, due to the heavy internal acoustic damping.
The vibrations of the acoustic shield (the “box”) also destroy the Tone.
If one wants to hear real bass tone, he must take all those sound-poisoning
vibrations out of the picture. The material that really do this job well is the
sand. I know there are some people who state, the acoustically dead/inert
cabinets destroy tone. And the fun part is they really do not imagine it – it
really happens sometimes, but there is a reason why it happens. The reason is
because they are Гевреци/Gevreks (Morons)
and they use tonally dead drivers. What else could they expect, besides dead
tone when using tonally dead driver in acoustically inert enclosure?? So they
“fix” the problem by using mechanically resonant enclosures, calling them “musical
instruments”!! These wide-band resonators really make their dead drivers to
sound more alive and with more “tone” but in reality it is just the opposite
situation – they have nearly zero tonal complexity, because the bass tone is
highly polluted with cabinet colorations. The lower the parasitic resonances,
the more complex the tone becomes.
The type of filtering also affects LF tone. With first order
low-passing, the bass channel sounds like the missing part of my
widerange/upperbass channels combo, but with sharp filter it sounds much like a
self contained sound that is not part of nothing. It lives its own life and
although it could be +/- integrated with the other channels, this integration
is light years away from 1st order filtering integration abilities.
There are many other factors, affecting the tone at LFs, but the people
who are really interested into this, are going to learn those factors by
themselves.
Best regards,
Petar Haralanov
"If you can't explain it simply, you don't understand it well enough." -A.E.
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