|Drivers are a mystery to most, understood by some, and mastered by only a few. The biased
"advertising" words and frequency response curves are often disturbingly
embellished/smoothed. However, there is hope. The earliest quote I remember was internalized in about 4th
grade. It is, "The truth is not apart from reality but lies within
it." I find this applicable to many things, and speaker
drivers are certainly included. There is some truth in what every
manufacturer says about their drivers. Some frequency response curves are
A waterfall plot can speak a thousand words. A frequency response (FR)
curve can speak only a few. Unfortunately most manufacturers (except
Raven, Jordan) don't publish their waterfall plots. They only publish a FR
curve. The degree of truth varies from manufacturer
to manufacturer. SEAS FR curves tend to be very truthful, and this is evident with
their metal cone drivers. With stiff cone drivers, there is cone resonance.
My first project was a prime example of this. The driver
had an excellent reputation, and a very stiff cone. It was not a SEAS
driver. The manufacturer's FR curve
seemed to indicate little/no resonance. The designer (not me) applied a
2nd order crossover to this driver at 2200hz. This all seemed quite
logical, but it failed profoundly. This is because the manufacturers FR
curve was quite misleading.
I didn't understand the issues concerning cone resonance initially. I
now understand that
uncontrolled resonance made the whole speaker sound really bad. It was
"nasally", "congested" and "bright". I figured the
resonance was very high, but even in
the male vocals the speaker sounded bad. Resonance effects the whole
system. The @$150 midwoofer (I will not
disclose which one) and @$80 tweeter didn't sound any better than my B&W
DM602. I was disgruntled until Dennis Murphy came to the rescue. He
had the same problem with this driver and previously spent quality time designing a
crossover for it. He conquered! Dennis has excellent crossover design
skills. He sent me the crossover plan he designed. It was a 4th
order parts count at 2100hz. I implemented it and "viola"
excellent sound happened. The difference wasn't subtle. It was
amazing! Stiff cone resonance requires a steep crossover to get
"beneath it". This entails great skill, and many crossover
parts. A good crossover is far more important than good drivers.
Most folks focus on the cone of the driver. It is visible, and it sells
speakers. The common "Joe" can readily grasp that a stiff cone will produce a
flatter wave-front and a cleaner resultant sound. A pretty cone seems to be equally important. If
it looks high tech then it sells well in hifi shops. At least 50% of the
cost in a commercial speaker goes toward marketing. But there seems to be
little marketing $ given to the motor structure of the driver. IMO, this is
more important than the cone material. I would rather have my $ go into a
good motor than a good cone.
I read an article in speaker builder copy 7 of 1999 titled "Navigating
Speaker Design: Sleuthing Driver Parameters" by Mark Wheeler. This
article is very good at describing how subjectively "fast" sound will emanate
from a given driver. Mr. Weeler suggests that one way to explain how fast a
driver will be is to multiply the magnetic force (bl) by the radiating area (Sd)
of the driver. Then divide this number by the weight of the cone (Mms). This
coefficient will yield the audible "speed" of the driver. This concept is very
similar to hotrod power (Bl) to weight ratio (Mms) with traction (Sd) used to
figure out a ľ mile time. This analogy might be useful when trying to
comprehend Mr. Wheelers assertions. While he is correct in his assertions, and
his article is very informative, he leaves out a very important facet of what
makes a driver audibly "fast". He omits magnetic
Lambda acoustics does a nice job describing
their motors and the issue of magnetic linearity. I believe that
magnetic linearity is very important too. I will make an attempt to describe magnetic linearity in crude fashion. I will assume an
overhung voice coil moving within the magnet to produce sound.
coil should operate within the confines of the pole piece. Some distortion
always occurs when the voice coil extends beyond the pole piece. When the
voice coil is within the confines of the pole piece there is some dilemma.
manufacturers will call the region where the voice coil is within the confines
of the pole piece the linear region, linear X-max, or
something of the sort. But just how linear is it?? My crude theoretical compendium is
that if the voice coil moves forward within the pole piece when responding to a
bass note (50hz) it is off center for a moment and effects the field of the
pole piece. Furthermore, the moving voice coil will induce current (eddy
currents) within the pole piece. While the motor's field is disturbed
by the voice coil, the disturbed motor must tell the voice coil to respond to an upper octave piano note
(@2000hz) the motor will, in varying degrees, smear this note. This is because
there are other currents in the magnet - eddy currents. The eddy currents
interfere with the primary electrical field of the motor. The midwoofer magnet needs a clear (uninterrupted) path for this
2000hz waveform to be most effective.
Asking a motor to perform 50hz and 2000hz is a tall order. Focus on the issue of
magnetic linearity (eddy currents) within the normal operating range of the voice coil becomes
important when the difficulty of the drivers task is considered. Just because
the voice coil is within the confines of the magnet doesnít mean that it is
operating in totally "linear" fashion. In truth, all motors are
effected by voice coil movement. Some are less effected than others. This fact
isn't profoundly clear in factory advertised specifications. The closest
number for assessing linearity is inductance. Better motors have lower
inductance, and poorer motors have higher inductance.
Thus, the primary motor function is the voice coil inducing a varying electrical field upon the
magnet/pole piece. But there is a reverse effect too.
The pole piece induces an electrical effect from the eddy currents that messes
up the magnetic field, and effects the voice coil. Shorting rings result in a drastic reduction of this reverse effect, and their
result can be measured in lower inductance and increased clarity.
A few common hifi driver companies have endeavored to conquer magnetic linearity issues. They are Focal (W series), Lambda, Scanspeak, Dynaudio, and SEAS (EXCEL line
only), and the Peerless (HDS line only).
These drivers are all on the expensive side (except for peerless). This isnít to say that all
other drivers sound bad, but it is my opinion (however infantile) that
magnetic linearity does make a difference. Thus, the "linear region"
isnít the same from one company to another. And Mr. Wheelers theory isnít
the end of the story.
Applying Mr. Wheelers raw power to weight ratio theory to Scanspeak drivers would lead you to believe that
many Focal and Audax drivers would outperform the SS by a wide margin.
This doesn't appear to be true. The Scanspeak drivers have a fairly poor power to weight ratio when compared to the
Focal and Audax stuff. The
Vifa stuff would also seem far behind Focal and Audax with their comparatively
poor (Mark Wheeler) numbers and their soft cones too. What appears true is that Vifa drivers are used in a disproportionately
large number of hifi speakers for their seemingly poor (Mark Wheeler) numbers.
Scanspeak drivers are used in some very high end systems despite their poor
(Mark Wheeler) numbers. Audax drivers are pretty scarce despite their good numbers. Focal is
represented outside of their own JM Labís line of speakers, but not to the
degree Mr. Wheelers numbers would indicate. Thus, some examination of what
drivers the commercial speaker manufacturers use seems to indicate that there is
something else afoot. There is more to this issue than Mr. Wheelers
numbers would indicate. I believe the missing issues are voice coil inductance
and magnetic linearity.
A very good place to start a study on drivers is at the LDSG
webpage. Upon entering this hobby, I spent about 100 hours internalizing the "LDSG".
It is a phenomenal web page where Bob Stout has spent a great deal of
unselfish time and effort.