D I O... P A P E R
High-Efficiency Full-Range Drivers
2004 by Steve Deckert
sounding low power tube amplifiers becoming integrated into the
hi-fi consciousness there is more then a casual interest in high-efficiency
full-range drivers. As a result there have been many speakers manufactured
based on these types of drivers. Unfortunately from a realistic
perspective many of them have a sound that is somewhat questionable.
Odds are not in one's favor when it comes to designing cabinets
for these drivers unless they are fully understood and properly
implemented. As a result odds are not in one's favor that there
will be any long term satisfaction from owning a pair. At least
not unless they're really good. Of course everyone will claim
theirs are really good so where does that leave us?
is a collection of observations and or insights that will help you
understand why these drivers sound the way they do. Hopefully
with a better understanding of these potentially wonderful little
beasts you'll be able to buy a good one, or tweak something you
may already own to a more satisfying level of performance.
thing to understand about these drivers is what makes them efficient
and what differentiates them from a more conventional audiophile
driver. The efficiency comes from the voice coil design and
stronger magnets combined with lighter cones and spiders. Higher
magnetic force controlling a less then normal moving mass.
A visual picture
of what this means can be easily seen by looking at the differences
in voice coil designs between the two types of drivers. The
high efficiency driver will have a thin or even flat voice coil
wire wound to a width that is close in size to the magnetic field
that surrounds it. This magnetic field exists between a metal
pole piece that fits inside the voice coil bobbin and a steel ring
that surrounds the outside of the voice coil. The magnetic
gap (or flux) is typically between ¼ and ½ inch long in the
majority of drivers made today. The large differences
between high and low efficiency drivers can be found in the width
of the gap. The other thing that varies between the two types
of drivers is the diameter of the voice coil wire and the winding
width. Many popular audiophile drivers and especially long-throw
woofers have wind widths that exceed the magnetic gap by as much
as 400% or more. This is called an “overhung” voice coil.
High Efficiency drivers typically have the opposite
type of coil design referred to as “underhung”.
The gap between
the voice coil and pole in a good high efficiency driver will usually
be so close that a dollar bill will just fit between the two. In
contrast a low efficiency driver (like the overwhelming majority
of drivers used today) will accommodate 1 or 2 business cards or
more. Now, if you’re building a driver from scratch
and your first attempt is to use a high efficiency approach, you’ll
measure it and see a frequency response that is full of peaks and
dips that can exceed 12 dB. This is usually handled
by weighting the response recorder or reducing the frequency sample
rate so that your response graph is plotted from less points. This
averages out the response and hides the spikes - on paper. It
won’t change the way it sounds however, so once the obsession to
get a smoother and flatter response takes over you find yourself
with the same driver using a wider gap and longer coil. Now
the response peaks are 5 or 6 dB instead of 12.
you jump up and down claiming victory? Considering your driver
has (as a result of your meddling) now lost 6 dB of efficiency and
will need 4 times the power to reach the same output it’s debatable
whether the result is a cop-out or a success. No matter
how you compare the two, the high efficiency driver will always
have 6dB more detail, and be 4 times faster making it FAR more linear
and for lack of a better term – accurate. The low efficiency
driver will always look smoother on a frequency response plot.
find it interesting to know that because of the resistance and capacitance
and inductance created by moving the voice coil inside the magnetic
gap (also known as Impedance) there is a phase shift that manifests
itself across the frequency response of the driver. For example,
if you started at 100 Hz and moved up the scale you would find that
absolute phase does not exist. By the time you reached 5 or
6 octaves the output of the driver can be delayed in time by as
much as 180 degrees or more. Boy that sucks, you think to
yourself, and yea it sorta does. The trick is to make it work
for you rather then against you but that’s another paper. You
wonder why it wouldn’t be possible to make a driver that maintained
at least a nearly flat phase angle across its bandwidth, and actually
some companies do. The problem is this – on a linear phase
angle as you go up in frequency the efficiency follows. That
means that a driver designed with linear phase response would get
louder and louder as it travels into the midrange and treble frequencies.
This of course wouldn’t sound too good, and it certainly doesn’t
look attractive on paper so manufactures design a phase shift into
the drivers to prevent the efficiency from rising too much at higher
frequencies, or put another way - the keep the frequency response
In a perfect
world, such a linear-phase driver would be used in a full size straight
horn where the horn itself would exponentially raise the efficiency
as the music goes down in frequency. This can nicely
offset the response of the driver and end with a result that has both
flat frequency response and a minimal phase shift. When
you start downsizing and or folding (usually both) the perfect horn
you open a can of worms that will include phasing problems. I
think I’ve just outlined a substantial reason for why Lowther drivers
in particular sound the way they do.
believe that this is the most overlooked part of a high efficiency
full range driver. In a nut shell, the cone is always going
to be very THIN. A business card is thicker then most of these
cones. It kills me when I look at some of the cabinet designs
being used with these drivers today. It’s a PUSH PULL device
folks. AC – Alternating. For every forward motion there
will be an equal and opposite rearward motion. Why do so many
people focus the majority of their attention on the sound that comes
off the front of the driver? There is just as much coming
off the rear of the driver. When you cram that sound into
a box of some sort what exactly do you think is going to happen
to it? On a low efficiency driver with a thick pulp cone you
can wait for the sound to ring around in there until the insulation
and dense pulp of the cone eventually absorb it. With high
efficiency full range drivers you don’t have that luxury.
At least 90%
of the sound that came off the back of the driver will reflect back
right through the cone and out into the air if steps are not taken
to deal with it. To illustrate this you can remove the high
efficiency full range driver from it’s cabinet and take a battery
operated portable radio and drop inside the cabinet. Make
sure the radio is turned on and playing at a normal listening level.
Now re-install the driver in the cabinet and sit back and
listen. You will hear the radio perfectly and hopefully after
the horror wears off you’ll have an epiphany and start modifying
the cabinet. This alone accounts for most of the “Bad Sound” you
hear from these types of speakers.
not perfect devises. However many people blame them for sins
they simply don’t commit. For example the famous or infamous
rather – Lowther “Shout” is often blamed on the whizzer cone when
in fact it is usually the problem described in the paragraphs above.
The whizzer cones on 8 inch drivers extend response above
5 or 8 kHz. They do not effect the sound in the 2K area where
the “shout” is perceived. Don’t believe it, cut one off and
measure the response. The biggest way to improve the performance
of a whizzer cone is the application and type of phase plug used.
Fixing a bullet shaped plug onto the pole so that it sticks
out past the voice coil instead of using a dust cap will usually
correct or at least improve many of the phase conflicts that exist
between the two cones. This in my opinion will make or break
a drivers sound quality. Of course there are many types of
phase plug designs. Treating the whizzer by sticking small
pieces of foam under it actually does little to the whizzer cone,
but rather damps the top end response of the main cone.
noticed also do not realize that a high efficiency speaker because
of it’s light weight voice coil and tight voice gap must warm up
before it sounds good. This is because the diameter of voice
coil increases as it heats up and reduces the gap between it and
the plate. True in all speakers, but when the gap is huge
the tiny increase in diameter is by percentages far less meaningful.
In a high efficiency gap the percentage of coil growth is
significant. In such a sensitive device, operating temperature
absolutely WILL change the sound and even the measured Theil and
Small parameters. I find it takes about 30 minutes of playing
at a normal listening level for voice coil to stabilize and that’s
about when things start really sounding right.
With any speaker,
where it sets in a room will determine it’s frequency response and
the more experimentation one does with placement the better
the odds will be for long term satisfaction. Many full range
drivers will sound best between 5 and 25 degrees off axis because
the frequency response will be flatter. This is partially
because of the whizzer cone and since most all full range drivers
have one I thought it worth mentioning.
speakers should be called high-resolution speakers. People
don’t comprehend how high this resolution actually is. No
system will ever sound better then it’s weakest link. You
will not be able to tolerate these types of speakers without finding
a high quality speaker cable. Same thing goes for interconnects.
Generic interconnects like those packaged with generic CD
players and lamp cord is simply not going to cut it. If you
can’t do the cables right you’re better off to wait to get into
this type of speaker until such a time that you can.
applies to amplifiers. Stay away from high power amps and
if you don’t like to gamble with odds, stick with tubes. Your
system can’t sound any better then your amplifier. If the
amp isn’t as good as ( or better then) the speakers and cables you
ARE WASTING your time.
also applies to preamps and your source. Recently a customer
came by with a wonderful sounding DAC. It was well above average
– I would place it in the top 10% at any price. We listened
to a good pair of high efficiency full range single driver speakers
with it for a few hours. Then I put my reference vinyl rig
on as the source. The customer's exact words were “Are the
same speakers playing?” The difference was obvious. The
sound took on a size and weight that actually did make it sound
like far better speakers were playing. The moral is that even
if you have the best source in the world it will probably always
be the weakest link so long as all the other ducks are in a row.
This is of course unless you can listen to 2-inch master tapes
directly off the mastering deck, then you have the best possible
source. In the real world that’s not likely to happen for
most of us, so as far as sources go it’s always a balance of compromise.
As a general
rule you don't put a rocket in you're ass and light it unless your
full range drivers if done right can have wonderful frequency balance
with spot on clarity. They can have full bass and nicely extended
highs without midrange glare if they are implemented in a good cabinet
design. There is no perfect cabinet design, or anything else
for that matter so success is usually found by those who do the
best job of balancing all the pros and cons. No one said it was
going to be easy but then if you were satisfied with your SUV you
shouldn't be looking at race cars.
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