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HORN ENCLOSURE
DESIGN
THEORY AND
PRACTICE
Loth-X Horn Loudspeaker
Technology
In
order to appreciate horns and back loaded horn
loudspeakers, it will be helpful to know a little
about their history.
Nutshell
History of Triodes and Horns
The
invention of the triode tube in 1923 made it
possible to amplify an electrical signal to greater
levels than had been possible, and the triode could
produce an audible output when used to drive a
rudimentary headphone device known at the time as a
"receiver." The receiver consisted of a fixed coil
on an iron anchor which produced vibrations in a
metal diaphragm close to the coil. The first horns
speakers were basically a horn attached to a
receiver, and although the sound quality was quite
bad it was better than what had been posisble prior
to the invention of the triode output tube.
Due
to the low power output of the triode, high
efficiency loudspeakers were required to produce
sound pressure levels sufficient to be heard in a
room, which happened in 1927 when two Bell
Laboratory engineers, Wente and Thuras invented the
compression horn driver, commonly known as a
driver. Their driver used a field coil to magnetize
the pole pieces (permanent magnets at that time did
not have a practical strength/size ratio to be of
practical use) and incorporated an under-hung edge
wound aluminium ribbon voice coil.
The
diaphragm was an inverted aluminium dome attached
to a self supporting voice coil. The Wente and
Thuras driver also had a phase plug; a device
placed between the diaphragm and horn throat which
enabled the sound waves from the diaphragm to merge
into a coherent wave front in the horn
throat.
As
compression drivers will not produce bass
frequencies, they are either mated with direct
radiators or horn loaded bass drivers. P.G.A.H.
Voight did important work on the tractrix type
horns and later teamed up with a UK company called
Lowther to form Lowther-Voight. Lowther still
exists and builds drive units according to Voight's
practices. Note: Loth-X loudspeakers do not use
Lowther drivers, they use Loth-X drivers, designed
and manufactured expressly for use in Loth-X
loudspeakers.
Direct radiator
drivers
As
time went on, Rice and Kellogg "invented" the
direct radiator speaker (conventional driver).
Horns began to disappear from the scene, being big
and unwieldy. Soon after World War II, more
powerfull push-pull pentode amps began relieving
the humble triode of its duties. Later on, the
ultimate power/dollar rating transistor amps
arrived. As the need for high efficiency
loudspeakers dimished, manufacturers began cutting
costs by using low efficiency smaller magnets in
their drivers. Although horns no longer had a major
share of the market, they survived in movie
theatres, PA applications, and other applications
where their high efficiency and low distortion
outweighed their typically large cabinet volumes.
Special
qualities of horns
What
special qualities do horns have that endear them to
enthusiasts besides having very high efficiencies?
The horn may be viewed as an acoustic impedence
transformer. When a diaphragm vibrates, pressure
waves are created in front of it. This is the sound
we hear. Coupling the motion of the paper to the
air is not an easy thing to do due to the very
different densities of the vibrating diaphragm and
air. This is can be viewed as an impedence
mismatch. We all know that sound travels better in
high density materials than low density materials,
and in a speaker system, the diaphragm is the high
density (high impedence) medium and air the low
density (low impedence) medium. The horn assists
the solid-air impedence transformation by acting as
an intermediate transition medium. In other words,
it creates a higher acoustic impedence for the
transducer to work into thus allowing more power to
be transferred to the air.
A
horn is a tube whose cross-section increases
exponentially. The narrow end is called the throat
and the wide end is called the mouth. The
transducer is placed at the throat. When the
diaphragm moves near the throat, we have a high
pressure with a small amplitude in a small area. As
the pressure wave moves towards the mouth, the
pressure decreases and the amplitude increases.
Excellent natural efficient
amplification.
As
mentioned earlier, horns have very special
properties, summarized as follows: Less distortion
generated. For an equivalent SPL, horns require a
smaller diaphragm. Distortion is directly
proportional to the size of the diaphragm. A large
diaphragm electromechanical transducer
(conventional driver) has to move much more than a
horned diaphragm in order to create the same SPL
(sound pressure level), and the larger the
excursion, the worse the distortion. So, for a
given SPL, a horn loaded system will generate much
lower distortion than an electromechanical
transducer.
Since
the diaphragm is smaller, it is lighter and thus it
accelerates and decelerates faster. This, in the
real world means superb, fast snappy
transients.
As
the excursion of the diaphragm is very small as
compared to an electromechanical transducer, the
voice coil is much smaller and again, this
translates to a lower moving mass and again,
results in fast transients. Small voice coils also
take full advantage of the flux in the pole piece
gap. This increases the efficiency of the
transducer allowing the amplifier to work with
greater ease.
Since
the amplifier has more headroom and the driver
handles peaks and high outputs more efficiently,
horns are able to produce much higher SPL's before
they distort. Thus, in the normal operating range,
they are faster, more dynamic, have a better
transient response with less distortion and stress
on an amplifier.
Large enclosure
size reduced by folded-horn design
Since
the horn mouth size is exponentially proportional
to the frequency one wants to reproduce, the size
of the mouth increases to ridiculous proportions
very quickly. Enter the folded horn. Here the horn
is flared within a cabinet by creatively folding
the horn within the cabinet. This allows for us to
achieve low frequencies without taking up too much
space. Unfortunately, the exponential equation used
in conventional horns does not apply in a folded
horn enclosure due to the folds. As a result, the
expansion rate has to be compensated for and there
is no fixed formula for this, thus, the standard
horn formula is only used as a starting point and
the rest of the folding/tuning is done by
painstaking listening and measuring iterations.
After 15 years of work, we have finally come up
with our own formulae for our various cabinet
folds.
Horns have
impact!
You
feel the music and you become part of the music and
the music becomes part of you. The full-range phase
coherent wave front of horns produces a solid image
and presentation, as opposed to the phase impaired,
smeared, and diluted imaging of much hyped low
efficiency speakers. Horns will never sound veiled
or compressed. The performers will be there, in
your room, performing for you in the same way and
location as the original recording venue. If the
performer was six feet from the microphone during
the recording session, he will be six feet away
from you when the performance is reproduced through
your system, not fifteen feet behind the speakers.
Why would one want the performers to be fifteen
feet behind the speakers if that was not how the
material was recorded? We want the performers to be
in the same room we are in, in front of us, so that
we an feel the music, front row
centre.
INHERENT
DEFICIENCIES OF MULTIPLE-DRIVER LOUDSPEAKER
DESIGNS
Phase problems
Multiple
drivers produce individual wave fronts which are
mostly out-of-phase with each other, whose
phase-coherent lobes require that the listener sit
in a speficied position with respect to the speaker
placement ("head in a vice"). In addition,
crossovers introduce phase anomalies which prevent
true and accurate phase coherence in the lobes
themselves, whether or not the listener is in the
optimum listening position.
The
fundamental problem of lack of phase coherence in
multiple driver loudspeaker designs causes several
types of insoluble problems, the most important of
which is loss and corruption of information between
the program material and the listener’s ears.
The types of information which are lost and
corrupted includes spatialization and localization
information, micro-dynamics, and acoustic
harmonics.
Lost
spatialization and localization
information
Multiple
driver loudspeakers produce multiple individual
wave fronts that result in loss of information when
these wavefronts cancel out out-of-phase
information in each other, resulting in loss of
detail and a smearing of the combined wave front.
Spatialization and localization information is thus
lost, and both stereo and soundstage depth
information is lost. This results in lack of body
in both instruments and voices, loss of stereo
placement information at various frequencies, and
the doubling of various frequencies, which smears
the localization of the instrument or voice even
further. Ever hear a saxophone whose lower
registers are on the left, and the upper harmonics
are on the right? This is a result of the
phase-impaired presentation of the typical multiple
driver loudspeaker.
Missing
micro-dynamics
These
are the "forgotten" elements of recorded music, due
in large part to the inability of multiple-driver
loudspeakers to respond quickly enough to low level
transient signals and continuous tones without
smearing and losing the details. The nuances of an
instrumental or vocal performance are never heard
on most systems, hence they are forgotten, or worse
yet, unknown by most listeners with multiple driver
systems.
Lost timbre and
harmonics information
Some
of the lower-level high-frequency content of
instruments and voices which provide the "timbre"
information that distinguishes an oboe from a
clarinet is also lost due to the inability of
multiple driver loudspeakers to respond quickly
enough, and to do so without phase anomalies which
cause the individual wave-forms of each driver to
cancel out important harmonic information. This is
analgous to the "lossy compression" of AC3, which
basically throws away lower volume program content
at the same frequency as louder program content,
basically, the "survival of the loudest."
Is the cure
worse than the disease?
In
order to overcome the phase-impaired loss of
spatialization and localization information, the
smearing and lack of body, and the loss of
micro-dynamics and loss of high frequency acoustics
harmonics, a boost in frequency response between
1,200 and 2,000 Hz is designed and implemented into
most mass-merchandised multiple driver
loudspeakers. Due to the phase-impaired nature of
multiple driver / crossover designs, it is much
easier and cheaper to merely put in a boost at a
specific frequency range than to reproduce the
original program content accurately and without
phase-anomalies. This artificially-enhanced tweeter
output serves to "etch" the stereo location of the
sound source, but is also creating an irritating
high frequency outline of the original program
material, emphasizing the sibilant portion of the
spectrum, and destroying any possibility of a true
and accurate tonality. In addition, this elevated
tweeter output causes listening fatigue: there are
more "squawk" boxes on the market than any other,
and, unfortunately, they have a large share of the
market, and a large mindshare among dealers and
customers alike.
As if
this wasn’t bad enough, there is a mutation of
the "squawk box", the "thud-squawker," discussed in
the next paragraph below.
Sizzle and
whump over substance
In
order to obtain an advantage in the noisy and
distracting mass-merchandise environment, some of
these same "squawk boxes" are also boosted in the
80-100Hz range to provide a perception of good bass
response, giving rise to the endemic
"thud-squawkers" that can be found in every
mass-merchanise outlet. Unfortunately,
"thud-squawkers" are also found in high-end
audiophile shops, whose prices are orders of
magnitude greater than their mass-merchandise
bretheren, but whose performance suffers from the
same fundamental flaws, no matter how artfully
disguised with cabinet cosmetics or
"techno-babble". Whether mass-merchandise or
high-end, all thud-squawkers ultimately fail to
provide long-term musical satisfaction, and, worse,
can actually turn the listener away from his
once-cherished music collection, since it is
literally painful to listen to it, due to the
irritating tweeter level. Similar to fast food
which has been liberally dosed with sugar, salt,
and fat to provide an imitation of something that
tastes good, so do the thud-squawkers make all
types of music sound alike, since they impose their
high-frequency boost and low-frequency hump on all
program material with which they played. Classical,
jazz, popular, chamber music, vocals, all types of
music have the same sonic signature imposed by the
thud-squawker. Similar to fast food that
doesn’t really satisfy, the thud-squawkers
leave the listener feeling disappointed and
uninvolved, knowing that there should be more
enjoyment and pleasure in the listening
experience.
AN
"OLD" PARADIGM PROVES ITSELF
Phase coherence
by design
Single-driver
horns are phase coherent by design, and do not
require any "patching" to fix an inherent design
problem, such as the phase-impaired paradigm of the
multiple-driver
design.
Horns will never sound veiled or compressed. The
ability of low-mass single drivers and low-mass
voice coils to respond immediately to the faintest
performance nuance in a phase-coherent manner
enables the listener to hear a true and detailed
presentation of every instrumental or vocal
performance. The performers will be there, in your
room, performing for you, not hidden behind a
curtain of smearing and distortion caused by
multiple driver wave front interferance and
cross-over induced phase anomalies. For this
reason, no short-cuts or thud-squawk patches are
required, no additive or subtractive colorations
are necessary to correct the inherently flawed
paradigm of multiple driver / crossover designs,
since all except two Loth-X designs only use a
single driver and have no crossover whatsoever. The
two budget models that do have an ambience tweeter
in addition to the six inch main driver, the
Ion
1
and Ion
2,
use only a single capacitor per enclosure, while
the Ion
3,
Ion
4,
Polaris, Minstrel,
Azimuth,
and Bard
use a single driver per enclosure, have no
crossover whatsoever.
THE
BEST TEST
LISTEN FOR YOURSELF
The
best way to experience Loth-X designs for yourself
is to contact O. S. Services to find the nearest
Loth-X dealer. If there is no dealer near you and
you refer a dealer that becomes a Loth-X dealer, O.
S. Services will provide a rebate of 10% of retail
for any Loth-X purchases from the dealer for a
year, in addition to any discount the dealer gives
you.
DEALERS
WANTED
Dealer
inquiries are welcomed - Loth-X is being introduced
to the North American market for the first time,
and we would like to develop a loyal, qualified
dealer network in North America. Although
solid-state electronics will work just fine with
Loth-X loudspeakers, there is nothing like the joy
of a single-ended triode
played through Loth-x loudspeakers. Loth-X will
have a complete tube electronic line eventually,
and currently is producing the $18,000
C-103
preamplifier. Loth-X also has an exquisite line of
first-rate turntables.
O. S.
Services also distributes Audion tube electronics
in North America, and qualified dealers are being
sought to handle Audion in North America as well.
Loth-X loudspeakers were designed with Audion tube
electronics, and evaulation tests have shown that
both products lines work very well with each other.
If you would like more information on Audion tube
electronics, please visit the O. S. Services
website at www.ossaudio.com,
or send e-mail to info@ossaudio.com.
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rights reserved © 1998 Loth-X
Audio