April 16th 05, 01:38 AM
>>Why Horns?
By Dr. Bruce Edgar
Edgarhorns
I remember hearing a very good horn loudspeaker for the first
time in 1978. It was sheer magic to my ears. I could not believe the
incredibly realistic sound that seemed to radiate effortlessly into the
room. But when I talk to other audiophiles about horns, the reaction I
hear is usually one of disgust. Like a bad hair day, everyone seems to
have had a "bad horn" day.
Rich Weiner of "Bound for Sound" recently described his "bad horn"
day. "As a neophyte audiophile, I once had the horrifying experience of
listening to a friend's horn-based speaker system. "Killer equipment.
You've never heard anything like it," he promised. Indeed I hadn't. I
found myself sitting about six feet from a pair of Klipsch corner horns
driven by Phase Linear 700 amps. My friend was right. I have never
heard anything like it, although I understand that standing directly
behind a 747 during takeoff is quite similar. Since that time I have
avoided horn speakers." (Bound for Sound '99CES Report).
But times are improving. In the same report, Weiner says, "Perhaps
it's time to try horns again...Edgar's Titan (Horn) system was quite
impressive...detailed and articulate...">> Weiner's experience is not
unique. Other reviewers and audiophiles are coming around to the
concept of an acceptable audiophile horn system.
So you may ask, "What is the difference between the vintage horn
systems and the new generation of horn loudspeakers?" I have been
researching that question for the last 20 odd years. In the process, I
founded Edgarhorn whose main goal is the design and manufacture of
audiophile quality horn loudspeakers. But I'm getting ahead of the
story. After I had heard my first good horn speaker, I began trying to
hear other horn speakers and other pseudo-horns. Most of the ones I
listened to didn't come close to that first system that I'd heard.
Eventually, I was able to analyze that good horn system to discover
what made it tick. It belonged to a Senior Engineer at the aerospace
company where I worked. At the time, I was a research scientist
specializing in radio wave propagation and signals analysis. The design
came from a group of amateur horn builders on the east coast lead by
Ben Drisko in the early 50's. This system used a Drisko folded corner
bass horn design, similar to the Klipschorn, and a JBL 375 compression
driver on a Western Electric midrange horn. The component that made
this system so good was the JBL 375 driver. My engineer friend recently
bought a set of my Tractrix horns for his 375 drivers. When we tested
them, I was astounded by their ruler flat response from 400 Hz to 10
kHz.
In the early 80's, I set about tracking down all the horn articles
and papers and analyzing all of the published and underground designs.
I was very disappointed. There was no clear way of horn design. What
was even more depressing was the periodic regurgitation of wrong headed
ideas of horn design in articles from year to year that has infiltrated
into some horn design software that is marketed today. So I embarked on
course of experimentation with horn design and construction. I figured
that with my background in wave propagation, spectrum analysis, and
experimental physics, it should be easy enough to arrive at a
satisfactory design. Was I ever wrong. After 20 years of horn building,
I'm still finding new ways of designing and building horns. It's the
Edison experience of having to go through all the different
combinations and permutations of drivers and horns. And my customers
keep coming in with different requirements that change my views and
shift the proverbial paradigm.
So back to the question, "What made the vintage horn systems sound
bad?" There are a variety of problems. I have run into most of the horn
problems either in designing my own horns or duplicating somebody
else's horn.
The first problem is electronics. Weiner talked about listening to
some Klipschorns with high power solid state amps. The amps were one
source of the bad sound. Horn systems typically have sensitivities of
100 to 108 dB SPL with one watt input. Even at the loudest sound that
you would realistically audition any speaker system (95-100dB), the
amplifier is only delivering a watt or so to the horn system at peaks.
Most of the time the amp is idling at 100's of milliwatts; yes, I said
milliwatts. At this level many high power solid state amps have real
problems with crossover distortion. For this reason, I tell people who
buy my horn systems to try different amps with them along with their
existing amp. The low power requirement of horns means that single
ended tube (SET) amps can easily fill a room with sound. However, not
all single ended tube amps are created equal. Some SET designs have
relatively high levels of distortion that can be easily heard on the
horn system. And of course, the horn gets the blame for the distorted
sound.
A second problem with horn loudspeakers is that they are easily
prone to resonant peaks in their response. I have heard some horn
tweeters that made me think that I was being drilled between the eyes.
Many horn midranges have a honky megaphone sound that comes from
resonant peaks. I have measured many old horn midranges with high
resolution spectrum analyzers and have found them loaded with high Q
spikes that give each horn speaker its own characteristic coloration.
Bass horns have their own resonance problems. Typically, a
well-designed bass horn that can go down to 35 Hz will have a total
volume of 20 cu.ft. or more. A company's marketing department will say
that it can't sell a big speaker like it. Moreover, the marketers say,
"Keep the 35 Hz flare but make it smaller." So the horn length is
truncated to make it smaller. The net result is long folded slowly
expanding tube that sounds more like a resonant tuba than a wide band
bass horn. The response plot looks as a series of harmonically related
resonant peaks.
Some resonant problems are caused by structural defects. At the
point where the sound comes out of the mouth of the horn, the edge of
the mouth can vibrate in a bell mode. If you have seen some old style
trumpet PA speakers, they were actually shaped like a round bell. A
bell will resonate at a frequency whose wavelength can be wrapped
around the circumference of the bell end. No horn is immune to bell
modes, but proper damping and tension bracing can eliminate the
problems.
A third problem is the use of improper drivers on horns. Good horn
drivers require heavy magnets and light weight diaphragms. Most regular
speakers used in typical box speaker systems have lower weight magnets
and heavy diaphragms. The use of regular speakers on horns will, for
the most part, yield restricted bandwidths and irregular responses.
Recently, Speaker Builder featured an article touting an easy horn for
your dome tweeter. I performed the experiment with a high quality dome
tweeter on a 800 Hz Tractrix horn. Without horn loading, the dome
tweeter went up to 20 kHz at a sensitivity of 88 dB. With the horn, the
efficiency increased several dB, but the response rolled off above 10
kHz. With such a horn loaded tweeter, any listener would ask, "Where's
the top end?" My survey of horn construction project articles has
yielded many other examples of using the wrong driver on a horn.
As a corollary to the wrong driver syndrome, an associated problem
is the use of PA horn systems in home stereo applications. For example,
I find many horn enthusiasts over the world using variants of the Altec
"Voice of the Theater" (VOT) speaker system. The VOT featured a
compression driver on a metal mid horn and a 15" woofer on a front
loaded 110 Hz midbass horn. The back of the woofer was loaded by a bass
reflex ported enclosure. In general, I have found the Altec compression
drivers and woofers to be good quality horn drivers, but the horn
design can be improved with my Tractrix horns. And I have done this for
numerous customers.<<
By Dr. Bruce Edgar
Edgarhorns
I remember hearing a very good horn loudspeaker for the first
time in 1978. It was sheer magic to my ears. I could not believe the
incredibly realistic sound that seemed to radiate effortlessly into the
room. But when I talk to other audiophiles about horns, the reaction I
hear is usually one of disgust. Like a bad hair day, everyone seems to
have had a "bad horn" day.
Rich Weiner of "Bound for Sound" recently described his "bad horn"
day. "As a neophyte audiophile, I once had the horrifying experience of
listening to a friend's horn-based speaker system. "Killer equipment.
You've never heard anything like it," he promised. Indeed I hadn't. I
found myself sitting about six feet from a pair of Klipsch corner horns
driven by Phase Linear 700 amps. My friend was right. I have never
heard anything like it, although I understand that standing directly
behind a 747 during takeoff is quite similar. Since that time I have
avoided horn speakers." (Bound for Sound '99CES Report).
But times are improving. In the same report, Weiner says, "Perhaps
it's time to try horns again...Edgar's Titan (Horn) system was quite
impressive...detailed and articulate...">> Weiner's experience is not
unique. Other reviewers and audiophiles are coming around to the
concept of an acceptable audiophile horn system.
So you may ask, "What is the difference between the vintage horn
systems and the new generation of horn loudspeakers?" I have been
researching that question for the last 20 odd years. In the process, I
founded Edgarhorn whose main goal is the design and manufacture of
audiophile quality horn loudspeakers. But I'm getting ahead of the
story. After I had heard my first good horn speaker, I began trying to
hear other horn speakers and other pseudo-horns. Most of the ones I
listened to didn't come close to that first system that I'd heard.
Eventually, I was able to analyze that good horn system to discover
what made it tick. It belonged to a Senior Engineer at the aerospace
company where I worked. At the time, I was a research scientist
specializing in radio wave propagation and signals analysis. The design
came from a group of amateur horn builders on the east coast lead by
Ben Drisko in the early 50's. This system used a Drisko folded corner
bass horn design, similar to the Klipschorn, and a JBL 375 compression
driver on a Western Electric midrange horn. The component that made
this system so good was the JBL 375 driver. My engineer friend recently
bought a set of my Tractrix horns for his 375 drivers. When we tested
them, I was astounded by their ruler flat response from 400 Hz to 10
kHz.
In the early 80's, I set about tracking down all the horn articles
and papers and analyzing all of the published and underground designs.
I was very disappointed. There was no clear way of horn design. What
was even more depressing was the periodic regurgitation of wrong headed
ideas of horn design in articles from year to year that has infiltrated
into some horn design software that is marketed today. So I embarked on
course of experimentation with horn design and construction. I figured
that with my background in wave propagation, spectrum analysis, and
experimental physics, it should be easy enough to arrive at a
satisfactory design. Was I ever wrong. After 20 years of horn building,
I'm still finding new ways of designing and building horns. It's the
Edison experience of having to go through all the different
combinations and permutations of drivers and horns. And my customers
keep coming in with different requirements that change my views and
shift the proverbial paradigm.
So back to the question, "What made the vintage horn systems sound
bad?" There are a variety of problems. I have run into most of the horn
problems either in designing my own horns or duplicating somebody
else's horn.
The first problem is electronics. Weiner talked about listening to
some Klipschorns with high power solid state amps. The amps were one
source of the bad sound. Horn systems typically have sensitivities of
100 to 108 dB SPL with one watt input. Even at the loudest sound that
you would realistically audition any speaker system (95-100dB), the
amplifier is only delivering a watt or so to the horn system at peaks.
Most of the time the amp is idling at 100's of milliwatts; yes, I said
milliwatts. At this level many high power solid state amps have real
problems with crossover distortion. For this reason, I tell people who
buy my horn systems to try different amps with them along with their
existing amp. The low power requirement of horns means that single
ended tube (SET) amps can easily fill a room with sound. However, not
all single ended tube amps are created equal. Some SET designs have
relatively high levels of distortion that can be easily heard on the
horn system. And of course, the horn gets the blame for the distorted
sound.
A second problem with horn loudspeakers is that they are easily
prone to resonant peaks in their response. I have heard some horn
tweeters that made me think that I was being drilled between the eyes.
Many horn midranges have a honky megaphone sound that comes from
resonant peaks. I have measured many old horn midranges with high
resolution spectrum analyzers and have found them loaded with high Q
spikes that give each horn speaker its own characteristic coloration.
Bass horns have their own resonance problems. Typically, a
well-designed bass horn that can go down to 35 Hz will have a total
volume of 20 cu.ft. or more. A company's marketing department will say
that it can't sell a big speaker like it. Moreover, the marketers say,
"Keep the 35 Hz flare but make it smaller." So the horn length is
truncated to make it smaller. The net result is long folded slowly
expanding tube that sounds more like a resonant tuba than a wide band
bass horn. The response plot looks as a series of harmonically related
resonant peaks.
Some resonant problems are caused by structural defects. At the
point where the sound comes out of the mouth of the horn, the edge of
the mouth can vibrate in a bell mode. If you have seen some old style
trumpet PA speakers, they were actually shaped like a round bell. A
bell will resonate at a frequency whose wavelength can be wrapped
around the circumference of the bell end. No horn is immune to bell
modes, but proper damping and tension bracing can eliminate the
problems.
A third problem is the use of improper drivers on horns. Good horn
drivers require heavy magnets and light weight diaphragms. Most regular
speakers used in typical box speaker systems have lower weight magnets
and heavy diaphragms. The use of regular speakers on horns will, for
the most part, yield restricted bandwidths and irregular responses.
Recently, Speaker Builder featured an article touting an easy horn for
your dome tweeter. I performed the experiment with a high quality dome
tweeter on a 800 Hz Tractrix horn. Without horn loading, the dome
tweeter went up to 20 kHz at a sensitivity of 88 dB. With the horn, the
efficiency increased several dB, but the response rolled off above 10
kHz. With such a horn loaded tweeter, any listener would ask, "Where's
the top end?" My survey of horn construction project articles has
yielded many other examples of using the wrong driver on a horn.
As a corollary to the wrong driver syndrome, an associated problem
is the use of PA horn systems in home stereo applications. For example,
I find many horn enthusiasts over the world using variants of the Altec
"Voice of the Theater" (VOT) speaker system. The VOT featured a
compression driver on a metal mid horn and a 15" woofer on a front
loaded 110 Hz midbass horn. The back of the woofer was loaded by a bass
reflex ported enclosure. In general, I have found the Altec compression
drivers and woofers to be good quality horn drivers, but the horn
design can be improved with my Tractrix horns. And I have done this for
numerous customers.<<