OPEN BAFFLE SPEAKER PROJECT
has been a peaked interest in Open Baffle Speaker designs on
our Forums. Threads on the subject have remained active for well
over year. Threads typically loose steam after only a few
weeks or months so needless to say there is real interest in it.
first question in the minds of those of us who haven't owned or
at least heard a pair is "Do they sound good, and if so - how?"
a speaker in a flat board rather than into a cabinet... On the one hand you have a di-polar
speaker that puts out as much sound behind it as it does in front
- usually good for imaging and sound stage depth. And of course
no box to color the sound. On the other hand you have no box to
prevent low frequency cancellation so no bass.
goes without saying that most open baffle designs
have more than one speaker, the second one is often placed in a
box to generate some tone and bass. Others are just simply
large in size with one or more 15 inch woofers helping out
a smaller driver. The larger cone area of a 15 inch woofer with
the added boundary effect from the floor can create enough tone
and bass extension to prevent the main speaker from sounding thin. In all cases it is a design challenge to
get any real bass out of a speaker stuffed in a board. Unless
the board is extra large the distance a sound wave would have to
travel to get from the front
of the cone and intercept the out of phase wave coming
off the rear of the cone is too short to prevent cancellation much
below 100 Hz. If you go with a small baffle so it won't take
up your whole room and like your speakers to sound like they disappear then you
can expect the figure to be even higher.
INTEREST IN OPEN BAFFLES
anything out of the ordinary, I find Open Baffle speakers intriguing.
I've heard many over the years, most I thought sounded better
than I expected and a few sounded worse.
I see it as a battle between bass and imaging -
to over simply it. I also notice in many designs the lack of
texture or richness in the lower midrange that a well done cabinet
either adds or preserves.
I like to see things stay no more complicated than
needed to achieve a goal when it comes to audio gear and speakers. Most Open Baffle guys are running at least two if not three
or more speakers in their set ups. The main driver, a supplemental
driver for more bass or tone, and a sub woofer.
I see the fastest growing niche in
the audio world as single driver full range
crossover-less loudspeakers that can be driven with only a few watts.
I would like to combine the huge interest in Open Baffle speakers
with this niche.
A single driver with no crossover means a single
glorious point-source. My interest in Open Baffle design would
be to preserve this purity. If you add a second larger driver
in the baffle you have all kinds of complications to address. The
phase angle at any given frequency will be different between the
two drivers, that's problem number one. Both drivers can't occupy
the same point in space, that's problem number two. Problem number
three is how to drive two drivers rather than one. You either
have to wire them together or use another amp, cables, crossover? Either option opens a Pandora's box of possible ways the coherency
of a single point source could be damaged.
A DESIGN GOAL (or FANTASY)
I would like an Open Baffle design that used a single
8 inch full range driver, and was small and sexy like a floor standing
cabinet might be. I don't want a lot of cabinet or wide board
to get in the way of imaging and the speakers ability to disappear. And I want it to have real tone and texture, like a well done
cabinet. My guess is audiophiles who have ever built
or designed an Open Baffle would ideally like the same things. It's
almost like wishing for something out of nothing. This is
no doubt why you don't see such an animal. Nevertheless it
would like to get as close to achieving this goal as possible. Not
only would I like it for myself, but I'd like to see the design (if successful) bridge the gap between the Open Baffle and Single
the years I've built a few Open Baffles. They ranged from sticking speakers
in boards, to sticking speakers in slightly fancier boards. It was
known going in that a sub would be used to help out the bottom.
As a picky audiophile I observed that you can't help something
out unless it is there to begin with. Even crossed over rather high around
120+ Hz there was a disjointedness to the sound. The subs always had
a character all their own (usually pretty bad and each one different) that simply did not
match the driver in the baffle. The problem was compounded when using
tube gear to drive the baffle and solid state to drive the subwoofer. Anything that the main driver did well was poisoned by the
non-matching driver, in this case, subs.
get me wrong, the OB's I tried didn't sound hateful, but I've gotten
more and more sensitive to harmonics as time passes and I'll have
to explain where I'm coming from before you understand what I'm
getting at. As someone who has run a small studio for the past 10
years that hosts an open stage on Thursday nights I stay very calibrated
to what live instruments sound like. I'm also a drummer who's
really picky about how the kit sounds and how it's tuned. I've recorded
every single session and often use those masters to evaluate speakers
you look at music, the thing that defines what you are listening
to in all its subtle details is harmonics. The timbre of a given
instrument would be impossible without the ultra complex harmonics
that stem from fundamentals of each note. This is true
anywhere in the audio band, and yes even if not especially in the
bass. My kick drum is responsible for the heart beat in the
music and most would perceive it as a bass note. However,
while the fundamental tuning on the drum is typically around 40
Hz, every recorded beat from that drum contains harmonics and overtones
clear up to 20 KHz. These harmonics are what make it sound
real. They are how you can hear what type of beater head is
being used on the pedals, and how you can hear differences in drum
heads even if they're always tuned the same. It is why there
are drum shells offered in Maple, Birch and a dozen other hardwoods
- each gives the drum a different sound.
my point with all this, is that if you want an instrument to sound
real when you play a good recording of it on your stereo, these
harmonics should be viewed as being like DNA and preserved in any way
possible. When you take the recording of the kick drum and
reproduce part of it through one speaker and part of it through
another speaker or system it will not sound like the real drum,
only a recording of one. You've basically taken the DNA of
that instruments sound and split it apart and tried to patch it
back together. This is not unlike Jurassic Park where they didn't
have all the pieces for the dinosaurs DNA so they used segments
from a frog's DNA to make it work.
preserve the DNA a single driver with no crossover must reproduce
the entire range of frequencies including the low frequencies.
You know it's hard enough to engineer something
that actually works well, but as an artist I'm also cursed with
making it considerably more difficult by insisting that it be pretty
- or esthetically pleasing. In my world the artistic
side challenges the engineering and drives the whole process. Engineering
things has nothing to do with brilliance, it's just plain tedious
work. Hard to get passionate about number crunching by
itself but easy when the artistic background theme is involved.
Well, getting back on track, I saw a
truly attractive Open Baffle
speaker recently. It had
an 8 inch full range driver in it, but also had an attached cabinet
with a 15 inch woofer inside. Staring at it I began to wonder
if it would be possible to make a similar looking speaker without
the supplemental 15 inch woofer inside (imagine that)! I pictured an empty
cavity that resonated in concert with the 8 inch full range driver.
The resonance of the cavity would achieve the tone and texture.
But would it work?
Having done something remotely similar with the
design of the Imperial
SO horn cabinet I knew that it was possible to achieve massive
amounts of bass in a horn flare without actually having a driver directly
coupled to it. Instead the driver fires out into the room
past a carefully positioned slot where the slot becomes the throat
of the horn flare. As far as I know this was and still is
an original concept in horn design. It has so many advantages
you'd have to read the papers on it to fully appreciate why I wanted
to apply it in some way to this Open Baffle research.
Actually, integrating a horn mouth into this design
really isn't an option. While possible, it would be too small
in size to achieve any low bass. I am shooting for a goal
of around 40Hz. A tiny horn flare that fits this profile would limit
response below 100Hz. kind of defeating the purpose.
In this design the cavity must easily resonate
when excited by a nearby speaker. I did a conceptual model
on the computer ito determine what it would take to make this cavity resonate across two octaves.
Specifically I was interested in how to couple the cavity
to a tuned slot just below the back side of the speaker. This
would involve creating a neck at the top of the cavity (like a coke
bottle with respect to Helmholtz) and I needed to work through potential shapes. The formulas I found
most helpful in determining the transmission loss characteristics
of the neck, which would couple the cavity and driver were in
a paper by Ahrnet Selamet and IIjae Lee from the Dept. of Mechanical
Engineering and The Center for Automotive Research at Ohio State
University The 2002 paper can be accessed at the Acoustical Society
of America website and is recommended reading for anyone designing Helmholtz
resonators for noise control or bass traps.
In fact, one could say that if this worked, it was
a bass trap added to an open baffle that created the bass. Hey,
we're getting into the Zen zone...
Armed with some idea of
how to design the neck, I did
a conceptual sketch of it and then built a rough prototype from
that sketch out of drywall to find out if this was even going to
Here is a transparent view... No
drivers will be used inside the cabinet.
Here is a view of the tuning slot just below the woofer. It allows you
to tune the resonant cavity by ear while the speaker is playing. Shut
this slot completely off and it becomes just another painful OB in
desperate need of a sub or helping hand woofer somewhere.
Here is a view of the drywall prototype with a modified FE206E
FIRST ATTEMPT OBSERVATIONS
The most fascinating part of this initial test was
the slot itself. Closed and the whole cavity is deactivated,
leaving you with a tiny completely bass-free open baffle for comparison
sake. I found opening the slot took the hardness out of the
midrange and the sound off the baffle. A massive improvement
in midrange openness resulted...
The other thing that came as no real shock was that
the bottom of the cavity had to be vented slightly to increase the
efficiency of the cavity resonance. (At least on this model it did.)
Once the speaker was adjusted, I found the sound
of it intoxicating. Not perfect, just doing several things
better than I expected, thus intoxicating. I did notice
during this time that some of the music had detectible bass that
you could easily hear and feel. This was of course exciting, but
other music that should have had bass was missing it. Since I hadn't
even stuffed the cavity I expected a very narrow band typical of
Helmholtz resonators. I just wanted to see how loud the resonance
would be and if it would be enough. I expected it to drop
a lot by the time I got the bandwidth wide enough to be usable.
After a few days of casual listening and playing
with it I decided to see what it was actually doing so I got the
frequency generator out and started sweeping. Results of the test
were expected but still rather horrifying. There was no detectible
bass at 100Hz. In fact there was no bass at 50Hz. There
was only bass at 80Hz.
This was a good time for a long pause. I figured
out that the widest Q that would still have enough output to be
considered usable was still too narrow. It was starting to
look like at least three resonators tuned 10Hz apart would be needed.
This added level of complexity wasn't a terribly exciting
option so with a bit more math I was able to talk myself out of
it determining the dips between each peak would become noticeable only
to some of us.
I really needed the rubber throat effect of a good
horn flare to get the smooth flat bandwidth I wanted but had already
determined there wasn't enough space. To that end I pondered another
day and then decided to meditate on it. I put the cabinet
on its side and lay on the floor staring into the open bottom
of the cavity. After two hours of this I felt like I understood
the cavity. After all, the formulas I used for complex neck designs
were basically how to
change the tuning of a given cavity without changing its size. Regrettably, as usual,
there were no formulas that could model the ideas I wanted to try
THE DESIGN THAT WORKS
In another evening's modifications, I created a
protruding into the cavity that terminated about 1/2 way down,
with a narrow slot just behind the driver. This is how I intended
to get the rubber throat effect I'm after rather than using a neck
tuned at a center frequency. To make
it work properly I created another small cavity with enough volume to fill
half the area of the tapered flare. This smaller cavity acts
as a capacitor storing enough air to feed the taper under resonance.
This keeps the piston of air resonance from directly
hitting the driver. Also the main cavity is vented with an
1/8" slot at the baffle through the base of the capacitor to
help excite both by preventing a pressure drop in the capacitor,
allowing it to grow in size on demand. The final modifications included
sealing the bottom of the main cavity and putting a round 3/4"
long port at the top of the cavity to set the low frequency cut
While listening to the modifications it was a thrill to discover
this actually worked perfectly. The cavity now resonates with a
flat response across over an octave between 40 and 80 Hz.
Unlike a ported speaker cabinet where the sound
inside the cabinet is undesirable if you place your ear next to
the port, this cavity sounds clear, not garbled, and contains exactly
all of the musical information and low frequencies that are missing
from the Open baffle part of the speaker. This in and of itself
is rare and remarkable as far as I'm concerned. Part of the
reason this is even possible is because the driver is not directly
coupled to the cavity. There is no pressure exerted inside
the cavity from the driver and the natural resonance of the cavity
is at a very low amplitude leaving it distortion free when compared
to speaker in a box that would cause the panels of the box to resonate
(aka color/distort the sound).
by themselves when compared to a simple open baffle of the same
size are most impressive. The difference in midrange and overall
imaging are remarkable.
The final challenge is how to get the sound inside
this cavity to equal the same level as the sound coming from the
driver. If you listen to the port at the top of cavity with
one ear it places the back of the driver only 12 inches away from
your other ear. In this position the frequency balance is
simply outstanding, sounding like you're hearing a live gig. The
deep bass is wonderful.
obviously we can't stick our head down to the port to listen to
the speaker so the most logical way to get the sound out of the
box without increasing the pressure inside the box is to passively
amplify it using a microphone feeding a second system. It
might sound crazy, but provided the second system is done correctly,
the purity of the design concept is preserved.
mic feeds a good quality microphone preamp that feeds an amplifier
matching the one driving the Open Baffle. This second amplifier
drives a second set of cabinets capable of reproducing low bass
notes. The second system will allow you to tweak the low end
in several ways ensuring you have as much or as little weight as
you want at any given time.
is this approach better or different than simply splitting the signal
from your source into two separate systems? Read the part
about harmonics again. Also understand that the second system
is producing a live
of the sound inside the cavity of the Open Baffle speaker.
this project progresses and if we decide to manufacture it, the
speakers would likely come with their own microphones pre-installed
in the proper way and a stereo microphone preamp. It will
be possible to get good results from supplying your own amplifier
and speakers for the live broadcast. We will develop the whole
system with matching electronics and synergistic broadcast cabinets
to evaluate the design in the best possible light. At this
stage of the game it becomes the next logical step.
you see another paper on this in the future you'll know it was a
More explanations of this overall concept were
given in the forums and re-posted below:
I suppose whatís complex about how everyone else is doing it
would be the result. Trying to get bass
from a subwoofer and have it blend flawlessly with the driver in the open
baffle isÖ complicated. Trying to fill a
mid bass hole with a second woofer isÖ complicated. Trying to get the same imaging from a 15 as
you would an 8 inch driver isÖ complicated. Harmonically handicapped might have been a better term.
While this is no less complicated from a gear perspective it
retains honest harmonic integrity by presenting the amplifier with a single
crossover-less driver that completes the full bandwidth from top to
bottom. All the bass, mid, treble you
hear are being reproduced by that driver. The low frequencies are sampled
naturally by a separate resonant cavity and have no negative effect on the driver unlike a box where the low frequencies are
creating high pressures against the cone and coloring its sound. And unlike a driver applying high pressures
and high frequencies to a box (modifying the wave fronts inside the box), coloring the sound in the box. (This is why
sound inside a speaker box always sounds garbled).
By separating the two, both the sound inside the cavity and
the sound of the driver are uncolored. Getting the sound out of the cavity is where the live broadcast comes
in. Using another set of speakers, a
pair of mics and a good mic preamp allows us to hear the sound that is inside
the cavity. Since this is mostly low
frequencies we have to compare this approach with the conventional OB mated
to a subwoofer, both fed from the same line level
signal. In that approach, the subwoofer
is reproducing the low frequencies from the recording. In this approach, the second speakers are
broadcasting a live, pure analogue, direct feed from the cavity, not from the
Careful matching of components, ie., the mic preamp, and
power amp and speakers for the broadcast will of course determine how much
better this approach is, but I have already tried it with random mid grade
components and it is still clearly working better than when we turned off the
cavity (by closing the slot) and used a subwoofer. The bass sounds real on the broadcast setup,
even with all solid state gear because I tried that too. The bass does not sound as
real on the subwoofer
Besides the sheer strength of a live broadcast as a source vs. a CD
player there is another big reason why this approach is proving better than
closing the cavity and using a sub. Itís
the cavity. The cavity gives the OB wonderful tone and timbre. Without it, there is, among other things, an
obvious lacking of something in the mid bass region. Depending on how accurately you adjusted the
level on the sub, a frequency response chart would show a shelf in the response
with a nice hole in the center of it. Regarding the texture and tone from the
cavity, itís clearly audible without the broadcast speakers.
Iíll admit this whole thing is a bit of a mind twister. It's been keeping me on my toes and in the
dark. Iím sure it will prove to be very
interesting as it develops. Iím anxious
to get the entire set up completed for some serious evaluation when Iíll be
able to hear why this was a good or bad idea.