The Slot Loaded Open Baffle Project
and the Heil Years
Once the tweeter had been launched in a commercial product,
thoughts turned to the notion of a low frequency version. Oskar had a design
which looked like a stack of pancakes with magnets on the top and bottom and a
series of parallel cones driven by rods which passed through small holes in the
cones. As a concept it was workable, but it did not look like something that
would be easily made.
While Oskar continued to develop that approach I put together
a prototype for the AMT-3 that used six regular woofers firing against flat
panels in a slot loading arrangement. It looked like this:
You will discern several things from the diagram, among them
that I was not employed as a draftsman. What I hope you can make out is an
arrangement where three woofers on each side fire into a flat surface and the
pressure generated flows out the front through a vertical slot. The rear wave of
the woofers is allowed to amble out rear of the enclosure. These were 8 inch
woofers, which gives you an approximate sense of the scale.
The purpose here, as with Oskar's transformer, was to have the
air velocity at the output to be much faster than the velocity of the driving
surface, in this case the loudspeaker cones.
The motional energy imparted to the moving air is proportional
to the mass of the air times the velocity squared. The "squared" part means that
if you triple the velocity you impart nine times as much energy, or about 9 dB
worth. This raises the efficiency of the speaker quite a bit, and it improves
the transient response as the apparent mass of the air moved is higher in
relation to the cone mass of the drivers.
The way this is accomplished is by "squeezing" the air through
an opening narrower than the surface of the moving diaphragm of the driver. In
the diagram above, the air pushed by the cones has to exit out the front of the
loudspeaker through a slot opening whose surface area is only about one third
that of the cone drivers, and so it goes out about three times faster than the
velocity of the cones.
The bass performance of this system was a big surprise; I
remember sitting in the lab hosting guests from Audio Magazine playing these
speakers, and I noticed that their eyes were focused on a shipping box sitting
on the floor. The open flaps of the box were synchronously with the warp on the
vinyl LP we were playing. And it sounded great.
In spite of this, the project did not make it to market. In a
phenomenon which occurs in manufacturing, it was killed when the sales manager
intoned, "I can't sell a speaker with no back."
When such a prophecy is made, it tends to be self-fulfilling.
Instead, a conventional box (which looked the same) but with
two woofers on the front baffle went to market. Oskar continued to work on his
woofer, and eventually presented it in an open baffle. Since it does not seem to
have been a commercial success, the sales manager was perhaps proven right,
although we were both gone by then, and my early prototype speakers were torn up
to salvage the 12 woofers they contained.
Second, they are free of many of the internal resonance issues
that have to be considered with an enclosed box. The rear wave created by the
loudspeaker doesn't bounce around the interior of the enclosure, altering the
fundamental resonance and creating a host of additional resonant effects. These
higher frequency resonant waves partially make their way back out to the front
through the cone or any opening. They are not usually wanted.
Third, because the back wave is free to "illuminate" the
wall behind the speaker and also the rest of the room through reflection, you
get a very spacious acoustic effect that works very well if the room is big
enough and the placement of the speakers is done carefully. This effect is very
important to open baffle fans, and certainly was not lost on Dr. Bose many years
ago. It is also an important feature of most electrostatic and ribbon designs,
and of course the fine products of Magnepan.
This factor often leads to the second major downside,
humorously known as the WAF, or wife acceptance factor. Good bottom end from an
open baffle usually means a very large baffle, more than most living rooms will
tolerate. It also means the baffle needs to be at a distance from the rear wall.
I have made baffles out of 4 ft by 8 ft sheets of plywood, and
they have delivered pretty good bass without equalization. For such a large
baffle the question is not only whether they will fit in your space, but at some
point you start to notice that you are no longer effectively listening to that
rear wall illumination, as it is being blocked by the baffles. In this case, you
might simply try mounting the drivers in the wall. And some people do.
and Rear Sides
In the AMT-3 prototype, this is not the case. The air coming
out the front is squeezed at a ratio of about 3 to 1.The rear wave not so much.
Remembering that the motional energy is proportional to the square of the
velocity, we expect more energy out the front than the back.
The response is not symmetric – the rear wave that wants to
come around and cancel at bass frequencies is not as strong as the front wave,
and its ability to cancel is diminished. This is the raison d'etre of the slot
loaded open baffle – greater velocity out the front and less out the back.
After about thirty-eight years, I decided to recreate the
vision of that original prototype. In the summer of 2010 I had Roth Wiedrick
helping me out, and he began construction of a modified version which he
finished the following summer.
Here is a diagram showing how this woofer is laid out:
As an aid to understanding, I have highlighted the direction
of air motion in this system with green arrows. The woofers are wire in phase
and move toward and away from each other squeezing the air between them, and the
pressure makes the air flow fast out the front. The air in back is not squeezed,
and so flows more slowly out the rear.
It is pretty simple stuff. The baffle surface is made of MDF
and measures about 30 inches high by 36 inches wide. The six woofers are mounted
in a chamber formed by laminating five layers of 0.75 inch particle board, and
are arranged to squeeze the air out a front slot 2.25 inches wide and having
about one third of their combined piston area. The edges of the slot are rounded
to reduce turbulence. The eight-inch woofers are wired in either three in
series, two in parallel to form a 12 Ohm load, or three parallel, two in series
to form 5 Ohms. Because the wiring is slightly easier, I chose the 12 Ohms, but
if you are considering using a passive crossover here, you would definitely go
for the 5 Ohm arrangement for level matching.
We placed another baffle on top of this, adding another 30
inches of height and mounted a Lowther PM6A at 36 inches height. None of the
dimensions or construction details are critical. In any case, the intent here is
to explore the idea without getting too fancy.
Here is a picture of Roth assembling the front panel (safety
glasses raised for this photo opportunity only). Here you can see the Lowther
sitting on happily top in its own cozy baffle and you can just make out the top
pair of woofers in the slot:
Here is the view from the rear / side showing three woofers vertically aligned on one side, all three wired in series. Why is there a big space cutout behind the Lowther? To accommodate the new field coil version of the driver from Jon ver Halen (Mr. Fullrange).
Here is the rear view, illustrating a little more
constructional detail. Nothing but the best wire used for this project...
Of course one of the first things I did was haul them out into the room to measure the response difference between front and back. Here is the (smoothed) comparison measured near field on the woofer system after the crossover network has been applied. What you see is about about 9 dB pressure difference, equal to my 9 dB thumbnail calculation.
Unfortunately, this is only the difference you will experience in the close near field. As you move away from the slot, this will decline. In my listening position at three meters, the difference between front and back pressure measures only about 2.5 dB, a 78% increase.
When you build a slot loaded cavity like this, you will run
into a cancellation frequency that occurs when the wavelength is twice the
depth, where the pressure from the part of the woofer which is deepest in the
slot is one half the wavelength and cancels out the pressure from the portion of
the woofer near the slot opening. Here the cancellation occurs at 500 Hz.
Some equalization was called for on both drivers. The woofer
received a high Q high pass filter at 20 Hz which removed subsonics and gave the
response a 6 dB bump up at 20 Hz. The Lowther enjoys 4 dB of step correction
above 2 kHz or so to take down the overall top end. If you are a little
off-axis, you might decide that 2 dB is better. Here's the voltage response fed
to the finished system:
Notice that more gain is required for the bottom end, although if you're not crazy about bottom end you can probably get by with wiring the woofers for 5 Ohms, which will give you most of the difference if you want to run passive crossovers. Here's the acoustic response:
I had the advantage of using a B5 crossover ready-made for the
task, featuring lots of adjustments and spiffy discrete JFET buffers and stuff
If you want to DIY this, then here is the equivalent not-so-adjustable circuit rendered with op amps employed as buffers, except on the woofer high pass filter. where 6 dB gain is required.
Taken as a whole, here is the response of the result:
And here is a nice picture of the whole system:
The latter two items are self-apparent at this point, but you
need to hear some words assuring you that the audio experience is worth the effort. These
are the open baffles that have enough bass to push my JBL L300's off center stage. If
you look carefully in the background of the rear-view photo you will see one
They have a fairly deep bottom, and are very punchy in the
sense that transient attacks come at you with all the troops in formation, a
nicely aligned shock wave. They are well behaved with regard to mating with
full-range speakers, and with the crossover presented you can run both top and
bottom in-phase, which makes for an improved sense of space and imaging. It also
helps retain that full range driver character, but with bottom end.
Pardon me for having fun building such monster speakers. You
are probably looking at this and thinking, "That's nice, but I can't deal with
it." My actual point is that you can use this technique to get more bottom end
out of any open baffle, large or small. You may sacrifice some deep bottom end,
but you can still build a good OB woofer using a more reasonably sized
You can also make them with single-woofers and a little imagination. If
you're not afraid of two resistors, a coil and a capacitor you might be
surprised how much you can accomplish with a really simple single-pole passive crossover. Got
a saber saw?
Go ahead, make my baffle.
© 2011 Nelson Pass
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