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September 2011

The Slot Loaded Open Baffle Project
Article By Nelson Pass

Difficulty Level

Intro:ESSand the Heil Years
In 1972 I had the good fortune to begin working for ESS,arriving a few weeks before they met Oskar Heil and his air motion transformer design. Youcan imagine the excitement that followed. The first Heil tweeter was a dipoledesign like an open baffle and it operated from about 800 Hz to above 20 kHzwith high accuracy and efficiency.

Once the tweeter had been launched in a commercial product,thoughts turned to the notion of a low frequency version. Oskar had a designwhich looked like a stack of pancakes with magnets on the top and bottom and aseries of parallel cones driven by rods which passed through small holes in thecones. As a concept it was workable, but it did not look like something thatwould be easily made.

While Oskar continued to develop that approach I put togethera prototype for the AMT-3 that used six regular woofers firing against flatpanels in a slot loading arrangement. It looked like this:

You will discern several things from the diagram, among themthat I was not employed as a draftsman. What I hope you can make out is anarrangement where three woofers on each side fire into a flat surface and thepressure generated flows out the front through a vertical slot. The rear wave ofthe woofers is allowed to amble out rear of the enclosure. These were 8 inchwoofers, which gives you an approximate sense of the scale.

The purpose here, as with Oskar's transformer, was to have theair velocity at the output to be much faster than the velocity of the drivingsurface, in this case the loudspeaker cones.

The motional energy imparted to the moving air is proportionalto the mass of the air times the velocity squared. The 'squared' part means thatif you triple the velocity you impart nine times as much energy, or about 9 dBworth. This raises the efficiency of the speaker quite a bit, and it improvesthe transient response as the apparent mass of the air moved is higher inrelation to the cone mass of the drivers.

The way this is accomplished is by 'squeezing' the air throughan opening narrower than the surface of the moving diaphragm of the driver. Inthe diagram above, the air pushed by the cones has to exit out the front of theloudspeaker through a slot opening whose surface area is only about one thirdthat of the cone drivers, and so it goes out about three times faster than thevelocity of the cones.

The bass performance of this system was a big surprise; Iremember sitting in the lab hosting guests from Audio Magazine playing thesespeakers, and I noticed that their eyes were focused on a shipping box sittingon the floor. The open flaps of the box were synchronously with the warp on thevinyl LP we were playing. And it sounded great.

In spite of this, the project did not make it to market. In aphenomenon which occurs in manufacturing, it was killed when the sales managerintoned, '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 withtwo woofers on the front baffle went to market. Oskar continued to work on hiswoofer, and eventually presented it in an open baffle. Since it does not seem tohave 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 upto salvage the 12 woofers they contained.

The UpSides
They've been around forever, but open baffle enclosureshave a few charms that keep them semi-popular. First off, they are very easy tobuild. Anyone who can manage a hand held saber saw can make one out of a pieceof plywood. There is no exact measuring or tight joint fitting. At a minimum,they are not allowed to fall over, and other improvements are optional.

Second, they are free of many of the internal resonance issuesthat have to be considered with an enclosed box. The rear wave created by theloudspeaker doesn't bounce around the interior of the enclosure, altering thefundamental resonance and creating a host of additional resonant effects. Thesehigher frequency resonant waves partially make their way back out to the frontthrough the cone or any opening. They are not usually wanted.

Third, because the back wave is free to 'illuminate' thewall behind the speaker and also the rest of the room through reflection, youget a very spacious acoustic effect that works very well if the room is bigenough and the placement of the speakers is done carefully. This effect is veryimportant to open baffle fans, and certainly was not lost on Dr. Bose many yearsago. It is also an important feature of most electrostatic and ribbon designs,and of course the fine products of Magnepan.

TheDown Sides
There are two downsides to open baffles. Because the rearwave of the driver is free to wander, at low frequencies it comes around tovisit the front of the loudspeaker, and being out of phase, cancels the frontwave to a varying extent. As a result, open baffle designs have less deep bottomend, depending on their size. In general, the low frequency response is aninverse proportion to the dimensions of the baffle surface. Twice the size, onemore octave of bass.

This factor often leads to the second major downside,humorously known as the WAF, or wife acceptance factor. Good bottom end from anopen baffle usually means a very large baffle, more than most living rooms willtolerate. 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, andthey have delivered pretty good bass without equalization. For such a largebaffle the question is not only whether they will fit in your space, but at somepoint you start to notice that you are no longer effectively listening to thatrear wall illumination, as it is being blocked by the baffles. In this case, youmight simply try mounting the drivers in the wall. And some people do.

The Frontand Rear Sides
There is something important to note about the diagram ofthat AMT-3 prototype. In the original Heil dipoles, there is symmetry betweenthe front and back waves, specifically the air coming out the back is squeezedto the same velocity as the air in front. In this regard they

In the AMT-3 prototype, this is not the case. The air comingout 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 thevelocity, we expect more energy out the front than the back.

The response is not symmetric – the rear wave that wants tocome 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 slotloaded open baffle – greater velocity out the front and less out the back.

After about thirty-eight years, I decided to recreate thevision of that original prototype. In the summer of 2010 I had Roth Wiedrickhelping me out, and he began construction of a modified version which hefinished the following summer.

Here is a diagram showing how this woofer is laid out:

As an aid to understanding, I have highlighted the directionof air motion in this system with green arrows. The woofers are wire in phaseand move toward and away from each other squeezing the air between them, and thepressure 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 MDFand measures about 30 inches high by 36 inches wide. The six woofers are mountedin a chamber formed by laminating five layers of 0.75 inch particle board, andare arranged to squeeze the air out a front slot 2.25 inches wide and havingabout one third of their combined piston area. The edges of the slot are roundedto reduce turbulence. The eight-inch woofers are wired in either three inseries, two in parallel to form a 12 Ohm load, or three parallel, two in seriesto form 5 Ohms. Because the wiring is slightly easier, I chose the 12 Ohms, butif you are considering using a passive crossover here, you would definitely gofor the 5 Ohm arrangement for level matching.

We placed another baffle on top of this, adding another 30inches of height and mounted a Lowther PM6A at 36 inches height. None of thedimensions or construction details are critical. In any case, the intent here isto explore the idea without getting too fancy.

Here is a picture of Roth assembling the front panel (safetyglasses raised for this photo opportunity only). Here you can see the Lowthersitting on happily top in its own cozy baffle and you can just make out the toppair of woofers in the slot:

Here is the view from the rear / side showing three woofersvertically aligned on one side, all three wired in series. Why is there a bigspace cutout behind the Lowther? To accommodate the new field coil version ofthe driver from Jon ver Halen (Mr. Fullrange).

Here is the rear view, illustrating a little moreconstructional detail. Nothing but the best wire used for this project..

Of course one of the first things I did was haul them out intothe room to measure the response difference between front and back. Here is the(smoothed) comparison measured near field on the woofer system after thecrossover network has been applied. What you see is about about 9 dB pressuredifference, 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 runinto a cancellation frequency that occurs when the wavelength is twice thedepth, where the pressure from the part of the woofer which is deepest in theslot is one half the wavelength and cancels out the pressure from the portion ofthe woofer near the slot opening. Here the cancellation occurs at 500 Hz.

The ActiveCrossover
It is easier to use an active crossover when you wantquick results. You just adjust the crossover filters, measure, listen, and thenrepeat the process until you're happy. It did not take me long to get goodresults because the mating of the woofer and tweeter were well behaved. I used a2 pole low pass filter for the bottom at 80 Hz and a 2 pole high pass filter at160 Hz, and because I am lucky the proper response was with both drivers inphase.

Some equalization was called for on both drivers. The wooferreceived a high Q high pass filter at 20 Hz which removed subsonics and gave theresponse a 6 dB bump up at 20 Hz. The Lowther enjoys 4 dB of step correctionabove 2 kHz or so to take down the overall top end. If you are a littleoff-axis, you might decide that 2 dB is better. Here's the voltage response fedto the finished system:

Notice that more gain is required for the bottom end, althoughif you're not crazy about bottom end you can probably get by with wiring thewoofers for 5 Ohms, which will give you most of the difference if you want torun passive crossovers. Here's the acoustic response:

I had the advantage of using a B5 crossover ready-made for thetask, featuring lots of adjustments and spiffy discrete JFET buffers and stufflike that:

If you want to DIY this, then here is the equivalentnot-so-adjustable circuit rendered with op amps employed as buffers, except onthe 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:

Conclusions
They sound really good. They measure well. Theywill totally dominate your listening space..

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The latter two items are self-apparent at this point, but youneed to hear some words assuring you that the audio experience is worth the effort. Theseare the open baffles that have enough bass to push my JBL L300's off center stage. Ifyou look carefully in the background of the rear-view photo you will see onelurking impatiently).

They have a fairly deep bottom, and are very punchy in thesense that transient attacks come at you with all the troops in formation, anicely aligned shock wave. They are well behaved with regard to mating withfull-range speakers, and with the crossover presented you can run both top andbottom in-phase, which makes for an improved sense of space and imaging. It alsohelps retain that full range driver character, but with bottom end.

Pardon me for having fun building such monster speakers. Youare probably looking at this and thinking, 'That's nice, but I can't deal withit.' My actual point is that you can use this technique to get more bottom endout 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 sizedenclosure.

You can also make them with single-woofers and a little imagination. Ifyou're not afraid of two resistors, a coil and a capacitor you might besurprised how much you can accomplish with a really simple single-pole passive crossover. Gota saber saw?

Go ahead, make my baffle.

© 2011 Nelson Pass