Full Range Driver Open Baffle

Being a simple-as-possible low-cost extraordinary sound open baffle convert, I mounted the drivers on 25 mm x 750 mm x 730 mm boards, although the designer recommends baffles 760 x 610 mm with 380 mm side wings.

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:

Video marketing blaster pro tutorial. 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..

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. Wilson deep red ti forged 400 drivers for mac download. 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.

Spring 2010

DIY Open Baffle With Widerange Drivers
A.K.A. The third ugliest amazing speaker you'll ever see.
Article By Jeff Poth

Difficulty Level

This is the best speaker I've built at the time of this writing (that is mostly January, 2010). The speaker is almost two years old but the woofer.. much older. Older than some of you reading this! Let us dive right in: The drivers are the JBL 123A (or, preferred, 123A-3/2213) and Fostex FF85k (preferred FF85KeN).

OK, Out Of The Pool!
Dirty trick, eh? We'll do some backstory. I have to mess with my writing format to keep you fickle Internet clicky types involved! About the time of this speaker's concept, I had just moved cross-country. All my tools and hi-fi gear were living with my wife, 3,000 miles away, and I needed a speaker, both as a project to keep me busy (and hence out of trouble!) and as a way to listen to music for a few months until my beautiful wife and daughters joined me in semitropical paradise. It had to be easy to build, as I had a very limited number of items to work with.

The Woofer
At and about this time, I was reading Troels Gravesen's site when I came across his pages relating to the L100. For those of you who don't know, the JBL L100 is one of the top selling speakers of all time, and was insanely popular in the 70s and 80s. A number of versions exist, up through the L100T3, which is a much superior design to the original L-100, but also is a totally different speaker. The original L-100 was a set of superb drivers in a very poor overall design.What caught my eye was the 123A 12-inch woofer. Troels had done some measurements on a low-diffraction baffle, and got impressive smooth, extended bandwidth from this driver. As I didn't have my tools, I needed to keep the project simple, and a smooth, extended bandwidth tends to allow a simple crossover which can be tuned much more easily. So began the hunt for 123A. I got a pair each of 123A and 2213, the 123A-3 equivalent. JBL often had both pro and consumer part numbers for the same driver in these days.

This woofer has a 'Lansaplas' coated, ribbed, curvilinear cone, a high-ish Qes, and a low Fs. It also has a high quality AlNiCo magnet structure and JBL build quality. The combination of a curvilinear cone with low inductance and a lot of damping material not only gives it a fairly well damped, extended top end, but also a well extended low end- in the right box.

These drivers are available; the L-100 was the top selling speaker in history, in its day. You'll just have to hunt for a nice pair. A known good pair can be usually had for $200, you can find cheaper if you really look. $200 is a perfectly fair price for the level of quality these drivers have, both sonic and build. If you come up with L100s for the woofers, please try not to destroy good examples of the speaker- if the cabs are in good shape, you can offer them for resale, whole, and find another set of woofers. Also, be aware that if the cabs are NOT in good shape, the LE-5 midrange and tweeters are quality drivers too.

The 2213
The 2213 is a modest upgrade with a slightly improved motor design. The frame is nicer on 123A however, so if you're looks conscious of the back end.. 123A. Sonically, 2213 is just a little cleaner and 'bigger' sounding.

Note: These AlNiCo drivers have the potential for being de-magnetized. This leads to a higher Qts and less efficiency. You will need to be able to test these to confirm. I used a Dayton woofer Tester 3 for this task. Proper values should be approximately Fs: 25Hz Le: 0.6mH Qts .5

The Midtweeter
I wanted to do a simple crossover for this project, as mentioned above. This requires an extremely robust tweeter. The Fostex FF85k was the obvious choice and available from Madisound. I had previously used the FF225k, which is a widerange driver (not really full range, but covers 100 Hz to 10 kHz very nicely). Its little brother is a very slick 3' extended treble Fostex's cool UDR surround technology with a copper sleeve over the pole. This is a 3' 'fullrange' that can be thought of, for the purposes of this project, as a tweeter that can play down into the midrange cleanly. The awful sound of so many 1' dome speakers in your typical 'hi-fi' 6.5' midwoofer and 1' dome tweeter speakers is due to the 1' running out of steam. This driver has about nine times the surface area, and thus needs to move much less. For the extreme highs, it has a direct coupled aluminum dome dustcap, which acts much like a more common whizzer cone construction. This trick has been done since the 1950s and JBL's legendary LE8T fullrange (amongst others, E130 being particularly popular amongst guitar players).

For those of you who want a significant upgrade, the fully modified FF85KeN from planet_10 hifi is a big improvement, with better clarity, dynamics, and a more complete disappearing act. This is one amazing driver, taking the FF85k a significant notch above it's already extremely impressive sound quality. More information on the modified driver at this link.

Don't Make Me Box Your Ears!
Having found a woofer that was compelling and a lovely midtweeter to match, the enclosure design needed to be addressed. In this case, an 'unclosure' was the better choice. The 123A is a monster woofer, with prodigious low end output within its modest 7mm Xmax. A box for this driver wants to be 5 cubic feet or larger, the size of a large mini-fridge. An open baffle of the proper design, however, will play down to the 30s and 40s, with careful design. Open baffles are not known for having deep bass. This design relies upon floor reinforcement to buy some extra juice down low, without going to a much larger baffle.

The Cowpushers
Time spent playing with Xlbaffle, thanks to Thorsten Loesch for designing and Dave Dlugos for hosting, gave me a size approximation. Using acoustic theory and working with Tolvan's The Edge, I came up with the angled edge design. As it turned out, it worked out very well. The speakers are, necessarily, tilted backwards to aim the drivers towards the listening position. The amount of tilt is wholly dependent upon the listener position. You want the plane formed by the baffle tilted back slightly more than directly facing you when you look down at them from your listening seat. If you were to draw a line from the dustcap of each driver to your ears, you'd want the lines to be equal lengths. This is important, as a fairly high crossover point was utilized in this design. Adjust to taste but don't deviate much. This is also critical to getting the sound dispersed for use as background music, as if they are too far forward, the soundfield will be largely limited to the bottom half of the room.

Crossover
Crossovers are the hardest part of any speaker design, and this one was no exception. I got good results with a compensated series crossover at 500 Hz, but was still not quite happy. After a lot of experimentation, I came up with a combination of acoustic and electric crossover that produced an exceedingly flat response from 30 Hz to 20 kHz. By exceedingly flat, I mean in room over a 60 degree window, +/- 3dB, and only deviates from this within a 90 degree window vis a vis >10 kHz rolloff. This speaker interacts very favorably with a real world room, as it's very consistent- extreme off axis angles are controlled via dipole cancellation and acoustic absorption.

The great part here is the crossover is very simple. A first order filter is used on the 3-inch, 10uF. Use a good quality cap. The 12' does not work with a first order filter via series inductance; its small inductance interferes with both efficacy of a series inductance, as well as creating phase wrapping that cannot be corrected for with electrical solutions. My solution was an acoustic one. A felt (adhesive backed F13, part number 8745K53 at McMaster) pad was implemented on a metal grille from Parts Express that created a low-pass acoustic filter. This allowed excellent phase tracking at the crossover point and an ideal textbook first order filter behavior (read: transient response is superb).

One lump remains. A 200Hz lump, to be specific. This is the result of the baffle reinforcing some frequencies, as well as a reduction in capacitor efficacy at Fs on the Fostex. This is the hardest part of the filter, fortunately, it's not that hard. You WILL want to spend money for a very high quality inductor and capacitors here. I used Solen 10 AWG aircore coils and a composite (several values in parallel) capacitor bank to get what I needed. I recommend avoiding any cored inductors here if possible, and keeping the DCR low. It's a big part, an expensive part, and a part that you don't want to skimp on. 12 AWG would be good, 14 may be sufficient, but absolutely no smaller if you're using aircore. If you're using cored, 14 is okay, but I recommend aircore. Remember, this part is the one that's passing the first few, most energy dense, octaves through it. The capacitors are also quality important, but are likely less of an issue here. Several quality poly capacitors in parallel, 100V or more will do the trick. The resistor can be a generic 10W sandcast type; it is not going to be passing significant current apart from at the notch frequency. Sandcast resistors do have some significant inductance, but this isn't really an issue here. When using these resistors for high frequency circuits take care.

Directionality
Open baffles are a very different case than 'box' speakers, in a variety of ways. Some of their strengths are: