This is an overview of building a Modified Open Baffle Loudspeaker Built Around 4 JBL D123 Drivers.
First some design philosophy that will leave most of you as baffled as these speakers! Sometimes I design around enclosures. For instance, a couple of very solid old wooden pyramids come my way. Not sure what they were used for, but I notice how acoustically dead they are and wonder what I can do with that shape. Though a series of iterative tests an improvements using only my skilled listening and woodworking, I come up with an omnidirectional little beast with an 8" downfiring woofer and 4 silk dome tweeters arrayed on each side of the pyramid. Then I add a false bottom with about an inch clearance for the bass to radiate from. I have built several speakers around variously shaped enclosures and most compare well with the better commercial factory built, computer designs in terms of accuracy and convenience.
Other times, I begin with a driver and build the eclosure to bring out its best. This requires a different attitude. Because I am going for charm and beauty, not accuracy or easy placement, etc. For instance, I have built folded horns for 1950s Electro-Voice coax drivers such as the 12TRX. I wanted the finished design to sound like the most lovely 12TRX in Heaven. Not some modernized edgy cleaned up (or watered down) compromise. I have to add that the magic of these drivers is in vocals and re-creating a convincing stereo stage placement. And in being able to play loud with 5 watts of class a valve power. One has to give up on the idea of really deep bass, accurate cymbals, and handing the inner voices in very complex passages. These drivers can't do that. If you try to force them to try, then they will sound worse because what they can do so well will be compromised. So a folded horn design something like the EV Aristicrat cabinet is called for. Not too big. Perfect for vocals, combo jazz, singer songwriters.
The current design, I just finished building the first mockup for was designed around 2 16 Ohm JBL D123 drivers in parallel per cabinet. So we wind up back at 8 Ohms. These drivers are 12" fullrange models. They have an aluminum dust cap that acts as a kind of high frequency radiator. They are less than 4 inches deep. A 3" voice coil. None-the-less, they can deliver very convincing brass and percussion. My best guess is that they start to fade at about 13.5K and are gone at 16K. Bass probably starts to fade at 80 and is gone by 35. BUT, and this is important, the bass is actual notes and the treble is surprisingly clean. It does not screach; it is smooth and fades politely anf gradually. SO the sound is much more fullrange sounding that it probably measures. I usually give them a bit of a boost at about 50 Hz and 15 KHz depending on the room and placement.
I've read that these JBL D123s were Jerry Garcia's favorite drivers. And it makes sense. They are very smooth and inoffensive drivers with just a small plateau in the upper midrange, maybe 700 to 3K, but just a small lift and generally they are very pleasant sounding with just a bit of added "presence". A cloth speaker grill helps even out this plateau. I could have added a supertweeter and/or subwoofer and some electronics to slice up the signal. But that would have wrecked what is best about these charming vintage stoner drivers. And The Dead's "Sugar Magnolia" would not have sounded and better. Nor would Duke Ellington's "Piano in the Background". No one in their right mind would use these drivers for funk, disco, hiphop, or anything else that calls for more than 20 watts. So why go there? Play to their strengths of delicacy, detail, and smooth easy ear massaging.
To do just that, I went with a modified open baffle design. The top speaker is fully open baffle, but offset in a 2" x 2" flat board. No sides of back. About 3 feet off the floor. It even had a 1 inch by 8 inch slit directly above the driver, so it is really a mix of open baffle and no baffle. Now you would think there would be way too much bass cancellation with this design. Not so. The driver is offset in such a way that it still produces significant bass radiating out toward the side walls. Additionally much of its down-back firing energy is reflected back up at the ceiling. So one gets lots of deep bass coming off the back wall and ceiling. Strange but true.
The lower driver is about 8 inches off the floor located in the center of a large upsidedown triangle over 2 feet tall, 3 feet wide at top, and 16 inches at bottom. It has non-parallel sides about a foot deep, but no top or bottom. These sides focus the bass down and compress it into the floor as well as radiating all the frequencies up and out toward the back wall and the ceiling.
The BIG PICTURE is this: rather than bouncing the sound from the rear of the driver around in a box or tube and waiting for some chosen bass frequency to turn 180 degrees so it can reinforce the front wave, what I am doing is bouncing sound back and forth and all around outside of the box. But I am still getting it to turn around 180 degrees. But not just one frequency. Lots of frequencies. So this design is, as we say in Maine, unGodly effeicient. Honestly, 5 clean watts will make your ears bleed. This is because one is getting nearly the full back and front wave of 4 really efficient 12 inch drivers with almost no cancellation. Try to do that trick with a closed box!
And an added benefit is that the stereo imaging is about the best I have ever heard. There is absolutely no hole in the middle effect even with the speakers 8 feet apart and the listener sitting 5 feet in front of them. This is because of all the sound radiating from the ceiling, floor, and back wall. In fact, the effect is stunning. Vocals seem nailed in place between the speakers with none of the wavering and quavering of more boxy designs. And when a sound is located far off to one side or the other the illusion of space is abrupt and starting. I knw this sound a bit odd, but the only way I can think of to get across the feeling is that most nice speakers are like a warm shower, but these are a long luxurious lazy surrounding jet massage in a hot tub. They are that good.
As it is 1:30 AM, I will wrap this up with a few photos. Later I might add some detailed specs on the design. But I do not think those details are the critical thing. Once you get the idea of how to bounce sound around outside the box rather than insice to get the backwave back in phase, well the rest is a snap and probably has a thousand different implementaion details. I have already dreamed up several more ways to accomplish the same thing. I guess the big drawback so far is that most of these design ideas are fairly large. And have placement limitations. But they are worth a little minor inconvenience, for such glorious, smooth encompassing musical pleasure!
First some design philosophy that will leave most of you as baffled as these speakers! Sometimes I design around enclosures. For instance, a couple of very solid old wooden pyramids come my way. Not sure what they were used for, but I notice how acoustically dead they are and wonder what I can do with that shape. Though a series of iterative tests an improvements using only my skilled listening and woodworking, I come up with an omnidirectional little beast with an 8" downfiring woofer and 4 silk dome tweeters arrayed on each side of the pyramid. Then I add a false bottom with about an inch clearance for the bass to radiate from. I have built several speakers around variously shaped enclosures and most compare well with the better commercial factory built, computer designs in terms of accuracy and convenience.
Other times, I begin with a driver and build the eclosure to bring out its best. This requires a different attitude. Because I am going for charm and beauty, not accuracy or easy placement, etc. For instance, I have built folded horns for 1950s Electro-Voice coax drivers such as the 12TRX. I wanted the finished design to sound like the most lovely 12TRX in Heaven. Not some modernized edgy cleaned up (or watered down) compromise. I have to add that the magic of these drivers is in vocals and re-creating a convincing stereo stage placement. And in being able to play loud with 5 watts of class a valve power. One has to give up on the idea of really deep bass, accurate cymbals, and handing the inner voices in very complex passages. These drivers can't do that. If you try to force them to try, then they will sound worse because what they can do so well will be compromised. So a folded horn design something like the EV Aristicrat cabinet is called for. Not too big. Perfect for vocals, combo jazz, singer songwriters.
The current design, I just finished building the first mockup for was designed around 2 16 Ohm JBL D123 drivers in parallel per cabinet. So we wind up back at 8 Ohms. These drivers are 12" fullrange models. They have an aluminum dust cap that acts as a kind of high frequency radiator. They are less than 4 inches deep. A 3" voice coil. None-the-less, they can deliver very convincing brass and percussion. My best guess is that they start to fade at about 13.5K and are gone at 16K. Bass probably starts to fade at 80 and is gone by 35. BUT, and this is important, the bass is actual notes and the treble is surprisingly clean. It does not screach; it is smooth and fades politely anf gradually. SO the sound is much more fullrange sounding that it probably measures. I usually give them a bit of a boost at about 50 Hz and 15 KHz depending on the room and placement.
I've read that these JBL D123s were Jerry Garcia's favorite drivers. And it makes sense. They are very smooth and inoffensive drivers with just a small plateau in the upper midrange, maybe 700 to 3K, but just a small lift and generally they are very pleasant sounding with just a bit of added "presence". A cloth speaker grill helps even out this plateau. I could have added a supertweeter and/or subwoofer and some electronics to slice up the signal. But that would have wrecked what is best about these charming vintage stoner drivers. And The Dead's "Sugar Magnolia" would not have sounded and better. Nor would Duke Ellington's "Piano in the Background". No one in their right mind would use these drivers for funk, disco, hiphop, or anything else that calls for more than 20 watts. So why go there? Play to their strengths of delicacy, detail, and smooth easy ear massaging.
To do just that, I went with a modified open baffle design. The top speaker is fully open baffle, but offset in a 2" x 2" flat board. No sides of back. About 3 feet off the floor. It even had a 1 inch by 8 inch slit directly above the driver, so it is really a mix of open baffle and no baffle. Now you would think there would be way too much bass cancellation with this design. Not so. The driver is offset in such a way that it still produces significant bass radiating out toward the side walls. Additionally much of its down-back firing energy is reflected back up at the ceiling. So one gets lots of deep bass coming off the back wall and ceiling. Strange but true.
The lower driver is about 8 inches off the floor located in the center of a large upsidedown triangle over 2 feet tall, 3 feet wide at top, and 16 inches at bottom. It has non-parallel sides about a foot deep, but no top or bottom. These sides focus the bass down and compress it into the floor as well as radiating all the frequencies up and out toward the back wall and the ceiling.
The BIG PICTURE is this: rather than bouncing the sound from the rear of the driver around in a box or tube and waiting for some chosen bass frequency to turn 180 degrees so it can reinforce the front wave, what I am doing is bouncing sound back and forth and all around outside of the box. But I am still getting it to turn around 180 degrees. But not just one frequency. Lots of frequencies. So this design is, as we say in Maine, unGodly effeicient. Honestly, 5 clean watts will make your ears bleed. This is because one is getting nearly the full back and front wave of 4 really efficient 12 inch drivers with almost no cancellation. Try to do that trick with a closed box!
And an added benefit is that the stereo imaging is about the best I have ever heard. There is absolutely no hole in the middle effect even with the speakers 8 feet apart and the listener sitting 5 feet in front of them. This is because of all the sound radiating from the ceiling, floor, and back wall. In fact, the effect is stunning. Vocals seem nailed in place between the speakers with none of the wavering and quavering of more boxy designs. And when a sound is located far off to one side or the other the illusion of space is abrupt and starting. I knw this sound a bit odd, but the only way I can think of to get across the feeling is that most nice speakers are like a warm shower, but these are a long luxurious lazy surrounding jet massage in a hot tub. They are that good.
As it is 1:30 AM, I will wrap this up with a few photos. Later I might add some detailed specs on the design. But I do not think those details are the critical thing. Once you get the idea of how to bounce sound around outside the box rather than insice to get the backwave back in phase, well the rest is a snap and probably has a thousand different implementaion details. I have already dreamed up several more ways to accomplish the same thing. I guess the big drawback so far is that most of these design ideas are fairly large. And have placement limitations. But they are worth a little minor inconvenience, for such glorious, smooth encompassing musical pleasure!
Last edited by a moderator: