JF, did you get to finish your OB subs discussed here? If yes, did the design/build achieve all or most of what you were after?
Any unexpected deficiencies or other performance issues?
Please describe the build in some detail. Did you go with MJK’s 18” Goldwood
driver and in an h-frame?
How far from the rear wall do you keep them? What amplifiers to drive them?
How might one 18” drive in an h-frame differ in performance from dual 12” in a W-frame or dual 12” in a slot-loaded?
What solution did you use for low pass filter ing to blend with your midwoofers?
And to adjust delay (phase)? miniDSP? Dspeaker?
Did you find any benefit to using those OB subs with of your sealed monopole subs?
Bass, Bass, Bass....
How to get great bass.... That is the question. If you are trying to reproduce low frequency bass, I would not even consider using open baffles since the economics (more drivers, more amp power) for doing so are so bad. If indeed your mains can get you to 70 hz, then I would go another route than open baffle. Also, one key to reducing room modes is to have multiple boxes and place them in different places around the room.
I think that I should share some of my experiences with reproducing bass. Years ago, had Lowther DX4 drivers in Hedlund Horns, which can reproduce down to about 70 or 80 hz. Wanting lower bass, I purchased a Genesis subwoofer (15 inch aluminum cone driver, servo controlled). Initially, the bass was mediocre with this arrangement, but Genesis said that they found a better way to attach the accelerometer to the cone which reduced distortion. I sent the driver to them and sure enough the change dramatically reduced distortion. Between the Hedlund Horn (with very well-braced backhorn) midbass and the Genesis subwoofer, the bass was excellent. The Genesis subwoofer had infinitely adjustable phase and crossover frequency which was key to match the crossover frequency and phase with the Hedlund horns.
You seem to think that the Bryston amp is better than the Class D amps that come with the Rhythmics. I would not assume that. I have an audio friend with Legacy speakers. He purchased a Bryston amp to drive the woofer drivers in his Legacy speakers. He tried a good quality class D amp and found it to be better in the bass than the Bryston. For midrange and higher frequencies, I am sure that the Bryston would beat the class D amp, but probably not for reproducing bass. I would certainly give the Rhythmics a go.
Well, the Genesis subwoofer died (the built in amp bit it), and I put my Lowther drivers on open baffles. I figured that I would try out open baffle bass. So I purchase a pair of Ascendent Audio Avalanche 18 drivers. These are XBL2 drivers with an FS of 16 hz and x-max of 27 mm - these are low frequency, low distortion, reproducers. I built W-style cabinets for them and mounted the drivers push pull to cancel second order distortion. I have a 500 wpc Hafler amp (P7000). Despite the large output capacity, these big speakers could not reproduce low frequency bass in open baffle. And trying to do so, increased distortion. I ended up building a box which is open on both ends and facing that sideways and bolting this on the back side of the W-style open baffle box. The chamber where the two drivers face into each other faces the back of the room and this open ended box bolts onto there (the chambers towards the outside of the box that each driver faces into, face the front of the room), basically extending the distance that the sound must travel and cancel which lowering the frequency where the open baffle cancelling starts and making this a U-shaped baffle. The problem with this is that it dramatically lowers the resonance frequency of the box to around 60 hz, or so. So now I have to deal with this, but I was able to achieve pretty good bass down to below 20 hz.
The way I dealt with the resonance is I then stuffed that open ended box 5 inches deep with insulation, thus changing the open baffle into a lossy closed box. This, of course overdamped the driver and no more 20 hz bass - no free lunch.
My plan going forward is one of two pathways. If I keep the Lowthers in open baffles, then I would likely put the Avalanche 18s in 100 cubic foot LLT subwoofers, and get them flat to something like 10 hz, and high pass them at 100 hz. If I put the Lowthers in front horns, then I will put the Avalanche 18s into tapped pipes (like tapped horns, but because the Qts of the Avalanche drivers are fairly high, there would be no expansion going from the throat to the mouth). In this setup, the Avalanche drivers could handle from 10 to 40 hz. Then I would need some sort of woofer system for 40 up to the 200 to 400 hz, depending on the size of front horn. I am thinking Karlson right now.
Enough for now.
Retsel
How to get great bass.... That is the question. If you are trying to reproduce low frequency bass, I would not even consider using open baffles since the economics (more drivers, more amp power) for doing so are so bad. If indeed your mains can get you to 70 hz, then I would go another route than open baffle. Also, one key to reducing room modes is to have multiple boxes and place them in different places around the room.
I think that I should share some of my experiences with reproducing bass. Years ago, had Lowther DX4 drivers in Hedlund Horns, which can reproduce down to about 70 or 80 hz. Wanting lower bass, I purchased a Genesis subwoofer (15 inch aluminum cone driver, servo controlled). Initially, the bass was mediocre with this arrangement, but Genesis said that they found a better way to attach the accelerometer to the cone which reduced distortion. I sent the driver to them and sure enough the change dramatically reduced distortion. Between the Hedlund Horn (with very well-braced backhorn) midbass and the Genesis subwoofer, the bass was excellent. The Genesis subwoofer had infinitely adjustable phase and crossover frequency which was key to match the crossover frequency and phase with the Hedlund horns.
You seem to think that the Bryston amp is better than the Class D amps that come with the Rhythmics. I would not assume that. I have an audio friend with Legacy speakers. He purchased a Bryston amp to drive the woofer drivers in his Legacy speakers. He tried a good quality class D amp and found it to be better in the bass than the Bryston. For midrange and higher frequencies, I am sure that the Bryston would beat the class D amp, but probably not for reproducing bass. I would certainly give the Rhythmics a go.
Well, the Genesis subwoofer died (the built in amp bit it), and I put my Lowther drivers on open baffles. I figured that I would try out open baffle bass. So I purchase a pair of Ascendent Audio Avalanche 18 drivers. These are XBL2 drivers with an FS of 16 hz and x-max of 27 mm - these are low frequency, low distortion, reproducers. I built W-style cabinets for them and mounted the drivers push pull to cancel second order distortion. I have a 500 wpc Hafler amp (P7000). Despite the large output capacity, these big speakers could not reproduce low frequency bass in open baffle. And trying to do so, increased distortion. I ended up building a box which is open on both ends and facing that sideways and bolting this on the back side of the W-style open baffle box. The chamber where the two drivers face into each other faces the back of the room and this open ended box bolts onto there (the chambers towards the outside of the box that each driver faces into, face the front of the room), basically extending the distance that the sound must travel and cancel which lowering the frequency where the open baffle cancelling starts and making this a U-shaped baffle. The problem with this is that it dramatically lowers the resonance frequency of the box to around 60 hz, or so. So now I have to deal with this, but I was able to achieve pretty good bass down to below 20 hz.
The way I dealt with the resonance is I then stuffed that open ended box 5 inches deep with insulation, thus changing the open baffle into a lossy closed box. This, of course overdamped the driver and no more 20 hz bass - no free lunch.
My plan going forward is one of two pathways. If I keep the Lowthers in open baffles, then I would likely put the Avalanche 18s in 100 cubic foot LLT subwoofers, and get them flat to something like 10 hz, and high pass them at 100 hz. If I put the Lowthers in front horns, then I will put the Avalanche 18s into tapped pipes (like tapped horns, but because the Qts of the Avalanche drivers are fairly high, there would be no expansion going from the throat to the mouth). In this setup, the Avalanche drivers could handle from 10 to 40 hz. Then I would need some sort of woofer system for 40 up to the 200 to 400 hz, depending on the size of front horn. I am thinking Karlson right now.
Enough for now.
Retsel
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Why not try the Rythmic subwoofers since you have them?
@Retsel:
I just built a system with a modified H-frame that has fantastic bass. Two 12" drivers are mounted to a 14" high baffle that stands on edge. To keep it upright, I created cradles for the magnets of each driver. One driver faces forward and the other backwards, so a magnet sticks out on each side and in the cradles the drivers+baffle is quite stable. Then I just plopped a 36"W, 40"D board on top to form a table, adding one leg in the front and rear on the midline to provide stability. I put my amps on top of the table, which helps to weight it down. The sides remain totally open, so it's not a true H-frame but more like a folded baffle. The "line resonance" happens around 150Hz but since it is the same front and rear I just EQ it out. The long pathlength (1m) makes for very good dipole bass response even though it is "open". Of course I do need to EQ the response and boost the low end but the quality of the bass is very nice (system is only 1 week "old" at this point).
Pic of underside is attached. This is only a prototype so please excuse the cheap materials and lack of finish.
@Retsel:
I just built a system with a modified H-frame that has fantastic bass. Two 12" drivers are mounted to a 14" high baffle that stands on edge. To keep it upright, I created cradles for the magnets of each driver. One driver faces forward and the other backwards, so a magnet sticks out on each side and in the cradles the drivers+baffle is quite stable. Then I just plopped a 36"W, 40"D board on top to form a table, adding one leg in the front and rear on the midline to provide stability. I put my amps on top of the table, which helps to weight it down. The sides remain totally open, so it's not a true H-frame but more like a folded baffle. The "line resonance" happens around 150Hz but since it is the same front and rear I just EQ it out. The long pathlength (1m) makes for very good dipole bass response even though it is "open". Of course I do need to EQ the response and boost the low end but the quality of the bass is very nice (system is only 1 week "old" at this point).
Pic of underside is attached. This is only a prototype so please excuse the cheap materials and lack of finish.
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