due to the nature of dipoles they are not optimally suited for low bass but they do have advantages in the modal response region. I think perhaps some comprimise can be made here.
I have a few ideas....
the main idea is two speaker(or any even number) dipoles
The first consists of two speakers in seperate enclosures of any type: acoustic suspension, bass reflex, bandpass, or passive radiator. The enclosures would be placed so that the speakers/ports would be facing in opposite directions then the signal fed to on speaker would be reversed polarity of the other. perhaps you could simply place them at a distance apart in a room and feed reversed polarity to one sub.(now I dont think this would help the low bass any, but it surely effects something)
I suppose double horn loaded (front and backloaded) horn dipoles would also be something to consider
or just two horns built very close and pointing oppostie directions with one fed a reverse polarity signal of the other.(once again I dont know that this would help the low bass as it would require extremely large horns to make the low bass directive or efficient)
Finally simply a sealed box with subs mounted on opposite sides. There would perhaps be a digital signal processing unit which would have a phase shift on one sub between a specific band of frequencys (probably right below where the room ceases to be modal and cabin gain comes into effect) which would make the sub go from a isobaric dipole to a push push sealed monopole. This would give the low extension BACK!
So who thinks any of this will work, or for the last idea, even be possible? I'd be glad to hear any thoughts ideas or experiences
I have a few ideas....
the main idea is two speaker(or any even number) dipoles
The first consists of two speakers in seperate enclosures of any type: acoustic suspension, bass reflex, bandpass, or passive radiator. The enclosures would be placed so that the speakers/ports would be facing in opposite directions then the signal fed to on speaker would be reversed polarity of the other. perhaps you could simply place them at a distance apart in a room and feed reversed polarity to one sub.(now I dont think this would help the low bass any, but it surely effects something)
I suppose double horn loaded (front and backloaded) horn dipoles would also be something to consider
or just two horns built very close and pointing oppostie directions with one fed a reverse polarity signal of the other.(once again I dont know that this would help the low bass as it would require extremely large horns to make the low bass directive or efficient)
Finally simply a sealed box with subs mounted on opposite sides. There would perhaps be a digital signal processing unit which would have a phase shift on one sub between a specific band of frequencys (probably right below where the room ceases to be modal and cabin gain comes into effect) which would make the sub go from a isobaric dipole to a push push sealed monopole. This would give the low extension BACK!
So who thinks any of this will work, or for the last idea, even be possible? I'd be glad to hear any thoughts ideas or experiences
No thoughts on this?
I thought my last idea was pretty good. It seems like exactly what we need. Dipolar response in the modal region and monopolar below.
Can a filter be made that would do a 180degree phase shift between say 30-40hz
I thought my last idea was pretty good. It seems like exactly what we need. Dipolar response in the modal region and monopolar below.
Can a filter be made that would do a 180degree phase shift between say 30-40hz
Based on your previous posts over the past months, I would be of the opinion that any idea you have is not likely to work.
now now, play nice. 
To me the big benefit of dipoles is getting your drivers free of boxes. I for one don't want to figure out how to put them back in boxes and somehow retain dipole radiation.
To me the big benefit of dipoles is getting your drivers free of boxes. I for one don't want to figure out how to put them back in boxes and somehow retain dipole radiation.
the advantage is, you get the dipolar response in the modal region, and monopolar below. It takes the best of both worlds
Or you could simply do what I do, which is use dipole bass (open baffle) down to 30-40hz, then monopole (sealed servo-sub) below. There's no significant advantage to dipole that low and the monopole sub is better suited for 20-35hz, in terms cost, size etc.
Well, that's what Bill gets for not actually reading the thread. 🙂
Bass,
I've experimented with just that idea in various arrangements, most promisingly in the form of opposed drivers in a sealed box. The idea is to use an all-pass filter on the rear driver such that at the sub's crossover point it is 180 deg out of phase with the front driver (assuming you have dipolar mains, which I think such a design is most suited for), and at the pressure mode transition frequency it is in phase with the front driver. Actually, the exact point where the phase becomes the same as the front driver is where I've spent a lot of time experimenting, and it seems that you still want some phase lag at the transition frequency.
Recently I've aimed for full monopole behavior by the time the sub has reached it's F3. The polar response transitions from omni, through a shifting cardioid, to figure-8 in the upper bass region.
This idea is very similar to studio mics that are selectable omni, cardioid, or dipolar response. They do this by placing two omni capsules back to back, and allowing a switch to set the phase offset. Thus, the idea is some form is very old. I've never seen it applied to subs though, which is why I had to do the research myself.
I'm not sure it is the most efficient use of drivers (a separate dipolar and monopolar sub with traditional crossover does quite well), but it does have the convenience of a one box solution. The biggest problems I've encountered are pipe-organ resonances in the box when in dipolar operation, and creating a satisfactory analog all pass filter with such a gradual phase shift (and more annoyingly being able to stop the phase offset at a chosen low freqeuncy instead of it continuing to drift, creating a reduction in output that you don't want as the frequency drops). Digital filters might be a nice solution, but thus far I haven't taken the idea that far, as I've had plenty of other things to work on.
Bass,
I've experimented with just that idea in various arrangements, most promisingly in the form of opposed drivers in a sealed box. The idea is to use an all-pass filter on the rear driver such that at the sub's crossover point it is 180 deg out of phase with the front driver (assuming you have dipolar mains, which I think such a design is most suited for), and at the pressure mode transition frequency it is in phase with the front driver. Actually, the exact point where the phase becomes the same as the front driver is where I've spent a lot of time experimenting, and it seems that you still want some phase lag at the transition frequency.
Recently I've aimed for full monopole behavior by the time the sub has reached it's F3. The polar response transitions from omni, through a shifting cardioid, to figure-8 in the upper bass region.
This idea is very similar to studio mics that are selectable omni, cardioid, or dipolar response. They do this by placing two omni capsules back to back, and allowing a switch to set the phase offset. Thus, the idea is some form is very old. I've never seen it applied to subs though, which is why I had to do the research myself.
I'm not sure it is the most efficient use of drivers (a separate dipolar and monopolar sub with traditional crossover does quite well), but it does have the convenience of a one box solution. The biggest problems I've encountered are pipe-organ resonances in the box when in dipolar operation, and creating a satisfactory analog all pass filter with such a gradual phase shift (and more annoyingly being able to stop the phase offset at a chosen low freqeuncy instead of it continuing to drift, creating a reduction in output that you don't want as the frequency drops). Digital filters might be a nice solution, but thus far I haven't taken the idea that far, as I've had plenty of other things to work on.
RHosch,
Great info! thanks for the reply. One Idea I had to reduce the standing waves of the box was to make the box an odd shape and make the distance between the two drivers as small as possible. Instead of two oppositely mounted drivers being 12 inches apart(from front to front) due to the magnets behind the speakers I would have the subs facing in with the magnets outside the enclosed airspace.
I wonder if a vented design using this could be made.
I was thinking that the filter/phase shift could be made possibly by using an all pass filter on one sub with a high pass(reversed polarity) and a stadard lowpass on the other. Thats a bit oversimplified though.
What would be strange is the region when the speakers are 90degrees out of phase. There when one sub is at full excursion the other will be in the middle of its gap, and the other way around. The net output is the same as one speaker.
Then when you get further down and the phase difference aproaches zero the output will be 6db more than a single driver monopole due to mutual coupling
Great info! thanks for the reply. One Idea I had to reduce the standing waves of the box was to make the box an odd shape and make the distance between the two drivers as small as possible. Instead of two oppositely mounted drivers being 12 inches apart(from front to front) due to the magnets behind the speakers I would have the subs facing in with the magnets outside the enclosed airspace.
I wonder if a vented design using this could be made.
I was thinking that the filter/phase shift could be made possibly by using an all pass filter on one sub with a high pass(reversed polarity) and a stadard lowpass on the other. Thats a bit oversimplified though.
What would be strange is the region when the speakers are 90degrees out of phase. There when one sub is at full excursion the other will be in the middle of its gap, and the other way around. The net output is the same as one speaker.
Then when you get further down and the phase difference aproaches zero the output will be 6db more than a single driver monopole due to mutual coupling
heh, similar thinking on possible enclosure design. I had modeled (dimensionally, not acoustically) an enclosure that was only a few inches thick but pretty tall and wide, with drivers facing out on front and rear faces. That pushed pipe-organ resonance way above operating range, and also created a fairly wide baffle that is more effective for dipole operation. Pretty strange looking, though, and I never built one to experiment with it. A simple notch filter might work, though I never tried it with one.
I think there is a bit of strangeness going on with this idea though, and I think it may be tied to an enclosure operating as both monopole and dipole when multiple frequencies are played simultaneously. I haven't been really satisfied with the results so far, but it seems promising enough to keep experimenting.
The all pass filter can be assembled by cascading simple all-pass filters used for VC alignment, and with work the range of phase shift can be made to accomodate this idea. I personally haven't been successful at "nailing it" though. I haven't really done much work on this in the past two years though, which is when I added capable measuring abilities, so I never have had the chance to closely examine what is happening with measurements. In the past it was through listening only.
I think there is a bit of strangeness going on with this idea though, and I think it may be tied to an enclosure operating as both monopole and dipole when multiple frequencies are played simultaneously. I haven't been really satisfied with the results so far, but it seems promising enough to keep experimenting.
The all pass filter can be assembled by cascading simple all-pass filters used for VC alignment, and with work the range of phase shift can be made to accomodate this idea. I personally haven't been successful at "nailing it" though. I haven't really done much work on this in the past two years though, which is when I added capable measuring abilities, so I never have had the chance to closely examine what is happening with measurements. In the past it was through listening only.
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