There is nothing wrong with SL's dipole model that you linked to. What made you think that?
Because you are mounting the drivers so close together, you likely don't have two separate dipoles that are needed for SL's model.
When SL says "but the two baffles might as well be placed next to each other." I think he meant side-by-side, and not your face-to-face arrangement.
Because you are mounting the drivers so close together, you likely don't have two separate dipoles that are needed for SL's model.
When SL says "but the two baffles might as well be placed next to each other." I think he meant side-by-side, and not your face-to-face arrangement.
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Sorry I don't follow that Charlie:
"The separation 2d2 between them has no influence on the total output as long as it is small compared to the wavelength of radiation. I see no compounding effect other than summing two dipoles, but the two baffles might as well be placed next to each other. "
"The separation 2d2 between them has no influence on the total output as long as it is small compared to the wavelength of radiation. I see no compounding effect other than summing two dipoles, but the two baffles might as well be placed next to each other. "
Charlie - this is a specific mathematically modelling of two dipole woofers placed back to back - no different from the clam-shell configuration I have used.
SL concludes by saying the summation in this configuration is no different to that of two drivers next to each other.
He also concludes that the space between the two woofers back to back or if you like between the clamshell has no effect on summation or output - this is the 2D2 distance he is quoting.
In other-words he concludes that placing two 15" woofers with no baffles side by side, is exactly the same as having two woofers placed back to back.
As long as they are naked.
He then states this is all good and well - but you may as well mount a single 15" woofer in an H frame!
As long as they are naked.
He then states this is all good and well - but you may as well mount a single 15" woofer in an H frame!
I think you may be confusing SL's sketch (at top) with how the system should actually be arranged. My guess is that he just drew it that way out of convenience. If he was still around we could ask him...
In any case, for your system the airspace between the drivers is not radiating at all because of the close coupling. This means that the point-source representation of the compound dipole on SL's page like this:
(-)(+).....(-)(+)
Is in your system actually just this:
(-)(+)
which is simply a dipole. You don't have a compound dipole, so that analysis doesn't apply.
In any case, for your system the airspace between the drivers is not radiating at all because of the close coupling. This means that the point-source representation of the compound dipole on SL's page like this:
(-)(+).....(-)(+)
Is in your system actually just this:
(-)(+)
which is simply a dipole. You don't have a compound dipole, so that analysis doesn't apply.
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This thread, plus another review of a speaker using dipole woofers (rotated 90 deg, push push, slot between radiates front, rear slot between blocked and cone backs radiate backward) made me start cutting wood for a pair of 18" H frames...
bushmeister, can you measure your woofers at some distance at least 1m away (to be away from the nearfield) then remove and disconnect one woofer and measure again at the exact same spot?
Sorry I don't read it this way at all. I am sorry Charlie - but I feel you are mistaken on this.
His first sentence is this:
"Completely open driver arrangements have been used by Celestion and Legacy Audio. A simple model to describe this case would be given by two drivers mounted on their own small baffles of effective radius d1 and separated by 2d2 from each other."
He is referring to the Celestion 6000 subwoofer which you may not be familiar with:
And the Legacy audio woofer arrangement:
Both of which are identical to my set-up.
2D2 is the separation distance in this configuration - which he demonstrates doesn't matter.
His first sentence is this:
"Completely open driver arrangements have been used by Celestion and Legacy Audio. A simple model to describe this case would be given by two drivers mounted on their own small baffles of effective radius d1 and separated by 2d2 from each other."
He is referring to the Celestion 6000 subwoofer which you may not be familiar with:
And the Legacy audio woofer arrangement:
Both of which are identical to my set-up.
2D2 is the separation distance in this configuration - which he demonstrates doesn't matter.
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I will give it a go Charlie - I have no horse in this race - so will be really interested in the measurements!
Those ones in post 40 were taken at 1M distance.
Well, sure, I could certainly be wrong! If you gotta be wrong, do it early and often I say. 🙂
Even so, just thinking phenomenologically, two dipoles separated by very little distance will experience large amounts of cancellation in between them at low frequencies. Compared to the wavelength, the two closest sources are essentially on top of each other, and so will have nearly complete cancellation. Honestly, I find these setups in commercial loudspeaker to more of a marketing gimmick than an effective design approach. At least with your setup in which one driver is reversed there might be some reduction in even order distortion.
Let's see what the measurement finds in terms of sensitivity.
Even so, just thinking phenomenologically, two dipoles separated by very little distance will experience large amounts of cancellation in between them at low frequencies. Compared to the wavelength, the two closest sources are essentially on top of each other, and so will have nearly complete cancellation. Honestly, I find these setups in commercial loudspeaker to more of a marketing gimmick than an effective design approach. At least with your setup in which one driver is reversed there might be some reduction in even order distortion.
Let's see what the measurement finds in terms of sensitivity.
LOL! I completely agree Charlie! And you could be right -which would actually prove SL wrong!
But in fairness -my logical brain states given the wavelengths these drivers are producing - 100hz is the highest -so 4 meters, I can't see how spacing them side by side 50cm apart or back to back 20cm apart can make much difference in cancellation.
Both 50cm and 20cm are small compared to even the smallest wavelengths involved!
But in fairness -my logical brain states given the wavelengths these drivers are producing - 100hz is the highest -so 4 meters, I can't see how spacing them side by side 50cm apart or back to back 20cm apart can make much difference in cancellation.
Both 50cm and 20cm are small compared to even the smallest wavelengths involved!
I will get on this tonight - as this does seem to be a long running somewhat controversial topic with one camp stating there is no benefit to this arrangement and the other saying it is the same as placing them side by side and therefore is useful for space saving.......
I will take one of my speakers apart and photograph it all so we can have some definitive answers!!!
I will take one of my speakers apart and photograph it all so we can have some definitive answers!!!
I want to quote this sentence from SL "A single driver in an H-frame would have the same output if the distance D between the openings is 2d1"
A H-frame has a higher SPL output at lower frequencies due to less cancelation, so it is used to extend the low end of dipole systems. So when SL writes that this compound dipole has the same output as a single driver in a H-frame, I think he acknowledges that the compound dipole has more output, still, that the same could be achieved with a single driver and some wood?
A H-frame has a higher SPL output at lower frequencies due to less cancelation, so it is used to extend the low end of dipole systems. So when SL writes that this compound dipole has the same output as a single driver in a H-frame, I think he acknowledges that the compound dipole has more output, still, that the same could be achieved with a single driver and some wood?
Exactly my interpretation Erik!
I didn't want the size of an H frame so I went for the compound dipole model!
I didn't want the size of an H frame so I went for the compound dipole model!
That's right - the two drivers are definitely acoustically close. And you may actually be right, and therefore I would be wrong, in that SL may have intended that the drivers were "in line" e.g. sharing the same excursion axis, just like in your system.
I took another look at SLs page on this, which you posted earlier:
If you take a look at the "conclusion" at the bottom, where he compares it to the equivalent H-frame, he writes that 2d1=D where D is the length of the H-frame and d1 is the effective diameter of each driver, whether nude or in a planar baffle. Note that d2, the separation of the two drivers in the compound driver model, does not appear. This makes sense when the two drivers are positioned acoustically close to each other.
In THAT case, if I am understanding SLs conclusions correctly, from two drivers you get something like a single driver at low frequency but with the effective baffle diameter TWICE as wide. But doubling the baffle diameter would increase sensitivity by a lot more than 3db. It's more like 6dB according to some quick modeling I did. And this is exactly what you measured. Go figure.
Anyway, I would be very interested in seeing a measurement with a single driver to compare the results and then putting that into the picture with all the other info. This type of compound dipole woofer system might turn out not be so "gimmicky" after all (well at least for low frequencies). Let's see what we can see.
I presume the cones are moving in the same direction? If so, I'm a little confused where the cancellation is coming from?
Also, I found this recent thread (from last year) where people commented on a similar compound dipole system.
Celestion System 6000 vs GR Research dipole bass
It seems to indicate what I wrote above in post 197, corroborating your interpretation of SL's driver arrangement. So I was likely wrong earlier. I would still like to see these measurements we have been talking about.
There is one interesting point that is mentioned in the post linked above: the poster mentions the difference between the close-proximity in-line arrangement like you use, and a side-by-side arrangement. Both seem to be +6dB sensitivity (assuming parallel connection, or driving each woofer separately, for the side-by-side drivers), but the side by side will be able to reach a 3dB higher max SPL because there is twice the radiating area and no cancellation between the drivers. But it would appear twice as large from the listening position as well, and that is not typically desired when the drivers are likely already large.
This is all interesting stuff. Some of it seems counter-intuitive to me, but there is no arguing with the evidence I guess!
Celestion System 6000 vs GR Research dipole bass
It seems to indicate what I wrote above in post 197, corroborating your interpretation of SL's driver arrangement. So I was likely wrong earlier. I would still like to see these measurements we have been talking about.
There is one interesting point that is mentioned in the post linked above: the poster mentions the difference between the close-proximity in-line arrangement like you use, and a side-by-side arrangement. Both seem to be +6dB sensitivity (assuming parallel connection, or driving each woofer separately, for the side-by-side drivers), but the side by side will be able to reach a 3dB higher max SPL because there is twice the radiating area and no cancellation between the drivers. But it would appear twice as large from the listening position as well, and that is not typically desired when the drivers are likely already large.
This is all interesting stuff. Some of it seems counter-intuitive to me, but there is no arguing with the evidence I guess!
