A good OB system does sound very good. I have a couple and really like them. Based on your measurements I believe yours would be right up there too.
Thanks for your time and analysis!
My drivers were £100 each, so the cost and time to build similar enclosures to SL isn't worth it for me. Also the clamshell allows for the nice isolating sling suspension - which is a fantastic bonus for mechanical coupling isolation, and something that would be more difficult without the 'clamshell'.
I appear to have a good 5-6dB increase in output as measured (you have achieved 4.5-5dB in yours and others have found similar) - which as you and SL both highlight - is not a very efficient use of the extra amp channels and drivers compared to a 'normal OB arrangement' - but is a nice, unexpected bonus, and I have both to spare!
I don't need a further 10dB headroom either with these having such high VD - they go much louder than I require in my man cave and dig deep - as my measurements show!
As I have said all along - this arrangement works for me - my man cave is not very large, and I have only just managed to squeeze naked 15" drivers into the space - I like the clamshell aesthetics too which is nice
It appears then we can draw a line under this?
There is nothing new under the sun!
Yeah, I think its a concept that would work well for many people. Maybe not optimum but good efficiency for zero visual baffle
No worries. You gave me an itch I needed to scratch.
BTW, what country are you in?
I think it is aesthetically quite striking. The advertising people would love that!
I'm more form follows function but that doesn't cut it for some I consult for. Commercial loudspeakers sell for their aesthetics, not their measurements. Even more so in the high end.
LOL! Well sir - Well scratched!
I live in the middle of sunny (at present) England!
Other side of our beautiful planet
Oh, I do appreciate all the efforts that are made in this thread trying to understand the concept and the output.
How I wish I could... unfortunately, I don't have a pair of 15" subs lying around the house! I'd love 4 to try this thing!
And in my little corner of the world, those drivers are hard to get, or way too expensive to ship.
I do have four 5x8 SBA woofers I could give a try. Will need to disassemble a pair of OBs I have at the moment (they are in WFW setup) but I was planning to do so anyway.
Thanks for taking the time.
I think it is aesthetically quite striking. The advertising people would love that!
I'm more form follows function but that doesn't cut it for some I consult for. Commercial loudspeakers sell for their aesthetics, not their measurements. Even more so in the high end.
There is truth to that but at the same time aesthetics are important to many people. Function and form need to be balanced in a domestic environment; particularly when it is shared...
They don't exactly "disappear", that's for sure.
LOL Jazz man!!!
Not visually, but they sure do acoustically
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I think your example above is completely correct but not a very appropriate one - here's why. You certainly have a valid point that a couple of inches more of baffle is no big deal when compared to the cost of another driver AND another amplifier channel or halving the load seem by the existing amp (for a parallel connection). But the drivers you are using are rather small and the frequencies in which they would be used rather high. I foresee the advantages of the compound dipole to be primarily for dipole subwoofers and woofers below e.g. a couple of hundred Hertz.
Let me give what I feel is a more appropriate example: let's say the driver is already in the largest practical baffle but "what if we need that extra push over the cliff. What can we do? We can turn it up to 11..." by switching to a compound dipole. Sure, you have to use a second driver, but unless you want to double your baffle size (making the visual impact much larger) you may not have any other good options to increase both sensitivity and output. For a more concrete example, if your baffle is 24"x24" you can keep the visual size the same (from the listening position) by adding another 24"x24" baffle placed behind the first one and get 6dB more out of the pair. For low dipole bass, 6dB can make a real difference. In this case, increasing the baffle size to 48"x48" is really not a viable option.
So IMO the compound dipole is useful but not so much at higher frequencies where you can simply use the next larger driver, a more sensitive one, etc.
On the other hand, I think your measurements are great because they hint at the "phantom baffle" concept that may lie behind why the compound dipole has 6dB higher output, and is a phenomenon that isn't' apparent at first glance.
Can you explain your thinking behind this please, I still find it confusing? Thanks.
Could a shallow (say 100mm) U or H frame get close to the same boost but allow crossover at a higher frequency - maybe 500hz?
Or can the interior of a deep (200mm plus) U frame be lined with thick dense acoustic lining (Like 50mm Martini XHD) to make the U frame walls disappear acoustically for frequencies above say 150hz while still boosting the low end and allowing the dipole pattern to form?
Or can the resonances from the U frame be corrected with DSP for higher crossover? FIR only?
Or can the interior of a deep (200mm plus) U frame be lined with thick dense acoustic lining (Like 50mm Martini XHD) to make the U frame walls disappear acoustically for frequencies above say 150hz while still boosting the low end and allowing the dipole pattern to form?
Or can the resonances from the U frame be corrected with DSP for higher crossover? FIR only?
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Sure, let me explain:
The drivers are arranged "in line" meaning that the centers and axis of the cone movement are on the same axis for both of them. It doesn't matter whether they are face to face or not, but they are wired in order to operate so that the cones move simultaneously in the same direction. Looking from the side, you would see the cone of both drivers moving e.g. to the right together, and then back to the left together.
We can represent this with point sources. Both drivers are, individually, dipoles since they are not enclosed in any way. So the equivalent point sources would look like this
(-)--d1--(+)...............(-)--d1--(+)
Above, we have four monopole point sources, two per driver, representing the sign of the pressure being generated by the front and back of each driver's cone. d1 is the distance between the front and rear of the cone, which is the same as the diameter of the driver (let's say the driver is not mounted in any baffle for this example). The center-to-center distance between these two drivers is given as 2*d2 (not shown). This is consistent with SL's nomenclature.
In between the two drivers you can see that the left driver will be producing a positive pressure wave from the side of the cone that is towards the middle, while the right driver will be producing a negative pressure wave from the side of the cone that is towards the middle.
Although there is some finite distance between these two "inner" sources, compared to a long wavelength of 3.5 meters (for e.g. 100Hz) or more (for lower frequencies) this distance is small, so we call it "acoustically small" and you can say that in essence the two sources are in the same place (I used to term "on top of each other") even though they are not physically so. Why? Because their separation is much less than the wavelength at low frequencies, the phase after the two "inner wavefronts" propagate and meet will be approximately the same as it was before propagation. So they almost perfectly "cancel out" and there is no net pressure in the far field.
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No.
The issue with U- and H-frames that limits their upper range has to do with what is called a "tunnel resonance", meaning that the air enclosed by the frame acts as a slug that can resonate and cause peaks and dips in the response as well as differences between the on and axis responses at a given frequency at and above the frequency where these resonances begin to appear. Wall treatment of the tunnel will do nothing to change that.
Correction via DSP (IIR or FIR) is not useful because peaks and dips occur at different frequencies depending on the axis you look at.
You mentioned using a shallow frame and a goal of using the compound dipole up to 500Hz. You can just place the drivers very close together and get up to 500Hz already, so this is kind of a non-problem.
If you look at the plot below (my data) for example the yellow line would be fine up to about 500Hz.
Right Charlie, thank you, I understand what you're saying. I think the excursion needs to be measured, I have a sneaking feeling it may have increased with the two speaker set up.
If anyone is interest the set up can be simulated with the ripple tank by using the "linear quadrupole" and rotating one of them.
If anyone is interest the set up can be simulated with the ripple tank by using the "linear quadrupole" and rotating one of them.
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It is possible to measure excursion at home, and without any "special" equipment. Here's how:
This is yet another gem from SL's web site: the wedge micrometer. It's a folded trangular piece of paper with some marks on it. You can actually just color in a section of it in e.g. black. Then you run the driver with a modest amount of power (or more) at some low frequency such that you can get the cone to move by several (the more is better) millimeters.
The wedge micrometer is attached (taped I think) to the top of the curve in the surround, with the colored or marked side facing outwards. It would be helpful to attach another piece of paper to the frame with some reference marks, e.g. millimeters or 1/8 inch or something. This reference card should be attached to the frame right next to (as viewed from the side) the wedge micrometer.
With the driver running with whatever input power level and frequency you choose, the movement of the wedge micrometer will be blurred by the eye and the colored area that is moving up and down wildly will look like a solid. You can then compare this to the reference card to get a pretty good idea of the magnitude of the cone excursion. If you use a long enough exposure on your camera, this should be able to capture the same thing.
So, you can try this at home if you wish. Set up a single driver with the wedge micrometer on it and try to record its excursion. Then position a second driver so that the two are in a compound dipole arrangement, power the second driver with the exact same signal as the first one, and re-check the cone excursion via the wedge micrometer.
Let us know what you find.
Here is information about the wedge micrometer from SL's web site, with the following description:
This is yet another gem from SL's web site: the wedge micrometer. It's a folded trangular piece of paper with some marks on it. You can actually just color in a section of it in e.g. black. Then you run the driver with a modest amount of power (or more) at some low frequency such that you can get the cone to move by several (the more is better) millimeters.
The wedge micrometer is attached (taped I think) to the top of the curve in the surround, with the colored or marked side facing outwards. It would be helpful to attach another piece of paper to the frame with some reference marks, e.g. millimeters or 1/8 inch or something. This reference card should be attached to the frame right next to (as viewed from the side) the wedge micrometer.
With the driver running with whatever input power level and frequency you choose, the movement of the wedge micrometer will be blurred by the eye and the colored area that is moving up and down wildly will look like a solid. You can then compare this to the reference card to get a pretty good idea of the magnitude of the cone excursion. If you use a long enough exposure on your camera, this should be able to capture the same thing.
So, you can try this at home if you wish. Set up a single driver with the wedge micrometer on it and try to record its excursion. Then position a second driver so that the two are in a compound dipole arrangement, power the second driver with the exact same signal as the first one, and re-check the cone excursion via the wedge micrometer.
Let us know what you find.
Here is information about the wedge micrometer from SL's web site, with the following description: