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Old 15th July 2007, 04:12 AM   #1511
dmason is offline dmason  United States
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Contact Andy Lankester at www.capilanohornspeakers.com He is in White Rock, British Columbia

He was having troubles posting the info on his site. He will be happy to forward to you, the information.
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Old 15th July 2007, 04:58 AM   #1512
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I've been taking a little vacation from hifi.

You guys want pictures, huh? Well, check these out (it's OK, they're work-safe). Looks like we're in the middle of a UFO flap - these happen every few years.

As a connoisseur of the fringes of consensus reality, I present for your reading pleasure, the account from "Isaac" of his years at the Palo Alto CARET laboratory. The 4th-Quarter Report of the CARET team is certainly a lot more interesting than what I was doing at Tektronix in 1986.

Well, I can't claim to have reverse-engineered antigravity from recovered alien artifacts, but I think I've made a (rather small) contribution to the dipole discussion over all of these posts. To summarize:

1) Dipoles can make up the intrinsic efficiency loss below the 1/f rolloff frequency through equalization, an increase in emissive area, or both. To keep distortion the same as an equivalent monopole system, the cone area needs to increase 4 times for every octave below the 1/f frequency.

For example, if you compare monopole and dipole systems using 12" woofers, with the dipole having a 1/f of 160 Hz and meeting a subwoofer at 80 Hz, the dipole system needs to have the area of 4 woofers at 80 Hz to have comparable distortion to the single-driver monopole system that covers the same frequency range. In practice, by taking advantage of the floor image, this requirement can be relaxed to using 2 woofers at 80 Hz - assuming both woofers close to floor level.

It can be seen relying on the low-distortion properties of selected audiophile woofers may not be sufficient - the distortion probably isn't going to be several times lower (as it needs to be), and power-handling hits the SPL/excursion wall much sooner than the equivalent monopole system. Like cars, there's no substitute for cubic inches (of displacement).

Although 12 and 15-inch midbass drivers with smooth response to 1~2 kHz are readily available, the same is not true for 18-inch and larger drivers. Drivers in the 18-inch and larger category appear to be designed for subwoofer use only, with response that's quite rough above 200 Hz - bad enough that 24 dB/oct active crossovers are required.

Given this information, the best approach for dipoles made with existing drivers is to select several wide range, low-distortion midbass drivers in the 12 to 15-inch range. The number of drivers required is the result of the spectral distance between the 1/f frequency and the crossover frequency of the monopole subwoofer.

Although it is tempting to keep the number of drivers down by using a high subwoofer crossover (right at the 1/f frequency, for example), this re-introduces the problem with box colorations. Much of the reason to use dipoles is to remove box coloration; re-introducing it from the subwoofer defeats much of the "boxless" advantage.

This is a good reason to keep the subwoofer as physically small as possible, use the lowest crossover practical, and use a fairly high rolloff slope. The lower the subwoofer crossover, of course, the more drivers required for the OB, so it's a tradeoff. That, for me, is a primary advantage of the 12" closed-box Rythmik - the box is small and rigid, the driver distortion is low, and they can be used in groups of two or four, as needed.

2) Although there are no box standing-waves to contend with, diffraction is still with us, thanks to the sharp boundaries at the edge of the baffle. A lossy mesh softens the transition, spreading out the area (and time) of the boundary.

The simplest method of spreading out the boundary might be nothing more than a large number of narrow tangential (or is it radial?) slots cut into the baffle, radiating out from the center of the widerange driver. If the slots start at the edge of the driver, extend to the edge of the baffle, and there are enough of them (20, 30 or more), there should be a measurable improvement in diffraction.

3) Although this thread opened with much optimism about coaxial drivers, the frequency response curves for the woofer portion of these drivers have not been very promising, never mind the doubtful response of the horns. In particular, the 12 and 15-inch woofers for the coaxes I've looked at so far appear quite a bit worse in the critical 1~5 kHz region than equivalent, monitor-grade midbass drivers. Most of the advantage of a coax is defeated if the rough response of the drivers dictates a 24 dB/octave crossover.

The best drivers are typically the ones that have the mildest demands on the crossover, fashion trends notwithstanding. A 24 dB/octave crossover slope, although high, is not infinite, and high-Q resonances can still be audible after substantial attenuation. Although equalized-flat speakers can sound OK on first listen, there can be a subtle fatiguing quality that builds up over time - this is the biggest reason I avoid drivers with really severe peaks, or rough rolloff curves.

P.S. You tell me.
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Old 15th July 2007, 07:55 AM   #1513
ScottG is offline ScottG  United States
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Old 15th July 2007, 12:08 PM   #1514
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Quote:
Originally posted by Lynn Olson

1) Dipoles can make up the intrinsic efficiency loss below the 1/f rolloff frequency through equalization, an increase in emissive area, or both. To keep distortion the same as an equivalent monopole system, the cone area needs to increase 4 times for every octave below the 1/f frequency.

For example, if you compare monopole and dipole systems using 12" woofers, with the dipole having a 1/f of 160 Hz and meeting a subwoofer at 80 Hz, the dipole system needs to have the area of 4 woofers at 80 Hz to have comparable distortion to the single-driver monopole system that covers the same frequency range. In practice, by taking advantage of the floor image, this requirement can be relaxed to using 2 woofers at 80 Hz - assuming both woofers close to floor level.
I think you have to be a little careful here. One point that is often missed with dipole systems is what happens at Fequal. Fequal is the frequency at which a dipole and monopole have the same level. Fequal = C/(6D) where C = sound speed and D is the dipole separation. What this says is that a dipole composed of two omni directional sources, each radiating at an intensity of I, will have the same on axis intensity at Fequal as a single omni directional source radiating with intensity I. But a funny thing happens when we use a conventional driver on an opened baffle. Say the driver has a 2Pi sensitivity of 90 dB/W/M. Put the driver in a box and at low frequency (in free space) the sensitivity drops to 84dB. I.e. it’s an omni directional source with 84dB sensitivity. Now, place the driver on an open baffle such that at Fequal the driver would be operating in its omnidirectional frequency range and you will find that at Fequal the dipole sensitivity will be 90 dB, just as if the dipole were composed of to omni directional point sources in free space with 90 dB sensitivity. That is a little bonus. (pretty easy to verify with a measurement.)

The reason for this is what happens at the baffle edge which goes back to the argument that Earl and I were having some time ago. Just to clear that up a little, I would agree with Earl that the strength of the wave wrapping around the baffle in a dipole is twice that which would result from edge diffraction in a conventional speaker. But what is and isn't diffraction then becomes somewhat of a semantics argument depending on the point of reference. The bottom line is that Earl and I actually aren't in disagreement, but were we’re looking at different aspects of the same problem with different interpretations.


Quote:
Originally posted by Lynn Olson


2) Although there are no box standing-waves to contend with, diffraction is still with us, thanks to the sharp boundaries at the edge of the baffle. A lossy mesh softens the transition, spreading out the area (and time) of the boundary.

The simplest method of spreading out the boundary might be nothing more than a large number of narrow tangential (or is it radial?) slots cut into the baffle, radiating out from the center of the widerange driver. If the slots start at the edge of the driver, extend to the edge of the baffle, and there are enough of them (20, 30 or more), there should be a measurable improvement in diffraction.

P.S. You tell me.

Here again I urge some caution. Suppose that you have a circular baffle. If there are no slots then the rear wave can be though of as a ring radiator at the baffle edge. On axis this reduces to a single source at the observation point. That is, the delay from all points around the circumference of the baffle to a point on the dipole axis will be the same. Cutting radial slots in the baffle would be something like spreading the edge ring radiator out over radius of the baffle. That is you are time smearing the rear wave. This may well have some smoothing effect above the first dipole peak (what you refer to as above the 1/f point) but is will also likely move the 1/f point higher in frequency since it will make the baffle look smaller to low frequency. I have some old circualr baffle lying around. Maybe if I have time this week I'll cut some slots in one and post the results. But the point here is that what the slots are doing is time smearing the rear wave. You get a similar effect by going from a circular baffle to a rectangular of other shaped baffle where the path lengths to the baffle edge vary around the circumference.

However, there are other issues you might want to consider with the slotted concept. First, since the slots will go all the way in to (or close to) the edge of the driver, the intensity of any "signal" that “leaks” through the slots will potentially be of greater amplitude that that which would exist at the baffle edge. (Attenuation due to propagation distance). Second, the slots will alter the structural integrity of the baffle and may introduce numerous individual resonances since the material between the slots could resonate at different frequencies.

Lastly there is this issue of using the driver above the 1/f point. Again, consider a circular baffle. There is a desire to use large baffles to improve the low frequency sensitivity by placing the 1/f point at a relatively low frequency. However, in doing that the driver tends to still be operating in an omni directional manner above the 1/f point and the peaks and null in the response above 1/f are severe. This behavior is also indicative of a multi-lobed, frequency dependent radiation pattern.

On the other side of the issue is using a smaller baffle. With a smaller baffle the 1/f frequency is raised and we suffer the loss of sensitivity at low frequency. However, by pushing the 1/f point higher we hopefully get it into the range where the driver is becoming directional. Since what controls the strength of the rear wave (and also any other diffraction effects) is the strength of the driver's front and rear response at 90 degrees off axis, as the driver becomes directional the response above the 1/f point naturally becomes smoother, even for a circular baffle. Additionally, pushing the 1/f point higher means that it approaches the crossover point to the tweeter and what is happening above it is no longer as much of a concern.
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Old 15th July 2007, 12:12 PM   #1515
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Old 15th July 2007, 03:54 PM   #1516
Aengus is offline Aengus  Canada
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Default Lossy mesh

Cymat is a foamed aluminum material I've been looking at in another context - it might make a good panel boundary material. It's interesting looking stuff, as well.

Apparently when you cut it, the cell edges are sharp, so this would have to be resolved somehow to prevent the panel edge from being hazardous: epoxy fill, perhaps, or a thin wooden frame at the outer edges?

Regards.

Aengus
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Old 16th July 2007, 03:05 AM   #1517
gedlee is offline gedlee  United States
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Quote:
Originally posted by Lynn Olson


The simplest method of spreading out the boundary might be nothing more than a large number of narrow tangential (or is it radial?) slots cut into the baffle, radiating out from the center of the widerange driver. If the slots start at the edge of the driver, extend to the edge of the baffle, and there are enough of them (20, 30 or more), there should be a measurable improvement in diffraction.


Think Stealth (F-117) here. This is exactly the technique used to reduce the diffraction from unavoidable edges on these airplanes.
But please remember that this technique does not reduce the diffracted energy, only its amplitude is spread in time.
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Old 17th July 2007, 06:08 PM   #1518
BudP is offline BudP  United States
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Hi all,

Wondering if I might get some roughing up here. I have attached an EDGE file that seems to be quite nice in it's response characteristics. I intend to use a Lowther 15 ohm PM6A on top (EnABL'd, of course)

The two lower drivers will be 8 ohm Eminence Beta eight drivers. The top one firing in open baffle and the lower one feeding a dual tube 9 foot by 4 inch diameter transmission line, with the driver set at 1/4 pipe length. Only looking for 40 Hz or so and the two Beta eights will be in series, to match the Lowther.

Looking like I can cross at 6 db per octave just about wherever I want to, though likely not higher than 300 Hz.

The Beta eights will also be EnABL'd. Ditto the baffle, which will have the "wings" bent back at about 75 degrees on either side at the 0 to -.5 meter area on the left and at the .3 to .35 meter area on the right. The stabilizer legs will allow the entire thing to rest with about a 10 to 15 degree angle to the front of the baffle.

I would appreciate any comments about obvious design flaws and general stupidity.

Bud
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File Type: zip final space ship baffle.zip (1.1 KB, 128 views)
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Old 17th July 2007, 08:55 PM   #1519
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Hi Bud,

The Eminences will have quite different impedance characteristics imposed by their different mounting/loading, so they cannot be expected to work in series - only parallel. However in parallel they will present a much more respectable load.

Surely the combo will go down to 25Hz via the pipe though be displacement limited.

This is something similar which I am doing myself at the moment. 6x9 drivers in front and back for bipole. Folded line between drivers is 8ft long. External = 10" wide by 9" deep by 52" high.
Dowel rod now either edge of both front and rear panels, like a rounded baffle edge; this does make a difference with mid peaks if you take them away.
Lots of holes now drilled in sides behind the front driver (only) to make the front driver semi-OB whilst simultaneously venting the line.
Both the drivers and the vent have phase matched output at 32Hz, with respectable output down to 25Hz, esp with room gain.

Work continuing, but high WAF and good sound so a definite keeper.

Cheers ........ Graham.
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Old 17th July 2007, 09:41 PM   #1520
mige0 is offline mige0  Austria
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Hi

Quote:
Think Stealth (F-117) here.
Quote:
Wondering if I might get some roughing up here.
Bud, looking at the pic's below, I think you will need either a lot of Earls recommendation for that shape – or astronomic distances for your listening position.



Click the image to open in full size.
Click the image to open in full size.



What I do with EDGE to get an idea about a specific shape is to put the mic into a 2m distance and make a rough plot
- at axis
- at 30 deg horizontally off axis
- at 30 deg vertically off axis
- and two positions in between

This almost ALLWAYS reveals that you have only the first peak that stays more or less in shape – everything at higher frequencies you better forget about.
To ease crossover design any simple baffle shape is advantageous as shown with the square baffle as an example

Its interresting to do the same with existing designs as well.


Greetings
Michael
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