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Multi-Way Conventional loudspeakers with crossovers

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Old 21st June 2007, 12:16 AM   #31
AJinFLA is offline AJinFLA  United States
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Quote:
Originally posted by gedlee


There are two unavoidable polar response lobe holes in the vertical direction - this is the worst area for power response because of this, but it's not as bad as many, if not most speakers. A lot of time was spent with the crossover to optimize this response aberation. One of the lobe "holes" is aimed at the floor bounce.
The low XO certainly helps here. A coincident or unity driver arrangement has the edge there, but of course have their own issues.

Quote:

From the impulse response, the time alignment of the two drivers is quite close - the difference is in the usecs. The combination of physical offset and time delay from the LP filter makes the matchup of the delays almost exact. The woofer could go forward an inch or so to be ideal, or the waveguide back, but thats very difficult to do at this point and for a few usecs its probably not worth it.
To a fixed point relative to the 2 AC's. Yes.

cheers,

AJ
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Old 21st June 2007, 12:25 AM   #32
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Quote:
Originally posted by gedlee



There are two unavoidable polar response lobe holes in the vertical direction - this is the worst area for power response because of this, but it's not as bad as many, if not most speakers. A lot of time was spent with the crossover to optimize this response aberation. One of the lobe "holes" is aimed at the floor bounce.

There is no problem at all in the horizontal plane. From the impulse response, the time alignment of the two drivers is quite close - the difference is in the usecs. The combination of physical offset and time delay from the LP filter makes the matchup of the delays almost exact. The woofer could go forward an inch or so to be ideal, or the waveguide back, but thats very difficult to do at this point and for a few usecs its probably not worth it.

Very smart to "aim" the lobe at the floor bounce. I like that idea. I do wonder about the horizontal response and axial response due to combing. You don't fine this a problem, or at least a high priority problem? A lot of fuss has been made over combing on and offaxis of MTM's and it's resulting coloration and I don't see how they would differ in principle with a large WG mated to large woofer at a conventional frequency.

BTW this is a great thread! I'm sure I'll it'll end up "favorited" by many by the time your done!
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Old 21st June 2007, 02:45 AM   #33
gedlee is offline gedlee  United States
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Quote:
Originally posted by pooge
Earl,

I think you would do well to rename the foam phase plug to something like "HOM absorber foam" or the like to distinguish from the phase plug in the compression driver. You are welcome to use my trademark "HOM absorber" if you like.

I've never called them a "phase plug". I called them a "refractive index absorber" in the patent, but thats a bit complex. Nothing really comes to mind.
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Old 21st June 2007, 02:50 AM   #34
gedlee is offline gedlee  United States
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Quote:
Originally posted by augerpro

I do wonder about the horizontal response and axial response due to combing. You don't fine this a problem, or at least a high priority problem? A lot of fuss has been made over combing on and offaxis of MTM's and it's resulting coloration and I don't see how they would differ in principle with a large WG mated to large woofer at a conventional frequency.
No we don't find this at all. When I get to Thailand I'll try to post the actual measurements as they clearly show that there are no issues like this. I don't have the measurements here with me now.

There is a single axial hole that is caused by the mouth diffraction, but nothing (horizontally) caused by the driver to waveguide spacing.

I am going to try the idea of foam on the mouth radius to cut down on the diffraction - see if it works.
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Old 21st June 2007, 02:53 AM   #35
gedlee is offline gedlee  United States
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Quote:
Originally posted by AJinFLA


To a fixed point relative to the 2 AC's. Yes.

To a fixed "plane" relative to the 2 AC's you mean. Yes, exactly and its the time differences in the vertical plane that cause the two "holes".
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Old 21st June 2007, 02:56 AM   #36
gedlee is offline gedlee  United States
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Quote:
Originally posted by catapult
I modified my simple oblate spheroid spreadsheed so you can set the initial throat angle using the Goal Seek function in Excel. No effort was made to make it "pretty" but it works.

http://www.crestviewcable.com/~catapult/Geddes.xls

Unfortunately this doesn't quite do it. Thats because when you simply shave off the end, when the angle is correct the throat radius is no longer correct. You have to find a new throat radius and offset such that when the angle is correct the radius is also correct. I doubt that Excel has a simple function for this. It took me a couple of pages of math to get it right.
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Old 21st June 2007, 12:13 PM   #37
mazurek is offline mazurek  United States
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I'm curious whether it may be possible to satisfy your equation using graphical synthesis in a parametric CAD program. I took a graduate level Mech Eng class which used parametric CAD to generate advanced linkages that satisfied certain position, acceleration and velocity constraints. This may end up being an easier solution.

Lee
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Old 21st June 2007, 12:30 PM   #38
MBK is offline MBK  Singapore
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FWIW I recently had a 15" oblate spheroid WG CNC machined out of wood/plywood. Coverage angle is 120 degrees, throat is 1.4", and the WG profile worksheet is JoshV's found in
this post,
using for all I know Earl Geddes' formulas. I then added a LeCleac'h calculation for the flare and matched it by hand to the diameter necessary for the desired directivity lower cutoff of 800-1000 Hz. Drawings are in
this post.

Since I have no compression driver available I tested it with a dome tweeter. The exit of the nominal 1" tweeter happens to match the 1.4" throat (the surround takes up that much space). I made no attempt to correct for entry angle, it is at 0 degrees, but with the dome tweeter all bets are off anyway and the test is only very preliminary, to get a handle on lower directivity frequency limits. I did test it without the faceplate, so that the diaphragm is actually in the throat, but differences in response were small.

Measurements were done with WG mounted on a large baffle to prevent edge diffractions, and are shown
in this post
and the posts following it. I find polar response good, though with a dip on-axis that may or may no be significant. Directivity matches expectations. Comments welcome.

And one more question to Earl Geddes: most if not all 1" compression drivers I have seen were rated for a 1.6k crossover minimum. How did you manage to have the 1" driver go below 1000 Hz?
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Old 21st June 2007, 01:21 PM   #39
Paul W is offline Paul W  United States
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Quote:
Many drivers show good HF response out to 20 kHz on a plane wave or on a non-CD horn, but when put on a true CD device like an OS waveguide the response dies above 10-12 kHz for a 1.5 " driver and 9-10 kHz for a 2" driver.
The larger drivers roll off at HF but I understood response on a plane wave tube to be equivalent to power response...or the same as a "perfect" CD waveguide. Why greater rolloff on a CD waveguide than a plane wave tube?
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Old 21st June 2007, 01:28 PM   #40
gedlee is offline gedlee  United States
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Quote:
Originally posted by MBK


And one more question to Earl Geddes: most if not all 1" compression drivers I have seen were rated for a 1.6k crossover minimum. How did you manage to have the 1" driver go below 1000 Hz?
I'd love to see your polar measurements.

This rating for drivers is a safety factor put on by the manufacturer to keep "Dummy's" from overexcurting the diaphragm and hitting the phase plug - goodbye diaphragm! In a true CD waveguide you have low end to spare and it is no problem to take the driver down to 900 Hz or so. In my systems the waveguide is padded out at the lower end by at least 10 dB (more like 12 - 15 dB) That's already a lot of protection for the driver. We have not seen a problem yet. But there are a lot of "dummy's" out there who will drive these systems with 1000 watt amps - clipping! We will see.
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