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Old 26th June 2007, 03:09 PM   #1301
AJinFLA is offline AJinFLA  United States
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Quote:
Originally posted by johninCR
I'm not sold on the idea than an infinitely thin baffle should result in 0 edge diffraction, especially since I've tried it and get greater diffraction.
Based on measurements or supposition?

Quote:
Originally posted by johninCR

My only explanations for the increase in vibrations are increased edge diffraction, or at the driver cutout energy is transmitted into the baffle and somehow the shape of the edge focuses that energy to escape right at the pointed edge.
What?

Not quite sure if I follow that one.

cheers,

AJ
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Old 26th June 2007, 03:54 PM   #1302
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Quote:
Originally posted by Paul W
Hi John,
For a typical cone driver, is the underlying assumption of a perfectly anti-symmetric source a fairly big leap? Acoustic center offset, probable diffraction at the rear inside edge of the driver mounting hole, basket diffraction, as well as spider and/or magnet interference seems to generate a fairly chaotic environment for this to work up to a typical mid/tweeter crossover frequency. (panel ESL etc = different case.)
Paul
Yes there are other effect to conside, particualry as the frequency rises. However, I was trying to response to Earl's comment that diffraction is worse for a dipole because of the of pressure being zero at the baffle edge. If the response isn't symmetrical you don't have a true dipole and you don't have zero pressure.


Quote:
Originally posted by johninCR
JohnK,

Respectfully, I'm not sold on the idea than an infinitely thin baffle should result in 0 edge diffraction, especially since I've tried it and get greater diffraction. The wavelengths involved are long enough that asymmetries don't explain the increase. Since out of phase sound waves net to zero only if they are travelling in the same direction, I see only 2 explanations.

No, that isn't correct. If two waves are out of phase at the point of summation it makes no difference which directions they are traveling. Summation only concerns amplitude and phase. Not direction or velocity.

But before you can quantify that diffraction is worse or not you have to ascertain whether or not the response is a dipole response. You have to measure the response at 90 degrees and see if there is a good, deep null there. If there isn't, you don't have a dipole response and the front and rear radiation isn't symmetric. So what you get is not a dipole diffraction effect.


As an aside, perhaps what Earl was thinking of was the strength of the individual diffracted waves and not the sum of the two (front and rear). It is true that the diffraction of the front (or rear) wave will be stronger for a zero thickness baffle at higher frequency than it would be for a box speaker where the box depth is much greater than the wave length in question. This is because for the box speaker (with square edges) the short wave lengths only turn a 90 degree corner and then must propagate to the rear of the box where they would turn another 90 degrees. With a zero thickness baffle these same waves would turn 180 degrees, resulting is a stronger diffracted signal. But the front and rear diffraction sources will still cancel, regardless of strength, for a true dipole response generated from symmetric front and rear sources.
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Old 26th June 2007, 03:57 PM   #1303
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Quote:
Originally posted by johninCR
JohnK,

My only explanations for the increase in vibrations are increased edge diffraction, or at the driver cutout energy is transmitted into the baffle and somehow the shape of the edge focuses that energy to escape right at the pointed edge.

Perhaps the baffle vibrates because the pressure on onside is + sin(wT) when it's - sin(wt) on the other? :-)
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Old 26th June 2007, 05:00 PM   #1304
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John,

They're just small flat baffles, so they're about as close to dipole as I can get. I used a nautilus shape when viewed from the front, so the resulting different driver to edge difference gives them a smooth net response, however, the edges are lit up quite obviously a secondary sound source.

The only thing further I'm going to try is open up the driver cutouts more and add some kind of damping material to prevent sound from going directly from the cone into the wood. Since it's less than an inch away the spl's are quite high, so sound may very well be getting into the baffle there.

Since the effect seemed to increase after I bevelled the edges to a point, I suspect the real answer is that the edge diffraction is just a single source instead of 2 that are out of phase. It would then be greater than with a box because as the compression part of the wave sees the pressure drop from the larger space for expansion, it meets the low pressure portion of the rear wave and vice-versa (a greater pressure change). A further increase may also come from the true full space for the pressure change instead of just 3/4 space at the edge of the box.

A less likely possibility is that since the edge, where the front and rear waves meet, has the highest particle movement that it stimulates more vibration in the relatively thin edge.

I'll report back if the non-drying clay I will use to dampen the driver cutouts, but otherwise I'm not going to spend any more time with this design approach since it didn't work for me. The smooth response due to the infinite driver to edge distances is nice, but not at the expense of quite ugly edge diffraction.
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Old 26th June 2007, 05:56 PM   #1305
DDF is offline DDF  Canada
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Quote:
Originally posted by mige0
Hi




Dave, to reduce the OB to a circular test baffle is something that I thought of to concentrate on the edge diffraction effect when trying to optimise it for an ideal thickness, roundover shape or the mesh idea.



Greetings
Michael

Hi Michael,
As a starter before you cut wood, if you want to look in the time domain, the following might be a good short cut.

SLs posted frequency response captures with circular baffles at:
http://www.linkwitzlab.com/diffraction.htm

If you buy into the premise that edge diffraction is min phase, then SLs curve can be converted to a table using:
http://www.pvconsultants.com/audio/utility/spl.htm

This freq domain can then be converted to an impulse response using just about any free ware measurement/analysis system that implements the Hilbert Transform.

Cheers,
Dave
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Old 26th June 2007, 06:09 PM   #1306
DDF is offline DDF  Canada
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Quote:
Originally posted by panomaniac


Thanks Ottawa Dave for all that cool stuff! A lot to chew on there.

I have been reading Dr. Dave's (the big kahuna at Lexicon) papers lately because the subject of spaciousness is dear to me. Why do some (few) systems do it well and other do not? Size helps, but isn't the final determinant.

Much studying to do.....

My pleasure Maui Michael, glad to help.

DG's site is great, he's so generous with it. I'm a fan of his research and spent a good deal of time there myself, and only absorbed a fraction of it.

DG seemed to nail down the root cause of spaciousness as velocity (not pressure) differences between the right and left ear. How to use that to advantage is the hard part.

This might be an argument to try two gradient subs, aimed at each other from opposite sides of the room, if maximizing LF spaciousness is the goal. Mind you, the standing wave signature may get ugly in such a scenario.

Cheers,
Dave
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Old 26th June 2007, 06:46 PM   #1307
ScottG is offline ScottG  United States
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Quote:
Originally posted by john k...



Perhaps the baffle vibrates because the pressure on onside is + sin(wT) when it's - sin(wt) on the other? :-)
Another thing I had mentioned!


Quote:
Originally posted by johninCR
John,

They're just small flat baffles, so they're about as close to dipole as I can get. I used a nautilus shape when viewed from the front, so the resulting different driver to edge difference gives them a smooth net response, however, the edges are lit up quite obviously a secondary sound source.
JohninCR - I'm having a hard time visualizing your baffle & transducer (and its range of operation).

IF its a small baffle then its highly unlikely that the radiation is symmetric at the edge (unless of course you are using two small (sd) drivers (like tweeters) with equal spl.s - the front driver in-phase and the rear out of phase relative to the front driver).
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Old 26th June 2007, 07:24 PM   #1308
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Quote:
Originally posted by ScottG
JohninCR - I'm having a hard time visualizing your baffle & transducer (and its range of operation).

IF its a small baffle then its highly unlikely that the radiation is symmetric at the edge (unless of course you are using two small (sd) drivers (like tweeters) with equal spl.s - the front driver in-phase and the rear out of phase relative to the front driver).
Here's the only good picture I have. This one is for the 8" B200, which I haven't even bothered to mount. I made 3 pairs almost identical except for the driver cutout, but for the smaller drivers the shortest edge is much sharper since I had more material to work with. The open ring is just decorative, since the nautilus shape alone was way too ugly. I tried mounting both the FE108EZ and FE167 with similar results. Both were quite dipole in response with deep nulls directly at the sides. The 108's were probably closest to exact, since the cone was pretty well centered within the baffle thickness. I don't recall the low end cutoff I used, but that was more as protection from overexcursion, so I matched the baffle roll-off with a corresponding shallow slope on the woofer before rolling off the woofers more steeply around the baffle cutoff of the main driver.

I liked the look, but disliked the sound. I chalked it up to being a good idea that didn't work. I made some out of cardboard first, but ignored that they vibrated a lot expecially at the edges. I thought it was just a materials problem, not a design problem.

I did try a foam ring around the edge which helped to some extent. It was too ugly on it's own, and I didn't like the look of covering the whole thing in grill cloth either. I got disgusted and moved on to my dipole WG's, which I'm happy enough with to stop experimenting.

Click the image to open in full size.
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Old 26th June 2007, 07:28 PM   #1309
ScottG is offline ScottG  United States
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Mon Graci!

Yup - that won't behave as a dipole at the diffraction freq.
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Old 26th June 2007, 08:09 PM   #1310
Pano is offline Pano  United States
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Quote:
Originally posted by DDF
DG seemed to nail down the root cause of spaciousness as velocity (not pressure) differences between the right and left ear. How to use that to advantage is the hard part.
Hey Dave,
It is hard to do. I've heard the soundfield reproduced with with such incredible vastness and detail that all listeners present just blinked, shook their heads and look at each other in disbelief.

For over 20 years I've tried to reproduce that, but haven't. I want to know why.

So I'm trying to put together some intelligent questions for Dr. Dave about the subject. But that's a whole other topic for another thread!
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