The Great Waveguide Shootout!!! - Page 6 - AVS Forum
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post #151 of 190 Old 09-11-2013, 07:58 AM
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Quote:
Originally Posted by mtg90 View Post

The measurements of native frequency response look cleanest without a baffle, while the large baffle does the directivity polars best.

Do you expect this to remain true with larger WGs?
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post #152 of 190 Old 09-11-2013, 09:24 AM
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That's an odd one Matt. I wouldn't expect any diffraction with such a large baffle. Where is the mic? 2m? Maybe try closer?
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post #153 of 190 Old 09-11-2013, 09:47 AM - Thread Starter
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Quote:
Originally Posted by Martycool007 View Post

Hey Matt, can you explain to us non-technical novices guys what those graphs are and how to interpret them?

That would be a huge help as I have been following this thread the entire time but honestly have no idea what these graphs are telling us.

The frequency response graphs with multiple lines just show what happens to the raw response as you move off axis.

That data is used to create the Polar Sonogram, for that the off axis responses are normalized to the 0 degree response. What this does is let you see how the dB level changes as you move further off axis at the different frequencies. The first contour line is at -6dB, this is what is normally used to determine the dispersion of a particular horn or waveguide. One with constant directivity control should look nearly flat across its entire range.
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Originally Posted by zheka View Post

Do you expect this to remain true with larger WGs?

I plan on trying this with the SEOS-12 too.


Tux - The mic for these so far have been at 1m. I have not tried 2m yet, though I expect it to look even worse further out. Only thing I can think of is to try is placing some sort of round overs on the edges of the baffle. Or I just measure on axis FR and distortion without a baffle then measure off axis for the polars on the baffle.
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post #154 of 190 Old 09-11-2013, 10:11 AM
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I hate to throw a wrench into things, but I remember reading a post by Dr. Geddes in which he said something to the effect that when measuring waveguides, you have to be at least 2 meters away. Maybe he'll chime in.

John
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post #155 of 190 Old 09-11-2013, 11:10 AM
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that is most likely when measuring waveguide speakers, so that the woofer and horn sections have time to blend. if you measure something like the summa with its large center to center driver spacing, you could get bad results.

Listen. It's All Good.
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post #156 of 190 Old 09-11-2013, 12:20 PM
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I'm mystified by how the polars for the large baffle, which show only a few dB of directivity at the low end ~1300 Hz, can be so different, and worse, than the others, which have 10 dB difference between 0 and 90 deg.

Also, this is a 6" WG? How can i hold directivity to such low freq?!
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Originally Posted by mtg90 View Post

OK as I thought the baffle was creating major diffraction bumps in the frequency response. It would be nice to have a hole dug in the backyard for a few months so I could drop the baffle in when needed but I still don't think it would go over too well tongue.gif

All three of these are adjusted the same settings, 6ms gating, 1/48th octave smoothing, and were taken at 1m (though I did not have drive level set the exact same). This is the DNA-150 on the EOS-6 waveguide.

First the original set 0-90 degrees (0, 10, 20, 30, 40, 50, 60, 75, 90) horizontal on the large baffle:


Now measured flush mounted to a speaker baffle (8"w x 14"h):


Now measured without a baffle:



Now the polars in same order:




I should mention that the large baffle polar was done with higher resolution having 5-15-25 degree measurements in there as well.

Now I have come to the conclusion that the polar on the large baffle may look slightly narrower because the diffraction bumps cause some spots on the 5 and 10 degree measurements to measure slightly higher in level then the 0 degree line which skews the 0dB reference point a little.

The measurements of native frequency response look cleanest without a baffle, while the large baffle does the directivity polars best.

Ahhhh what have I have gotten myself into. rolleyes.gif

Noah
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post #157 of 190 Old 09-11-2013, 12:50 PM
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Quote:
Originally Posted by noah katz View Post

Also, this is a 6" WG? How can i hold directivity to such low freq?!

Its amazing how much a baffle (diffraction) can add to apparent directivity. It can influence the woofer section as well.
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post #158 of 190 Old 09-11-2013, 02:45 PM
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The first dip in your first figure (large baffle) is at 1.7 kHz approximately. That corresponds to a distance of about 8". Cannot possibly be due to the large baffle. Must be the edge where the WG meets the baffle or somewhere close by.

If it was the large baffle causing diffraction, then we would have seen diffraction when you mounted it to the smaller baffle as well. But like you mentioned, that was a "flush" mount.

In other words, you would have to flush mount every WG smile.gif I say just do free air measurements... good enough. Besides, the way the WG performs on your small baffle, i.e, in practice, is closer to the free air performance than the large baffle performance. Well, unless someone decides to mount'em in a wall smile.gif
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post #159 of 190 Old 09-11-2013, 02:58 PM
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I was also thinking along those lines. How big is the square piece of mdf you mount the wave guides on, and is it flush mounted in the large baffle? If not i thin you have the sinner there.
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post #160 of 190 Old 09-11-2013, 04:44 PM - Thread Starter
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The large baffle has a 24" x 24" cutout which I drop in a set of sub baffle that support the 12" x 12" baffles on which the smaller waveguides are mounted and 12" x 16" baffles the larger waveguides are mounted to. Everything is flush mounted. I spent days making the baffles to flush mount all the waveguides, now I wonder I wasted all that time mad.gif

I agree the free air measurements are closer to what the waveguide measures when used in a real world enclosure. I am going to test the SEOS-12 those three different ways and if does the same thing I'll just switch to free air measurements.
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post #161 of 190 Old 09-11-2013, 06:37 PM
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Quote:
Originally Posted by LTD02 View Post

that is most likely when measuring waveguide speakers, so that the woofer and horn sections have time to blend. if you measure something like the summa with its large center to center driver spacing, you could get bad results.

Actually, I think he was referring to dedicated waveguide measurements:

http://www.diyaudio.com/forums/multi-way/103872-geddes-waveguides-377.html#post2047602
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post #162 of 190 Old 09-12-2013, 07:07 AM
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So with regards to those polar response graphs, the red, blue & yellow represent what?

I know that the right hand side of the graph is the Db (spl) and the left hand side is the axial degree, and the bottom is the actual frequency.

What do the colors represent and how are these graphs meant to be properly interpreted?

(Matt, thanks for the reply in my last post, but I am still trying to figure this out!)
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post #163 of 190 Old 09-12-2013, 07:47 AM
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The darker the color, the greater the dip in frequency response. It's like this:

except darker colors would be the same thing as lower points on the vertical scale. Think of it as a top down view of that chart.
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post #164 of 190 Old 09-12-2013, 09:26 AM
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post #165 of 190 Old 09-12-2013, 10:30 AM
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Any chance the some of the discrepancies are due to mechanically induced resoances from the waveguide to the large baffle? Large panels without support/termination on the edges tend to resonate easily.
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post #166 of 190 Old 09-15-2013, 05:27 PM
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Throw a lot of fiberglass on your 4'x8' baffle. I bet the response gets cleaned right up. Owens Corning 703, 2" thick, should do the trick.

http://www.bobgolds.com/AbsorptionCoefficients.htm
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post #167 of 190 Old 09-17-2013, 03:57 PM - Thread Starter
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I tried a few 1-1/2" 16"x24" egg crate foam sheets one on each side of the waveguide just because I had them handy but they did not do much. Here is the quick measurement I made with the SEOS-12, 0 degrees and 60 degrees vertical with foam (red) and without foam (blue).



I also compared the SEOS-12 without a baffle, in a speaker baffle (mounted to my 15" woofer enclosure) and on the big baffle. The graphs for these follow, DNA-360 was the CD used, 1/24th octave smoothing applied and a 6ms gate was also used.

FR graphs,

Big baffle:


Speaker baffle:


No baffle:


Polars same order:




I usually add a little more smoothing to the polars 1/12th octave or so like the EOS-6 ones a few posts above but forgot to here just in case you are wondering why they look a little rougher.
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post #168 of 190 Old 09-17-2013, 04:56 PM
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I wouldn't have thought they'd be so prone to diffraction. Speechless. I didnt do such a direct comparison, but my baffle wall didnt create such a change compared to my speaker boxes using the SEOS 12. But like I said, I didn't compare them nearly as detailed as you have.

Very enlightening, thanks.
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post #169 of 190 Old 09-17-2013, 04:57 PM
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Oh, have you tried normalizing the on axis FR and comparing the difference to what a program like EDGE would tell you?
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post #170 of 190 Old 09-17-2013, 09:38 PM - Thread Starter
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Yeah I am at a loss at why it is so bad on the baffle and I am surprised the foam had as little impact as it did. I'll probably start redoing the tests with the waveguides free air.


I feel like you have mentioned EDGE before, but I can't remember what exactly it is, Google was no help either redface.gif
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post #171 of 190 Old 09-18-2013, 09:41 AM
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How far away was the mic on these most recent tests? I'd really like to see, say 1 meter vs 3 meters, if that's not impractical.
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post #172 of 190 Old 09-18-2013, 09:52 AM
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EDGE is a diffraction modeler. I guy on diyaudio made it.
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post #173 of 190 Old 09-18-2013, 10:41 AM - Thread Starter
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Ok I found it, here is the simulated diffraction response vs. the baffle response normalized against the free air response. There are some slight similarities between the graphs but the magnitude of the dips/peaks is much greater in the measured response.




John - I have been doing them at 1m so far. I can measure the waveguide in the speaker baffle and free air at 1 and 3 meters and compare them but the big baffle is stuck at either 1 or 2 meters.
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post #174 of 190 Old 09-18-2013, 10:57 AM
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Does your mic setup change when you measure on the big baffle v free air?

Even with the SEOS-12, the first cancellation is at 1.7 kHz. So, it's not horn dependent. Something in the setup is causing the cancellation. A few pictures might help.
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post #175 of 190 Old 09-18-2013, 11:32 AM - Thread Starter
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Pictures of the baffle setup are on the first post, the free air measurements are done on a turntable a made that sits about 5' off the ground (I can make it higher too) with the mic in an adjustable mic stand.
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post #176 of 190 Old 09-18-2013, 02:18 PM
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Quote:
Originally Posted by mtg90 View Post

Ok I found it, here is the simulated diffraction response vs. the baffle response normalized against the free air response. There are some slight similarities between the graphs but the magnitude of the dips/peaks is much greater in the measured response.




John - I have been doing them at 1m so far. I can measure the waveguide in the speaker baffle and free air at 1 and 3 meters and compare them but the big baffle is stuck at either 1 or 2 meters.

Based on what Dr. Geddes said in his post that I linked previously, I really think that at least 2 meter measurements might be more accurate. That's just me relaying information- I'm not an expert.

I'm wondering if the free air measurements would be the most helpful- eliminate baffle diffraction altogether. Thanks for doing all this work! I know how much of a pain it is to do this kind of thing.

John
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post #177 of 190 Old 09-18-2013, 02:40 PM
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Matt, I had seen those pics, but never looked carefully. Now that I have, I have a feeling it's the wood strip the mic's mounted to that's causing the reflection. Would you say it's about 4 inches from the tip of the mic to the wood strip? If so, that's our first notch at 1.7 kHz.

Do you have the wood strip when measuring the horns in free air? That's what I meant when I asked about your mic setup in the earlier post.

Also, that round protractor thingy could also be causing reflections, especially in the absence of other reflections. But it would be pretty delayed, so the cycle would be much larger. In other words, the teeth of the comb would be spread apart further. Someone suggested fiberglass. It really works. About 2" on the reflecting surfaces should do it.

You might try suspending the mic off a string or something and see if the response is smoother. I have been known to use chandeliers and a string to suspend mics in mid -air for measurements eek.gif
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post #178 of 190 Old 09-18-2013, 03:04 PM - Thread Starter
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Well before I give up on the baffle I will try backing the mic out to the 2m position and adding some dampening material behind the mic to see if that strip is causing the reflection.
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post #179 of 190 Old 09-18-2013, 05:52 PM
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Suspending the mic from its cable is a great idea. Is your mic omnidirectional? If it's a small diaphragm condensor, then it's probably front address with a cardiod pickup pattern- that will work suspended from the cable as well. Plus, it's SOOOO simple to hang it. If you hang it like this, I don't think you'll need any foam or other extra material- just hang and go.

John
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post #180 of 190 Old 09-18-2013, 06:27 PM - Thread Starter
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Yeah but switched early on from a baffle that rotates and mic stays put to a mic that rotates while the baffle stays put. That way I did not have to glue in the fiberglass waveguides which don't have mounting holes. If I hang the mic it will always point strait down while it rotates instead of always pointing at the center of the baffle.
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