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Old 23rd December 2016, 01:35 AM   #11
Mark Kravchenko
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What software are you using for measurement?

For raw frequency response you could do near field measurements. About 6 mm away from the diaphragm. But you will have to hunt across the diaphragm both on the vertical and the horizontal planes to find the best response. This will give you almost as good as anechoic measurements.

Then if you want listening position measurements you can have a reference against the near field to see what is getting messed up by reflections.
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Old 23rd December 2016, 02:59 AM   #12
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Interesting. I didn't think you could equalize such a large wavelength relative to the radiating surface. I'll have to re-visit it.

Best,

Erik
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Old 23rd December 2016, 02:53 PM   #13
Few is offline Few  United States
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Erik,

As you know doubt learned from your reading, there's a lot of cancellation at low frequencies, just as your intuition would suggest. Fortunately, if your drivers can move enough air then equalizing the dipole cancellation is viable. The six 8" woofers in my OB panels have about the same surface area as a 19" woofer. On the other hand, my narrow baffle raises the frequency at which the cancellation begins, and the 8" woofers have less maximum excursion than would a typical very large woofer. So far, even with the bass flattened down to the low 30's Hz I've not seen large demands on woofer excursion when listening to jazz and classical music (no low frequency organ blasts yet). I have a pair of old NHT 12" woofers I can put into sealed boxes if I find I'm really missing something in the bottom octave. I'd rather not add two large sealed boxes to the rather large speakers my wife just accepted into her living room, though.

Thanks for the nearfield suggestion, Mark. I did just that with the woofers and of course got a very clean result. In the limited time I had I couldn't figure out a convenient way to use the nearfield measurement to compute what the response would be with the dipole cancellation included. John Kreskovsky suggests a method (invert and delay the response and add it to the main signal) but I couldn't figure out how to implement it just using Room Eq Wizard. With more time I can measure an impulse response and import it into Matlab where I have more control. I've also downloaded rePhase but I haven't worked with it yet so I don't have a good sense of its capabilities. I plan to use rePhase to figure out how to fix the phase response once I get the magnitudes worked out.

I haven't taken a nearfield measurement of the planar magnetic drivers yet but it's certainly worth doing. Thanks for the suggestion.

Few
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Old 23rd December 2016, 03:38 PM   #14
Mark Kravchenko
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There are a number of ways to deal with reflections in a room. Gating can be used. It will progressively limit the low frequency measurment.
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Old 23rd December 2016, 04:29 PM   #15
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Hi Few,

While I have never done an OB design before, I have followed them off and on. The closest I've seen to your designs is something like the Infinity arrays, which used very wide or U shaped baffles to reduce the cancellation effects. I'll go and see if I can find some models, it could be fun.

A good book on measurements is D'Appolito's Testing Loudspeakers.

Best,

Erik
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Old 23rd December 2016, 05:01 PM   #16
Few is offline Few  United States
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I've done some gated measurements, and tried a quick attempt at a ground plane measurement. I had high hopes for the ground plane but my first attempt didn't inspire much confidence. The trick, of course, is not just choosing the result that looks prettiest but instead finding the measurements that provide the most useful information. I'm still on that quest.

I have d'Appolito's book and have reviewed it a bit recently but really should give it a closer look now that I'm making measurements. One of the questions is how far away to place the mic when the speaker is 8 feet tall, and standing on a concrete floor (acoustic mirror). The response varies with distance, of course. Probably I need to go through the trouble of moving the speaker into the middle of the room and placing the mic equidistant between the floor and ceiling to try to squeeze as much reflection-free time as I can muster so that gating doesn't kill my measurement bandwidth.

In the meantime I'm attaching some nearfield results. The planar magnetic (PM) results are the result of a few different mic positions in front of the diaphragm, and one where I've changed the position of the speaker in the room because when I looked at the impulse response I found the back wave reflected from the rear wall was coming through rather strongly, even with the nearfield measurement approach. There were no crossovers and there was no equalization applied but I used 1/24 octave smoothing. Both the front and rear grills were in place so I'm measuring through the cloth. No doubt I'm losing a bit of high frequency response as a result but there is a roll-off even without the grills. Also, I'm using an old Behringer ECM8000 mic and haven't yet loaded any calibration files into the machine I'm using for measurements. That'll affect the high frequency response a bit as well.

I'm also attaching a single measurement from just in front of one of the open baffle (OB) woofers. Ignore differences between absolute levels---the gain settings were different when I measured the PM and OB speakers.

MODERATORS: Should I post a message in the planar forum to alert those readers about this planar magnetic discussion? Do you want to move the whole thread?

Few
Attached Images
File Type: jpg PM nearfield overlays.jpg (71.4 KB, 499 views)
File Type: png Screen Shot 2016-12-23 at 12.50.26 PM.png (161.8 KB, 477 views)
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Old 23rd December 2016, 05:13 PM   #17
Mark Kravchenko
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Awesome.

No smoothing.

Really good results.

What you see in marketing information is useless creative wishful hopes and dreams.

Learning to measure a dipole is jumping into the deep end of the pool.

Not easy even for me. Very dependant on reflections. And measuring without reflection doesn't characterize what you are hearing.
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Old 23rd December 2016, 06:09 PM   #18
Few is offline Few  United States
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Here are some awkward attempts at gated measurements of the planar magnetic tower. They're all smoothed at 1/24 octave, and gated at 9 ms. I did add a 6 dB/octave high pass at 60 Hz to attenuate the fundamental resonance a little bit but otherwise there's no crossover or equalization applied.

The differences between measurements are just mic heights above the floor (ranging from about 3 to 5 feet). Mic to speaker distance is about 6 feet.

I'm attaching a typical impulse response so others can get a sense of what's included and excluded by the 9 ms window. The details of the undesirable squiggles within the first millisecond vary with mic position. I'm assuming they reflect differences in the interference pattern formed by the radiation from the different point on the speaker diaphragms. I could shorten the window and kill that 5 ms glitch but that really limits the resolution and bandwidth. I probably should find a good way to do some spatial averaging before figuring out what I should try to fix with the miniDSP.

Few
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File Type: jpg PM 9 ms gate.jpg (67.8 KB, 457 views)
File Type: jpg PM impulse 9 ms gate.jpg (50.2 KB, 446 views)
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Old 23rd December 2016, 06:57 PM   #19
Few is offline Few  United States
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Well this is interesting...

I tried ground plane measurements again but they really got me nowhere, even if I tried to tilt the speaker to aim it at the mic laid on the floor.

On a whim I moved the mic so that it's against one of the 8' tall windows that form one wall of the room. I wasn't seeing a ton of floor or ceiling bounce so I thought I'd try seeing if I could tame the first wall bounce by using the window as a vertical ground plane. The measurements below are all taken without any gating at all; just 1/24 octave smoothing and the same first order 60 Hz high pass to attenuate the fundamental resonance. The only differences are mic height, ranging from 3 to 5 feet.

Below about 400 Hz the room modes are very consistent and clear. And they can be measured with good resolution when there's no gating. Above that frequency the response is surprisingly reproducible and clean considering it was obtained in the world's most reflective listening room, without gating, and with the mic placed 11 feet from the speaker. Maybe the tall emitter really is doing a good job killing the floor and ceiling bounce?

Few
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File Type: jpg PM wall plane no gating.jpg (67.6 KB, 441 views)
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Old 23rd December 2016, 10:48 PM   #20
Mark Kravchenko
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Quote:
Originally Posted by Few View Post
Well this is interesting...

I tried ground plane measurements again but they really got me nowhere, even if I tried to tilt the speaker to aim it at the mic laid on the floor.

On a whim I moved the mic so that it's against one of the 8' tall windows that form one wall of the room. I wasn't seeing a ton of floor or ceiling bounce so I thought I'd try seeing if I could tame the first wall bounce by using the window as a vertical ground plane. The measurements below are all taken without any gating at all; just 1/24 octave smoothing and the same first order 60 Hz high pass to attenuate the fundamental resonance. The only differences are mic height, ranging from 3 to 5 feet.

Below about 400 Hz the room modes are very consistent and clear. And they can be measured with good resolution when there's no gating. Above that frequency the response is surprisingly reproducible and clean considering it was obtained in the world's most reflective listening room, without gating, and with the mic placed 11 feet from the speaker. Maybe the tall emitter really is doing a good job killing the floor and ceiling bounce?

Few

You will find that a tall array is better at masking the effects of boundary reflections. As in you will have opportunity to be in the area where the signal is coming from the speakers more than from reflections. Horns do that is another manner. They limit reflections.

A line array will have a greater power response if it is a true floor to ceiling array. The loss due to doubling of distance will be 3db per doubling of distance rather than 6db. As to moving your mic around and finding a better response. That is basically how it is done ! Especially with a planar is it difficult to get a smooth response without some tweaking.

I did days of testing AMT's last time I was in China and done outside or inside a 1 meter response was always raged. Many companies use a near field response that looks marvelous and "correct it" by drop the displayed SPL level and use this as a marketing method.

If you want a really good flat response of your planars you will need to be in the sweetest spot.

Regarding your ground plane measurement. Did you place your mic at 2 meters? And tilt your enclosure carefully? This will be useful more for a response test below 500 hertz. Above that it is not so easy to either reproduce the test nor is it really a real world listening condition test.
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