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Old 14th May 2017, 11:40 AM   #1
WrineX is offline WrineX  Netherlands
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Default Can anyone shine a light on this ?

Heej

I am still playing with the planars. and i run into the following problem.


I have a small panel but at a certain volume i get some weird nasty distortion.

in the picture i measured one panel that has an openarea of around 10% single ended. plays fine. but when i add a backpanel so it works in push pull i gain 6 dB as expected. but this backpanel has lower open area around 4%. what i notice is that above 300hz all seems normal gain 6dB , but below 300 the distortion skyrocket upwards. particular 3rd harmonics. i did not change input volume, so it is not my amp. neither is it hitting the stator/magnets. you can also see at resonance it did not increase much is sensitivity.

when i put more power in the single ended version i run into the same problem be it slightly higher SPL then the push pull version

any one have an idea ? i would love to get that 6dB extra, but not when i get so much distortion for free.

few things i think it might be but i am not sure at all (hence the help needed)

- when in push pull there is not enough open area to let mylar move free and starts to self resonate?

- tension is so high on the foil and the magnets cant push it any further ?
(although i had panels with very low tension that did the same weird sounding distortion down low, they as well used the 4% open area)

hope someone experienced something like this ? if it is an open area problem this might occur in esl to.
It might save me allot of time.


RED - push pull (front plate 10%open area , backplate 4%open area)
BLUE - Single ended (front plate 10% open area, backside open)
Attached Images
File Type: jpg distortion planar low.jpg (282.8 KB, 242 views)
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Old 14th May 2017, 04:24 PM   #2
lowmass is offline lowmass  United States
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1- how small is the panel
2- at what volume does the distortion start to get ugly
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Old 14th May 2017, 04:31 PM   #3
WrineX is offline WrineX  Netherlands
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Hi low mass, the panel is rather small 280 cm2. at wich volume i cant say my mic is not calibrated for SPL. but yeah. they could be to small (prety sure) but i wonder why it sounds so disgusting at a certain volume.

here i did some measurements






First Pic Metal VS non Magnetic panel. 4% open area vs 10%

i could not get the metal membrane to stretch as far as the non magnetic did it by hand. so there should be slightly less distortion but not going to resolve what we see here .

Green is non magnetic HPL panel
Red is metal perforated sheet.

No the metal does give 1.5 dB more efficiency so it does increase the field. but at the same time it also screws up the distortion in the lower end.

PIC 2
When the 2 panels are combined we get a mix, distortion goes down everywhere, but there is still a peak from the metal variant.

RED Metal single stator
GREEN non magnetic single stator
BLUE Combined effort of both in series

SPL stays around the non magnetic only,

PIC 3
When i crank it up by 8 dB or something this is the result of HPL frame alone versus metal and hpl in series. at same output SPL. surface area wins, except for that nasty peak formerd by the metal version.

RED is single HPL
BLUE is HPL and METAL in series.
Attached Images
File Type: jpg metal vs HPL 5 percent 10 percent.jpg (187.0 KB, 219 views)
File Type: jpg hpl metal and both.jpg (276.6 KB, 220 views)
File Type: jpg dual compared to single hpl same spl.jpg (430.4 KB, 214 views)
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Old 14th May 2017, 04:39 PM   #4
WrineX is offline WrineX  Netherlands
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someone mentioned, Impedance radiation.

but i am not getting it , it can produce 150 hz but at a certain volume it starts to sound like ****. but there is excursion left in the panel. since hitting the stator sounds really different. this sound like a fart nasty. by adding panels in series helps it is also clear that the metal; perf variant has this problem more pronounced. but at the same time has lower distortion in the upper frequency.

Cant believe someone else never encountered this. or never tried
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Old 14th May 2017, 04:54 PM   #5
kevinkr is offline kevinkr  United States
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I think the hint is in the frequency response graphs? Below a certain frequency the air mass no longer loads the panel properly, excursion goes up (and is probably not uniform across the panel) and with it distortion.

This looks pretty good above a few hundred Hz, so why not high pass above 200Hz, or are you aiming for full range response from this panel?

I can't imagine linear excursion is much over a mm pp.. (Question, I don't know)

I lived with various Magnepans for almost a decade and remember excursion was extremely limited. (And they did not need to move much to couple either)
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Old 14th May 2017, 05:43 PM   #6
WrineX is offline WrineX  Netherlands
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Thx Kevinkr some good explanation !


For this panel excursion limit was 1.6mm (one way) almost as much as a full blown SMGA. with 1,8mm (3.6 pp). just what i had around in terms of spacers

i intend to make big ones just like the SMGA , but i noticed this artifact every time for each panel i made. so i wondered what it was.

trick is go big and with allot of tension on the foil so it seems, the lower it is the more crap it looks.
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Old 15th May 2017, 06:33 PM   #7
lowmass is offline lowmass  United States
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I suspect you are seeing distortions related to strong resonant activity. Flat diaphragms do not behave all that well below about 700 hz and often between 200 and 400 can be the worst. This issue is even more of a problem with small diaphragms and usually to taim them you will see damping added by solid attachment of some kind of cloth to magnet structure to create a resistance to air flow. I believe bolender does this. As well the Wisdom panels do too. Every planer I have ever built needed this resistance to control the very high Q resonance issues below about 300 hz and to tune the main resonance at lower freqs.

Speakers like the Apogees and Magneplaners do the resistance by using no cloth but making the perforated steel and magnet structure closed up enough to give the effect.

I never measured the distortion related to these resonances so take above with a grain of salt, BUT I have done tests with identical diaphragms but with different magnet structures, some very closed up, others very open. Using a stroboscope you could see huge standing wave issues on the ones with very open magnet structure. The more closed in ones showed much less activity AND they sounded fast and clean by compare.

Last edited by lowmass; 15th May 2017 at 06:37 PM.
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Old 15th May 2017, 07:08 PM   #8
WrineX is offline WrineX  Netherlands
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Pulling a SLooooowwwwmotion on the planar

playing sweep from 25 - 600 @ 240 frames a sec exported at 24

i wanted to see where it all goes wrong not sure if i managed but still is funny

resonance of the panel is at 45Hz distortion is High there and goes down to 200hz

on the left is the counter of frequency from the sweep i am not so sure how accurate that is. so i might do a manual once to see what really happens (poor neighbors)

https://youtu.be/_rb5mvXJVjg
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Old 15th May 2017, 07:13 PM   #9
WrineX is offline WrineX  Netherlands
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Quote:
Originally Posted by lowmass View Post
I suspect you are seeing distortions related to strong resonant activity. Flat diaphragms do not behave all that well below about 700 hz and often between 200 and 400 can be the worst. This issue is even more of a problem with small diaphragms and usually to taim them you will see damping added by solid attachment of some kind of cloth to magnet structure to create a resistance to air flow. I believe bolender does this. As well the Wisdom panels do too. Every planer I have ever built needed this resistance to control the very high Q resonance issues below about 300 hz and to tune the main resonance at lower freqs.

Speakers like the Apogees and Magneplaners do the resistance by using no cloth but making the perforated steel and magnet structure closed up enough to give the effect.

I never measured the distortion related to these resonances so take above with a grain of salt, BUT I have done tests with identical diaphragms but with different magnet structures, some very closed up, others very open. Using a stroboscope you could see huge standing wave issues on the ones with very open magnet structure. The more closed in ones showed much less activity AND they sounded fast and clean by compare.
funy you posted this and i posted a slow motion not the best quality ill try to make a better one with more light and higher resolution.

but yeah you are right about the Q and not pistonic behaviour. but one weird thing is the closed one has higher distortion as the more open one. ill try to do this over somewhere this weak maybe we can see more stuff


Thanks for thinking with me Lowmass.
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Old 15th May 2017, 07:22 PM   #10
WrineX is offline WrineX  Netherlands
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And no 50 hz light bulb if posdible
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