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Old 1st October 2018, 04:27 AM   #11
kazap is offline kazap  Australia
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Quote:
Originally Posted by bolserst View Post
Bob is not correct about the null.......

Thanks for the excellent info. I'm not sure I entirely follow your extrapolations from the theory?



It seems to me that for an ESL membrane to be excited by air there needs to be a pressure differential. A pressure differential can obviously created by air movement creating a negative pressure via a differential laminar flow, or positive pressure with eddies. However the membrane movement force created by velocity of air flow across a panel is surely directly related to pressure effects. It seems to me your positing that air flow can move or excite the diaphragm in a pressure null. That appears to be logically inconsistent over a diaphragm? It seems the theoretical basis is modelling free air effects rather then what will occur over a diaphragm?


Practically the design issue with the OB driver adjacent to the ESL membrane seems to be that the membrane becomes part of the dipole baffle with clear pressure and velocity effects impinged on the mylar. However most of the pressure and velocity effects can intuitively be expected to occur close to the driver down low and rapidly dissipate with distance leaving most of the ESL membrane relatively unaffected. This would explain the sound quality.


Still it appears to me the ideal design would have a slot between the OB bass driver and the ESL to allow for unimpeded air flow under the ESL, allowing the dipole pressure null to fully envelope the ESL panel?
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Old 3rd October 2018, 04:52 AM   #12
bolserst is offline bolserst  United States
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Originally Posted by kazap View Post
It seems to me that for an ESL membrane to be excited by air there needs to be a pressure differential.
Being excited by or sensitive to pressure differential, pressure gradient, or velocity are all ways of saying the same thing. Like ribbon microphones, ESL membranes are velocity sensitive. The velocity field and pressure field are related to each other. If you are mathematically inclined, most acoustic texts(ie Beranek, Olson, etc) provide examples of how to calculate one from the other for a variety of source types. Keep in mind that the pressure at a location in space is a scalar quantity whereas the particle velocity has both magnitude and direction.

Quote:
It seems to me your positing that air flow can move or excite the diaphragm in a pressure null. That appears to be logically inconsistent over a diaphragm?
That is exactly it, but think air particle vibration rather than flow. Take a pressure sensitive microphone and move it forward and backward past a dipole woofer and you will easily find a null or minimum pressure point(your ear is pressure sensitive). Do the same thing with a velocity sensitive microphone (oriented parallel to woofer piston) and you will find that there is no null as you sweep past the woofer. In fact the velocity is slightly higher at the point of the pressure null than at positions just in front or behind it. Since ESL membranes are velocity sensitive, as odd as it may sound there will NOT be a null in the excitation by the dipole woofer even with the ESL positioned at the location of the pressure null. Since the velocity vector is parallel to the woofer motion, rotating the microphone(or ESL) 90 degrees with respect to the woofer while at the null location will effectively remove the acoustic excitation from a velocity sensitive device.

If you are interested in measuring the velocity field but don’t have a ribbon microphone, a small magnetic planar like the BG Neo3 or Neo8 can be used as a microphone in a pinch. You can also use the ESL itself as a microphone to measure exactly what is happening. Peter Walker first suggested this in his AES paper and after a little prodding from golfnut I can say it works rather well for quantifying what is going on at low frequencies. On a related note, standing waves in pipes produce maximum particle velocity at pressure nulls/nodes.

Quote:
It seems the theoretical basis is modelling free air effects rather then what will occur over a diaphragm? Practically the design issue with the OB driver adjacent to the ESL membrane seems to be that the membrane becomes part of the dipole baffle with clear pressure and velocity effects impinged on the mylar.
If you can model a woofer’s velocity field covering the location of the ESL diaphragm without the ESL in place, you will find the velocity imparted to the ESL diaphragm is nearly identical to what was calculated. Borrowing words from Peter Walker, the ESL diaphragm is for all practical purpose acoustically transparent. Peter Walker demonstrated this during AES talk by holding up a stretched diaphragm in front of his face while continuing to talk. For low frequencies, you can demonstrate it to yourself by testing a small dipole woofer in 3 ways:

1) with minimal baffling
2) with a large flat wooden baffle
3) with a thin frame the size of the baffle in 2) but with a diaphragm stretched on it.

You will find that 3) will measure nearly identically to 1) rather than having the increased low end response from 2).
The only differences will show up at the resonance frequency of the diaphragm.

Quote:
…most of the pressure and velocity effects can intuitively be expected to occur close to the driver down low and rapidly dissipate with distance leaving most of the ESL membrane relatively unaffected…
Correct. As mentioned in the previous post, for low frequencies the velocity field falls as the cube of the distance. So, double distance and velocity falls by factor of 8. Distance is your friend when trying to minimize acoustic excitation of membrane.

Quote:
...it appears to me the ideal design would have a slot between the OB bass driver and the ESL to allow for unimpeded air flow under the ESL, allowing the dipole pressure null to fully envelope the ESL panel?
The gap will be more helpful in a line array situation with a column of dipole woofers adjacent to the ESL panel than with a lone woofer below the panel. Even then, its effect is much smaller than anticipated…not much more than just the added distance would give on its own. This surprised me the first time i tried it. But, the gap is also beneficial for reduction of mechanical vibration which may be equally important. Adding rearward baffle extensions on either(or both) sides of the gap to increase the path length will reduce the particle velocity local to the ESL diaphragm.
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Old 3rd October 2018, 09:42 AM   #13
bentoronto is offline bentoronto  Canada
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Quote:
Originally Posted by bolserst View Post
...velocity imparted to the ESL diaphragm is nearly identical to what was calculated...

... On a related note, standing waves in pipes produce maximum particle velocity at pressure nulls/nodes.
Appreciate the analysis, as always, not that I am able to understand it fully.

But back to the question of diaphragm movement, it seems the diaphragm isn't getting too much impetus to move after all. But for whatever imbalance of pressure it is experiencing from the woofer, an ESL panel seems transparent because it is moving the same as the air would be in the absence of the film. Not standing still. So wouldn't the motion be undesirable, imparting Doppler-like and other distortions?

About the second quote, is that right?

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Old 14th October 2018, 10:03 PM   #14
bolserst is offline bolserst  United States
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Quote:
Originally Posted by bentoronto View Post
… an ESL panel seems transparent because it is moving the same as the air would be in the absence of the film... So wouldn't the motion be undesirable, imparting Doppler-like and other distortions?
Yes. Any motion at LF imparted to the diaphragm by an adjacent dynamic dipole will produce IM sidebands around HF tones. This is no different than when a full range ESL is generating the LF diaphragm motion itself. But, with most hybrid ESLs, we are talking about 1/16” of motion or less so the Doppler related distortion tends to be a non-problem since sideband tones generated from that small motion are far down in level.

Quote:
About the second quote, is that right?
Yes. A couple websites on the topic.
Standing Sound Waves (Longitudinal Standing Waves)
http://www.physics.mcgill.ca/~guymoo.../lecture20.pdf

The Animation provided at the first link is rather nice. Easy to see pressure nodes at maximum motion points and vice versa.
Note also that the stationary end of the tube on the right is a reminder of why placing foam on a wall(where there is little air particle motion) does little to damp room modes. You need thick foam or foam spaced away from the wall to add some acoustic resistance where it will do some good.
Click the image to open in full size.

Last edited by bolserst; 14th October 2018 at 10:12 PM.
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