Hi
Sorry, I think we are talking at cross purposes. There are two things happening here (i) the membrane resonance and (ii) the frame response to the resonance.
My point was that the frame mass and stiffness (beyond a certain point) has no practical effect on the membrane resonance, since I understood that to be the question.
That is not the same as saying that there is no undesirable frame movement in response to the resonance - different question.
Note too, it is not the mass of the membrane itself (a few microns thick) that provides the mass that's responsible for the resonance, its the air sloshing around the side of the panel at low frequencies - like the air trapped under a bed sheet when you shake it - that appears to give the membrane mass. Keep in mind that a cubic metre of air has a mass of about 1.25 kg - way higher than the mass of the membrane.
The membrane resonance is the same as the resonance that occurs with drums. You can tune drums by tightening the membrane, or by making the drum frame deeper so that more air is trapped - as with different sized kettle drums. Extending the frame size, in much the same way as a conventional open baffle driver, will also alter the air mass responsible for the resonance.
As I indicated earlier in this thread, the stators and frame will move in a bunch of different ways:
> recoil - linear movement in direct opposition to the membrane
> the charge on the stators pulls the two stators close together causing square law distortion (second harmonic with a single tone),
> and almost certainly there are more complicated motions of the frames and stators - like standing waves along the lengths of the frames etc, and probably some ringing - the frames will have their own resonances.
Some of these frame movements will cause audible effects, others not. The linear recoil type movements probably have very little effect on the sound. The square law stuff from the stator attraction is probably horrible. The frames themselves will radiate sound too so standing wave effects in the frames can be expected to radiate unwanted sound.
Making heavier, stiffer, frames perhaps with damping, will help. The frames on my ESLs are rectangular aluminium box section - often thought of filling them with lead shot and wax.
Sorry, I think we are talking at cross purposes. There are two things happening here (i) the membrane resonance and (ii) the frame response to the resonance.
My point was that the frame mass and stiffness (beyond a certain point) has no practical effect on the membrane resonance, since I understood that to be the question.
That is not the same as saying that there is no undesirable frame movement in response to the resonance - different question.
Note too, it is not the mass of the membrane itself (a few microns thick) that provides the mass that's responsible for the resonance, its the air sloshing around the side of the panel at low frequencies - like the air trapped under a bed sheet when you shake it - that appears to give the membrane mass. Keep in mind that a cubic metre of air has a mass of about 1.25 kg - way higher than the mass of the membrane.
The membrane resonance is the same as the resonance that occurs with drums. You can tune drums by tightening the membrane, or by making the drum frame deeper so that more air is trapped - as with different sized kettle drums. Extending the frame size, in much the same way as a conventional open baffle driver, will also alter the air mass responsible for the resonance.
As I indicated earlier in this thread, the stators and frame will move in a bunch of different ways:
> recoil - linear movement in direct opposition to the membrane
> the charge on the stators pulls the two stators close together causing square law distortion (second harmonic with a single tone),
> and almost certainly there are more complicated motions of the frames and stators - like standing waves along the lengths of the frames etc, and probably some ringing - the frames will have their own resonances.
Some of these frame movements will cause audible effects, others not. The linear recoil type movements probably have very little effect on the sound. The square law stuff from the stator attraction is probably horrible. The frames themselves will radiate sound too so standing wave effects in the frames can be expected to radiate unwanted sound.
Making heavier, stiffer, frames perhaps with damping, will help. The frames on my ESLs are rectangular aluminium box section - often thought of filling them with lead shot and wax.
The quads have dust covers of thin plastic that are stretched on main frames on both front and back of the speaker. When replaced you carefully heat shrink the material til taught. You can run it without but the quads are dust magnets so don't. They are pretty transparent and only cause problems when they aren't stretched properly. Then they rattle.
In my opinion the biggest fault with the Quads is the very floppy frame the 4 panels are mounted on. They are mounted on pretty thin angle aluminum. They are held in place at the top and the bottom but are free in the middle. You can easily displace the panels in the middle by a 1/4" with finger pressure. They resonate around 200 hz. I add bicycle inner tubes filled with sand between the inner frame with the panels and the outer frames, top to bottom. They both add mass and stiffness to the panel and absorb any reflected waves. A well recorded orchestra bass drum can kick you in the chest after the mod.
So in answer to "how does Quad control resonances?" The frames don't. However there is a fine mesh cloth on the rear stators that does provide air damping for the diaphragm. I would guess that Peter Walker arrived at the right type empirically measuring the resonances of various types of damping.
The other thing unique to the Quads is the delay line that feeds concentric rings such that the speaker creates a virtual point source 30 cm behind the speaker. The center of the panels get the leading edge of the wave front and it spreads out from there. There are a lot of photos and details online about quads.
I recently had an Acoustat panel in my shop that was constructed of ceiling light grid panel. I never got around to using it. It was for a project of redesigning the Acoustat tube amp.
I love ESL63s with Futtermans. It doesn't get much better. My Quads are a diagnostic tool. I can hear what the electronics are doing very clearly with these. Transparent. They seem to disappear.
Have you measured the resonant frequencies that you are trying to damp? What are they?
Good luck.
In my opinion the biggest fault with the Quads is the very floppy frame the 4 panels are mounted on. They are mounted on pretty thin angle aluminum. They are held in place at the top and the bottom but are free in the middle. You can easily displace the panels in the middle by a 1/4" with finger pressure. They resonate around 200 hz. I add bicycle inner tubes filled with sand between the inner frame with the panels and the outer frames, top to bottom. They both add mass and stiffness to the panel and absorb any reflected waves. A well recorded orchestra bass drum can kick you in the chest after the mod.
So in answer to "how does Quad control resonances?" The frames don't. However there is a fine mesh cloth on the rear stators that does provide air damping for the diaphragm. I would guess that Peter Walker arrived at the right type empirically measuring the resonances of various types of damping.
The other thing unique to the Quads is the delay line that feeds concentric rings such that the speaker creates a virtual point source 30 cm behind the speaker. The center of the panels get the leading edge of the wave front and it spreads out from there. There are a lot of photos and details online about quads.
I recently had an Acoustat panel in my shop that was constructed of ceiling light grid panel. I never got around to using it. It was for a project of redesigning the Acoustat tube amp.
I love ESL63s with Futtermans. It doesn't get much better. My Quads are a diagnostic tool. I can hear what the electronics are doing very clearly with these. Transparent. They seem to disappear.
Have you measured the resonant frequencies that you are trying to damp? What are they?
Good luck.
To Golfnut - I didn't see your note until I posted mine. We are in agreement. My understanding of the stator frames you are talking of have very little surface area compared to Quads and are probably much stiffer so resonances might have a lesser effect.
Quads have bass resonance problems around 200 hz but when fully damped everything clears up because the panels aren't moving and phase modulating all the rest of the audio band.
I think if you put some additional weight and damped supports on the panel grids there would be a difference. Probably for the better. But that's a guess from my kitchen table very far away.
That's my theory and I'm sticking to it.
I went to the NYC Hifi show when I was around 12 when my aunt, who was looking for something to do with me saw an ad for it in the Times. The 2 things that stood out for me were the Crown recorders, the Acoustats in several rooms and everyone playing Herb Alpert and the Tijuana Brass albums. Spanish Flea. The Acoustats were the best speakers there. Just amazing. Still the world of tubes!
Quads have bass resonance problems around 200 hz but when fully damped everything clears up because the panels aren't moving and phase modulating all the rest of the audio band.
I think if you put some additional weight and damped supports on the panel grids there would be a difference. Probably for the better. But that's a guess from my kitchen table very far away.
That's my theory and I'm sticking to it.
I went to the NYC Hifi show when I was around 12 when my aunt, who was looking for something to do with me saw an ad for it in the Times. The 2 things that stood out for me were the Crown recorders, the Acoustats in several rooms and everyone playing Herb Alpert and the Tijuana Brass albums. Spanish Flea. The Acoustats were the best speakers there. Just amazing. Still the world of tubes!
So for the Acoustat panels, would the solution not be to simply stretch non driven membranes across both sides of the louver?
A buddy of mine had a pair of KLH 'stats. Sounded good but bass was thin, little more than perfunctory popping. Well, he sawed holes in the wall as big as the stats were and recessed the stats into the wall, completely isolating the rear wave from the living room. Rear wave in its entirety went into the bedroom. Bass in both rooms was exceptionally good!