So I don't quite understand yours either, my bad. I refer to the problematic resonance Acoustat installed the felt pads for on the backs of the panels.
My English isn't too bright... I sent a reply to Havun's question about fundamental resonance frequency shift at doubled membrane.
A little later I will test and show here "my" version of the absorption of parasitic bass panel resonances, I called it "floating volume". The essence is the acoustic load of the isobaric package. But not a hard load, but a "floating" one. But I won't let the cart go before the horse, I'll test it first 
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And why did you decide that Acoustat installed felt behind the panel for this purpose? Have you taken measurements with and without felt and found a clear spurious resonance that stands out from the rest?
It is well known it's purpose is to tame this excess resonance. Do you know something different? If so, please present it. This issue is discussed in numerous threads here. I have not taken measurements but others have. Also other materials have been/are being used as substitutes such as mesh and different types/thicknesses of felt.
I wrote above here that if you place any body behind the film, then it will cause reflection of certain frequencies and increase their return.
Yes that's a given. It's obviously a balancing act. Nonetheless you seem to be skirting the operative. See post #304, #301 which is in response to neodymium. You appear to be in agreement and taking me to task at the same time. I'll gladly tell you what I know but it isn't much. I can definitely hear the resonance without the felt pads as I mentioned the "tubby sound" without it, so It would appear to be upper bass/lower mid range, I'm guessing? I'm definitely here to learn as my set up is that in question here. If you refer to my thread I've linked to, I mention a vast improvement. However I love experimenting and intend to try what's been mentioned here, ie: hermetically sealing the panels which I have not done. With this approach I will triple the panels, so total 9 per speaker, hermetically sealed, with 4 driven by one amp and 5 by the other. I've read the Direct Drive Servo amps can handle up to 5 panels. I will then discover whether damping is required again on the back side.
What do you think?
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Hi,
the second ´parallel mounted´ membrane dosen´t obviate the base resonance at all.
It just changes the frequency and Q of the base resonance.
This is due to the changes of mass, driving force and compliance.
Damping isn´t part of it ... hence the resonance can´t simply ´disappear´.
As I mentined earlier - and Bolserst seems to have experimented on this issue also- it requires a specialized drive of the second membrane in that it should be driven in a ´partial antiphase mode´ over a small frequency range around the base resonance of the membranes and in phase over the rest of the audio range.
jauu
Calvin
the second ´parallel mounted´ membrane dosen´t obviate the base resonance at all.
It just changes the frequency and Q of the base resonance.
This is due to the changes of mass, driving force and compliance.
Damping isn´t part of it ... hence the resonance can´t simply ´disappear´.
As I mentined earlier - and Bolserst seems to have experimented on this issue also- it requires a specialized drive of the second membrane in that it should be driven in a ´partial antiphase mode´ over a small frequency range around the base resonance of the membranes and in phase over the rest of the audio range.
jauu
Calvin
Of course, the second, third, fourth ... membranes do not eliminate the main resonance of the whole package of membranes, they only increase the frequency of this parasitic resonance. In my case with six membranes, the main resonance is about 500Hz, although each individually 50/50 cm membrane made of 15 micron thick film has a main resonance in the region of 15-20Hz. Also, no damping materials are able to remove this resonance, which, moreover, can coincide with the harmonics from the resonance of the room and amplify even more. Therefore, this is a complex problem that also requires acoustic processing of the room as well as equalization. Therefore, if someone thinks that it is enough to put a piece of felt behind the column and the problem is solved ... then he is greatly mistaken.
Anyone explain "isobarik ELS panels"? I assume this is stacking 2 push pull panels, one behind the other so the effective area is the same as one.
What's the advantage?
I see many disadvantages, the most obvious & pertinant is reflections from the other panel which will be similar but worse than that off dustcovers as there are stators as well.
What's the advantage?
I see many disadvantages, the most obvious & pertinant is reflections from the other panel which will be similar but worse than that off dustcovers as there are stators as well.
At two panels:
First, and most important advantage is the doubled driving force at same signal voltage. The voltage rated sensitivity, and force limited output is 6dB higher, but reactive current is double. The "power" rated sensitivity will be 3dB higher.
This is not in the entire range, but mainly in the bass. High frequencies from the isobaric will suffer, they will be muddy and with a decline to the highest. Martin Logan made 10-15 years ago one model of a full-range electrostatic, they used isobaric in the bass register, but in the middle and high range they left their traditional transducer. Something I can’t find this model on the Internet, everything is there except for it.
Of course, it's true. At high frequency isn't too good the moving mass multiplication and a lot of reflection. I tried it with Final panels.
This is interesting. I found the opposite. Perhaps I'm being fooled by the mid range? Or could it be because mine are not true isobaric with a wall of felt in between? In that case it could be that using the right grade of mesh between panels will compensate the added HF rolloff due to the mesh's reflection, but without an interaction with adjacent panels?
Hi,
the double membrane principle was described in a Sony patent from 1983.
It claimed a up to +6dB increase in SPL between fo and ka<1.4.
Their proto utilized a 3-stator-2-diaphragm setup, similar to what ML later implemented in the CLX bass panel.
As mentioned earlier, this setup can only be used to a certain frequency limit due to the increasing phase shift of early high frequency reflections between the membranes.
The resulting phase shift and its influence on the amplitude response can easily be calculated and measured.
The higher in frequency the working range of the panel shall be the smaller must be the inter-diaphragm distance.
If You can keep the distance below a few mm (even better <1mm) there will be no audible drawback.
I implemented such a system into my ESL Mk2 and indeed could measure a up to 6dB increase in SPL just where it counted, with no ripples at the upper audio range.
While a single diaphragm panel of the same size clearly exhibited a drop in SPL due to the acoustic short the double diaphragm panel measured linear down to ~200Hz @4m distance.
The increase in SPL as well as having not to eq the panel (besides the proverbial base resonance notch) resulted in a very considerable increase in maximum output level and sound quality.
jauu
Calvin
the double membrane principle was described in a Sony patent from 1983.
It claimed a up to +6dB increase in SPL between fo and ka<1.4.
Their proto utilized a 3-stator-2-diaphragm setup, similar to what ML later implemented in the CLX bass panel.
As mentioned earlier, this setup can only be used to a certain frequency limit due to the increasing phase shift of early high frequency reflections between the membranes.
The resulting phase shift and its influence on the amplitude response can easily be calculated and measured.
The higher in frequency the working range of the panel shall be the smaller must be the inter-diaphragm distance.
If You can keep the distance below a few mm (even better <1mm) there will be no audible drawback.
I implemented such a system into my ESL Mk2 and indeed could measure a up to 6dB increase in SPL just where it counted, with no ripples at the upper audio range.
While a single diaphragm panel of the same size clearly exhibited a drop in SPL due to the acoustic short the double diaphragm panel measured linear down to ~200Hz @4m distance.
The increase in SPL as well as having not to eq the panel (besides the proverbial base resonance notch) resulted in a very considerable increase in maximum output level and sound quality.
jauu
Calvin