QuAD ESL 57 Question

[dude007]

- Was wondering what tools people are using for measuring diaphram tension of a Quad panel.

Is the tension different between the tweeter and base panels?

[StanJ]

Hi,

your question is certainly interesting to everyone who refurbishes or builds electrostatic panels. The tension itself is not a value of interest. What counts is the resonance frequency, that surely depends on several factors such as material and material thickness, mass, dimensions and the foil tension after the foil is glued to its frame and the weights are released.

The original Quad stretching jig uses steel weights which total to 45kg for the mid/high-panel and some 60 kg for the bass-panels. I have seen the original jig, which is now in Germany, and if I recollect correctly they have 30 weights. So, each weight is about 1,5kg for the mid/high and 2kg for the bass. If you apply these values you will be very close to the original way Quad is tensioning their panels. This is only valid if the tensioned area is in the vicinity of some 40 by 80 cm. The panel itself is apparently smaller. So if you intend to refurbish Quad panels or build new ones with the same dimensions this should put you very close to Quads specifications.

Coming to your question, Quad uses an indirect method to achieve, respectively to monitor the desired resonance. What they still do today, is to position the panel under test horizontal above a bass speaker ( in that case they use KEF B139). Again above the panel sits a microphone that measures the sound pressure coming through the panel. To be clear at this point the panel is not connected, it is the B139 that is working. Then they look for the maximum voltage at the microphone by running through the lower frequencies of the audio spectrum to find the resonance of the pre-finished panel. They apply this method prior to coating, assembly etc. By this they are able to decide weather the panels resonance is within the accepted tolerances. This also allows to match panels, if desired.

If you still would like to measure the tension, there are some different methods, most of them are not suitable for the hobbyist. What I did in the beginning was to lay down the one side of the panel that holds the mylar on a workbench and to measure the bending of the film by applying a constant weight (200g) in the very center of the film. To do so I used a micrometer. This for sure not a very scientific approach, but it works and the measured values correspond quite well to the resonance of the finished panel.

I hope this helps a bit and I would appreciate if someone could comment on the relation between the applied stretching weights and the achieved resonance – if there is a simple relation at all.

[oshifis]

I have no experience with replacing the diaphragm, but I replaced the dust cover of my ESL-57s a while ago. I followed this method: I layed down the dust cover foil on a flat surface (kitchen desk :-)). Then I applied glue on the wooden frame and pushed it down to the foil. After assembling together and sealed around the edge, I went through the surface of the dust cover with a hair dryer. I took care not to stretch too much, because I wanted to keep the tension (and the resonance) as low as possible. Theoretically, tension of the dust cover is necessary only to avoid rattle.
Is there any scientific explanation why is a relatively high tension used at the diaphragm? I guess even a relatively low tension can prevent the diaphragm from attracted by one of the stators.

[Fabien Lefebvre]

StanJ,

Concerning your remark:
<<I hope this helps a bit and I would appreciate if someone could comment on the relation between the applied stretching weights and the achieved resonance – if there is a simple relation at all.>>

This helps me at least ! This information is of great interest.
I am currently trying to make the link between the weights applied and the resonance frequency(s).
I already have a computing sheet but I need some information or confirmation:
* the tensioned area is always 80*40 whatever the panel ? (bass or treble)
* this 80*40cm area is flat during the process ? I mean the attachement between weights and PET sheet is in the horizontal plan. Or saying it differently the weights are evenly distributed on the full (80+40)*2 cm ?
* what is the density of the PET sheets ? I assume around 900kg/m3. Same for bass and treble ? Same question for the coating itself but I assume this part is neglectable
* do you have some estimation of the expected resonance frequencies ? I heard "supersonic" for treble meaning above 20kHz but this is achievable for horizontal direction and not vertical.
* finally I will need the effective dimension of the moving parts (=the coated part) but this will be easy to find

Thank you

[StanJ]

Salut,

it is an excellent idea to develop an algorithm that describes the relation between the applied stretching force and the corresponding resonance frequency. As far as know this has never been done before and published.

I am happy to answer your questions as follows:

1) Concerning the Quad stretching jig: 40 by 80 cm² is correct for both panels.


2) The sheets are flat while tensioned and glued to the frames


3) The specific density is the same for both panels. The value is –as far as I remember - 1,4 g/cm³ (check Mylar by Dupont). However Quad uses or at least used 6 ym for the mid/treble unit and 12 ym for the bass unit.

For the coating you may apply an extra 5-10 %.


4) The resonance of the Quad 57 bass panel is between 50 to 70 Hz.

Resonance supersonic? : You really mean that?: ...Impossible!


5) Dimensions: google!


Hope this helps a bit – and good luck,

StanJ

PS: I propose you open up a new “non-Quad-related” thread that focuses only on your subject. I am sure, if you do so, you will get plenty of help and appreciation.

[bear]

Originally posted by oshifis
<snip>
Is there any scientific explanation why is a relatively high tension used at the diaphragm? I guess even a relatively low tension can prevent the diaphragm from attracted by one of the stators.

Actually the tension has to be sufficient to keep the diaphragm from being attracted to the stators... then there is the relationship between the excursion at resonance vs. power input vs. overall frequency response. In the case of the Quad we just got a very good run down of where that optimal tension point is for the Quads... too high a tension and obviously the resonant point is too high as well... a balancing act between the factors...

_-_-bear

[Fabien Lefebvre]

First results of computation: first of all, let's make the difference
between intrinsic resonance(s) of the diaphragm and resonance
when stimulated with a source and considering coupling with air.

Using your figures, the stretched unbonded panel (80*40cm) will
have a main mode of resonance at 207Hz for treble and 169Hz for
bass. Meaning that hitting the diaphragm once it is set on the
stretching jig will give this sound. A nice way to check that it is
correctly stretched. This is by the way how it is done here:
http://loudspeaker-repair-service.re...rbish_quad.htm
Very interesting site wrt stretching and annealing !
These figures come from the classical computing of the resonance
modes of a square drum.

Once bonded, the diaphragm will give a different sound if
stimulated. Because the moving part is smaller. That was the
purpose of my question: the treble and bass panels are in fact
divided in three moving parts. But the exact dimension is not widely
available thus I cannot tell what will be the sound of the diaphragm
once bonded on the stator.

Next step will be to compute resonance frequency when stimulated
by an electric source and radiating in the air: these are not the
same frequencies as the ones given above because now the
"radiation resistance" will be implied.

Hoping I am clear !

[frank ziel]

Hi,
not only this will happen,
...if the diaphragm ist polarizied and between the stators,
it will attempt to attract the two stators,...
...it acts as if it has a negativ compliance...
...and will move the resonce frequency down...
(~The Art of Sound Reproducktion from John Watkinson)
Hope it helps
Regards,
Frank

[frank ziel]

If you want to read:

http://books.google.de/books?id=01u_...um=2#PPA182,M1

[Fabien Lefebvre]

I tried to add the effect of radiation through the air.
This brings an additional mass (which lowers resonance) but an additional spring effect (which highers resonance).
Note that all input figures are supposed to be verified except the size of the moving part (10*70cm below).

Any comment welcome !

Bass panel:

P0 101325
T0 293,15
R 8,32
Gamma 1,4

Weight applied with jig 60 kg 60 kg
Linear force 245,25 N/m

Stretched diaphragm Width 40 cm 0,4 m
Height 80 cm 0,8 m
Thickness 12 um 0,000012 m
Density 1390 kg/m3 1390 kg/m3
Mass 5,3376 g 0,0053376 kg
k (spring) 6051,301198
Resonance frequency 169,4616788 Hz
Equivalent radiating surface 0,129691115 m2
Additional mass due to air 15,81035605 g 0,015810356 kg
Additional k due to air 99121,6412
Resonance frequency with radiation effect 354,9261059 Hz

Moving part Width 10 cm 0,1 m
Height 70 cm 0,7 m
Thickness 12 um 0,000012 m
Density 1390 kg/m3 1390 kg/m3
Mass 1,1676 g 0,0011676 kg
k (spring) 17289,432
Resonance frequency 612,439864 Hz
Equivalent radiating surface 0,028369931 m2
Additional mass due to air 1,617572455 g 0,001617572 kg
Additional k due to air 4743,125409
Resonance frequency with radiation effect 447,6375819 Hz


Celerity 121,2569066 m/s