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Old 8th August 2008, 09:28 PM   #11
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Sensitivity will depend on equalisation. The figure of 95db is a rough calculation at 1kHz and asuming voltage driver rather than current drive. In practice you are likely to get a little less.

The spacers will provide mechanical segmentation and provide enough support for the film not to collapse into the stators (stability). For the tweeter part you'd also need electrical segmentation, e.g. that small strip of the stator must be electrically isolated from the larger stator. With a series resistor in the audio drive you can now alter the frequency response (introduce roll off). To do this the film can be just one piece, no need for different tensions.

For the spacers I would prefer acrylic strips of some sort. Foam tape has a limited lifespan and might not give you the structural integrity you're looking for.

Some manufacturers vary the dimensions of the cells to spread the resonance over a larger frequency range. For this purpose you can also work with dots of silicone to provide mechanical segmentation (e.g. break up resonance modes).

About film tension, your primary concern would be to get a stable diaphragm. This depends on polarisation voltage and stator spacing. Normally you would also have to take into account the low frequency limit, but this is not a problem as your panel will only be playing from 400Hz upward.

I could not make up from your post what exactly it is that you were asking, but I hope this answers your questions.
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Old 9th August 2008, 08:00 PM   #12
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The tape seems like a good idea in that it offers more flexibility in expermenting with D/S spacing, just peal the tape off. However, I worry about the longevity of it as oppose to acrylic. So I decided to use the tape as a means to experment, once could response has been acheived I will replace it with acrylic of same size, what do you guys think of my above stator prep?
Bryan
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Old 10th August 2008, 09:48 AM   #13
Calvin is online now Calvin  Germany
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Hi,

95dB@2.83V@1m is way out of the physical capabilities! 85dB is a already good value! But probabely AJ meant 95dB@2.83V@4m. When executed as strip source (line source) the SPL increases on the first 3-4m distance with increasing distance. You can reach the figure of 95dB, but only with a very elaborated metal sheet stator panel at a distance of 3-4m.

A good panel nowadays takes care of the distribution character. Imaging will still be excellent, but You´re not restricted to sit still like a mummy with Your ears fixed to a tiny micro spot! Changes of up to 20dB in amplitude within 1° of horizontal deviation are not a sign of good design!
A flat panel wo segmentation (i.e. flat metal stators) must be considered as inferior. Besides the very tight restrictions to the listening position the mechanical stability is low. You´d have to stabilize the panel somehow, but with the sheet´s advantage of thinness gone, there are superior alternatives like wire.
So if You want to stay with a flat panel, I recommend to change to wire stators with electrical segmentation. Easy to build, quite flexible with regard to eq´ing and easier to design ´safe´.
When using metal sheet stators the panel should be curved. Besides easing the sweetspot problem this considerably stabilizes the panel so that no additional bracing is needed. This gives You an easy and the thinnest possible solution with fine optics.
You only have to waste a thought or two about how to tension the membrane and which membrane material to use.

To take for building tolerances into account, the d/s of the ´backside´ should be slightly larger than the one to the ´frontside´. 1/10-2/10mm are fine (when the sheets are precisely curved and the insulative laquering is done 1st class). 3M offers the right tapes for this.
They are tightly tolerated in thickness. Acrylic spacers are not necessarily tolerated tighter in thickness, not to mention the tolerances of the ´handmade´ glue joints.
Ageing is no problem with the 3M brand and the ´softness´ helps in dampening membrane resonances and reduces ´rattle´. Actually the tapes completely miss out on the danger of ´cutting´ the membrane which could happen with stiff spacers and hard drying glues. The tapes are a clean and quick to work with solution and give a instantaneous strong bond, but ask for a calm hand and aren´t cheap.

You don´t neccessarily need to dremil each and every of the ~40.000 holes Sanding the rough side of the sheet (without pressure!!) and etching will do the job.
Its more important which laquer You use to insulate the sheets.
PVC, Nylon, PU and Acrylic are fine with regard to electrical parameters. Ask for a laquer that gives a nice even coating at roundings and edges (which rules out most water-based and one-component laquers). Be very sensitive with coloured laquers, especially grey and black ones, since those are often conductive under HV-conditions, eg. Krylon). If You use a conductive laquer, use it as a thin first or second layer and overcoat with several thick clear coats. A solvent based PU-Laquer used for coating boats or staircases is an excellent choice with regard to surface quality (optics), electrical parameters longevity and safety. When wet spray-coating expect to perform at least 8 runs on every side of the sheet. The best solution is to have the sheets thickly powder coated with a white Nylon powder, one thin layer of colour and a thick wet high gloss PU-coating. This yields ~0.4-0.5mm thick insulation that allows for voltage levels very close to field saturation (highest efficiency) and perfectly safe stators (up to 3-4 times the highest working voltages).

You use only one tension value for the complete panel. The spacer distance may vary slightly, though the claimed effect of ´resonance distribution´ can hardly be confirmed in praxis unless the distances vary widely and this may introduce other problems.

jauu
Calvin
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Old 10th August 2008, 04:02 PM   #14
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Hi Calvin,

I did a numerical solution of the generalised (for near field and off axis) Walker integral to calculate the theoretic SPL. Like I said, in pratice it's probably going to be a bit lower.

When executed as strip source (line source) the SPL increases on the first 3-4m distance with increasing distance.

I heard about this once before on a forum (maybe by you?) but never could find an explanation for this effect in the 'official' theoretic works. Nor did I ever measure (or notice) this effect on a real panel. Could you clarify the conditions that are required for this effect to take place? Do you know of a theory that explains it?

regards,
Arend-Jan
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Old 10th August 2008, 07:19 PM   #15
Calvin is online now Calvin  Germany
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Hi,

the effect of having a SPLmax around 3-4m (it depends on the length of the panel, but with usual sizes of 1m-1.5m the max. is at ~3-4m)
can be easily verified not only by listening but by measurement too.
The difference between 1m and 4m can reach up to 10dB.
I measured this effect myself and it was also measured at Aachen Unversity in their anechoic room.

jauu
Calvin
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Old 10th August 2008, 07:28 PM   #16
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Calvin,

I do like that I don’t glue using the tape bonds, I’m giving it serious consideration. Plus using tape is simpler for building curved cells, since the tape complies well to any shape, with the acrylic heat strips would have to be used to get the desired curve, and this could be hit or miss. If that weren't bad enough keeping the glue layer as uniform as possible will be impossible. I think ‘Ockham’s Razor’ applies here well.

Thanks for that nice recipe for prepping the stators. Forget about drilling the holes, I practiced on scraps I find with motor tool you do more damage than you had to begin with. I will get the stators coated by someone professional.

Bryan
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Old 10th August 2008, 08:18 PM   #17
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Hi

An effect that big (10dB) can not be overlooked in measurements.
So there must be a logical explanation why you are seeing this and I'm not. Perhaps it has to do with the curvature of your panels? Mine (Quad ESL panels) are slightly curved vertically (radius of about 3 meters), but otherwise flat.

regards,
Arend-Jan
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Old 10th August 2008, 09:09 PM   #18
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That is strange one would think that as you move further away from the sound source the levels would decrease. A lot of topic, reminds me of the corona heating problem. The surface of the sun is at a temperature of about 5800K as we move away from the surface the temperature increases to about 1.5 million kelvins. This is counter-intuitive as you move away from the heat source you would think that the temperature should decrease. I will do some thinking about this. Essentially the corona heating problem is one of the famous unsolved problems in solar-physics.

Jan, that is slight curvature. I imagine a cylinder of that radius, what angle did you curve through? I take it that with such a slight curvature you had little troubles tensioning the film as well as shaping the metal. Again thanks for all your input, maybe I will have pictures to share later. Also the etching solution cause the my sampled steel to rust slightly.
bryan
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Old 11th August 2008, 09:13 AM   #19
Calvin is online now Calvin  Germany
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Hi,

AJ, are You speaking of the 57? Well, I guess You should be able to measure the quoted effect, but since the Quad is rather low in panel height, the effect should only be measurable up to a smaller distance, maybe 1m and at elevated frquencies. But since the Quad has a great building width and the bass panels work as baffles for the tweeter panel, it isn´t a real strip source(!) and the effect would be countered by the low freq-lift because of the nearfield-proximity effect. I guess the Quad might not exhibit this effect as clearly as real strip sources like Audiostatics, bigger MLs or Finals with panels much longer than the Quad´s (the panle size has to be larger than the wavelength to produce the effect).

jauu
Calvin
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Old 11th August 2008, 12:10 PM   #20
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Hi,

I've got audiostatics myself.
Never heard of any increase in soundpressure at a greater distance. Certainly not in mono.
My book of physics (which explains theories of Huygens and Fresnel) doesn't describe this phenomenum.
The only thing what came in m y mind is a kind of interference pattern which amplifies sound upon distance.
Calvin, did you measure some frequency anomalies as well?
Are there minima as well?
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