Loudness etc. of ESL panels - diyAudio
Go Back   Home > Forums > Loudspeakers > Planars & Exotics

Planars & Exotics ESL's, planars, and alternative technologies

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 6th November 2008, 08:43 PM   #1
jjbunn is offline jjbunn  United States
diyAudio Member
 
Join Date: Nov 2008
Question Loudness etc. of ESL panels

Hi,

I'm a new member here, but not new to speaker building. My questions are about an ESL panel I am prototyping.

The ESL is made from stators of very open pressed steel mesh (I'd guess around 60% or more hole to steel ratio) sheets measuring 1ft by 2ft (see photo).

The insulating spacers are about 1mm thick (sorry to mix units!) for a stator-to-stator distance of 2mm. So, maybe about 1/16" each spacer for a total of about 1/8".

The membrane is shrink-on window plastic which I treated with powdered graphite to give an end-to-end resistance of a few MOhms.

The HV power supply is from a 9V AC wall wart attached to the 0-8V taps of a 0-384V power transformer. The 0-384V taps are connected to a voltage quadrupler giving around 1.5kV bias, the negative of which is connected via a 12 MOhm resistor to the membrane/diaphragm.

The HV positive end is attached to the centre of a pair of tube/valve audio output transformers for 8Ohm to 15kOhm impedance working. The 8Ohm taps are in parallel, the 15kOhm taps in series. This seems to give a step up of around a factor 100.

So, that is the setup. Here are the issues I'd welcome some advice on:

1) If I apply the full 1.5kV to the stators/diaphragm then I get some low frequency oscillation of the diaphragm. This can be removed by heating the plastic up and so stretching it more, but seems to come back after a while. If I use only 1.2kV then there is no oscillation.

2) If I charge up the diapragm, and remove the HV supply, then it remains charged for quite some minutes, which I believe shows low leakage of charge, and is good. This also means that, with audio playing through the panel, I can remove the HV supply and the SPL very slowly goes down. (Why have the HV powered up all the time: I suppose it could be on a timer.)

3) The panels are not very loud, and according to measurements made with pink noise, have an unacceptable and non-flat response that rises across the audio band. Perhaps I am not driving them with sufficient power? What is a sensible amplifier rating for ESL panels of this specification?

My main goal with the panel is to make a good centre channel speaker to complement the rest of my system (pair of JM Reynaud Millesime Vs, pair of Acoustic Triangulators, Speaker City subwoofer), so I don't need deep bass.

Thanks!
Julian

Click the image to open in full size.
  Reply With Quote
Old 7th November 2008, 12:35 AM   #2
Few is offline Few  United States
diyAudio Member
 
Join Date: Apr 2004
Location: Maine, USA
Welcome to the fun!

The first thing that strikes me abut your prototype is the large hole size. The hole size shouldn't be dramatically larger than the stator to diaphragm spacing or you end up with a large fraction of your diaphragm being only weakly driven. The 1 mm diaphragm-stator spacing is a good starting point if you can keep things flat enough, so I think your hole size is a major contributor to the poor sensitivity.

You could also check the transformer turns ratio to be sure it's behaving as you expect.

Don't forget that you'll have significant cancellation between the front and rear waves from the diaphragm--an effect that becomes more pronounced as the frequency decreases. This is inherent in the dipolar nature of ESLs that aren't mounted on enclosures (meaning the majority of them). The way to fix it is to introduce a circuit whose frequency response mirrors that of the ESL panel. You might check out the book by Sanders, if you haven't already done so, for some ideas. That book provides useful rules of thumb, even if some of the physics background is off the mark. Note that when you add the compensating circuit you'll need to be sure it doesn't also boost the drive to the ESL at the panel's fundamental resonance frequency (which is likely to be somewhere in the 50-200 Hz range, depending on how much tension you've managed to apply).

Ultimately, you'll probably want to switch diaphragm material to something thinner than what you're using now but it makes sense to use whatever's handy as you get the hang of things.

These aren't super detailed comments because I don't want to belabor points that might already be obvious to you. Post again if questions remain or my explanations aren't addressing what you're after.

Few
  Reply With Quote
Old 7th November 2008, 10:06 PM   #3
jjbunn is offline jjbunn  United States
diyAudio Member
 
Join Date: Nov 2008
Many thanks, Few.

The hole size is indeed rather large, and outside what one normally sees. On the one hand that would decrease the field strength somewhat towards the centre of each hole, but on the other should provide a larger volume of air displacement and so greater SPL ... that's what I theorized, anyway

Perhaps I could attach some denser mesh to each side of the panel to close up the holes somewhat.

My transformer pair is giving a factor 100 voltage step up, but I'm interested in what audio output level the amplifier should have. Right now I'm using a small amp that gives a few volts AC across the 8Ohm taps, so the audio voltage at the ESL is maybe a few hundred volts. Is that too little?
  Reply With Quote
Old 7th November 2008, 10:44 PM   #4
Few is offline Few  United States
diyAudio Member
 
Join Date: Apr 2004
Location: Maine, USA
Julian,
You clearly need room for air to move through the stator, but I think less room is needed than is often assumed. Even
Martin-Logan now touts their "Microperf" stators, which have a greater number of smaller diameter holes than their previous designs had. I really think you're better off erring on the side of more stator area and less room for air movement than the other way around, even though it seems a bit counterintuitive at first.

Also, note that often designers include some felt or other acoustic damping material immediately outside their rear stators. If you're going to add acoustic resistance, why not have it also add to the driving force?--at least that's my reasoning. Bear in mind also that the real advantage of an ESL is the uniform driving force experienced by the very light diaphragm. If the driving force isn't uniformly distributed across the surface of the diaphragm you risk exciting resonances higher than the fundamental. You may end up with buzzing and rattling sounds.

I don't think adding denser mesh outside your current stators is going to do the trick. To get decent sensitivity you need to minimize the stator-diaphragm spacing, and small changes can yield big differences. You really want the denser stator material as close to the diaphragm as you can get it without having the diaphragm smack into the stator at maximum loudness. Have you checked McMaster-Carr for perforated steel? That's where I got mine when I built perforated steel panels. I think the holes are about 1/16" diameter with an open area a bit less than 50%. I insulated them with epoxy spray paint intended for home appliances.

I'm now working on stretched wire stators at the moment but I'm still in the construction stage. The wire-wire spacing is going to be quite small (~1 mm), with an open area of around 45%.

It sounds like the voltage coming out of your step-up transformer is low. Do you have a higher power amp to try, or perhaps other taps on your current amp?

Few
  Reply With Quote
Old 8th November 2008, 07:20 PM   #5
jjbunn is offline jjbunn  United States
diyAudio Member
 
Join Date: Nov 2008
Hi Few,

You are probably right: my hole size is too big. Your comment about stretched wires makes me wonder whether I can thread some wire across my panels, in and out of the holes, so improving the field strength in the holes.

Or, I could just start a fresh pair of panels with some more suitable material My inclination is to fiddle around and experiment with what I have for a bit longer before doing that.

The amp I am using is a bit too weedy, but is what I have available. I've just scored a Kenwood Basic M1D on EBay which has 125W into 8Ohms, so when that arrives I can really pump up the panel and check.

Thanks again,
Julian
  Reply With Quote
Old 9th November 2008, 09:38 AM   #6
diyAudio Member
 
Join Date: May 2006
Location: Netherlands
Default Re: Loudness etc. of ESL panels

Quote:
Originally posted by jjbunn

1) If I apply the full 1.5kV to the stators/diaphragm then I get some low frequency oscillation of the diaphragm. This can be removed by heating the plastic up and so stretching it more, but seems to come back after a while. If I use only 1.2kV then there is no oscillation.

The tension of the diaphragm is too low and this causes instability. What happens is that the negative stiffness pulls the diaphragm towards one of the stators, there it touches and discharges. This keeps going over and over again until you either remove the HT or increase tension.

Quote:
3) The panels are not very loud, and according to measurements made with pink noise, have an unacceptable and non-flat response that rises across the audio band.

The output can be low for a lot of reasons. To get high enough output/sensitivity you need low stator spacing (2mm is good), consistent stator spacing (very important), and as mentioned the holes are way too big.

Rising response with frequency is normal for a voltage driven ESL. The output of an ESL depends on the current you feed to the stators. Electrically it's a capacitor so you have a natural rising response. You can put a big series resistor in the lead to each stator to flatten the frequency response, but this will give you a loss in output. See the Quad ESL Simulator
  Reply With Quote
Old 10th November 2008, 02:49 AM   #7
Few is offline Few  United States
diyAudio Member
 
Join Date: Apr 2004
Location: Maine, USA
Hey Julian,
I hope things are moving along for you---slowly, if your experience is anything like mine.

By all mean fiddle around with what you've created in order to get a sense of how everything works. I think, though, that threading wires into and out of the large holes in your current panels may not buy you much. The problem is that the maximum bias voltage you can apply, and the maximum loudness your speakers can generate, will be limited by the minimum stator-diaphragm distance--not the average, or the maximum distance. This implies that you really want all points on the stator to be equidistant from the diaphragm.

There are two trends you have to fight. First, if 95% of the stator is 1 mm from the diaphragm, but even one point is only 0.5 mm from the stator, then that one point is where the arcing will occur if you crank up the bias, and it's also going to be the point where the diaphragm touches the stator when you play the system loudly. The fact that 95% of the stator-diaphragm system has 1 mm spacing will only mean that 95% of the area will have lower sensitivity than it should, given the 0.5 mm minimum diaphragm/stator spacing.

So, the goal is to keep the stator-diaphragm spacing as constant as possible across the whole panel. Also, to maximize sensitivity, the spacing should be as small as possible, within the limits imposed by your crossover frequency and loudness expectations.

I'd be the last person to tell you not to fool around with your prototype to learn as much as you can. It's clearly the only way to learn some parts of the process. Nonetheless, you might learn more in less time if you can avoid learning everything the hard (time consuming, expensive, frustrating) way. I've done enough of that for both of us! Keep everyone posted as you experiment. No doubt we can learn a few tricks from your efforts as well.

Few
  Reply With Quote
Old 10th November 2008, 04:29 PM   #8
jjbunn is offline jjbunn  United States
diyAudio Member
 
Join Date: Nov 2008
Hi Few and Arend-jan ... many thanks for the encouragement. I am awaiting the arrival of a powerful amplifier with which to test the panel more thoroughly in my workarea.

I will test with higher audio driving voltages, resistance in series with the stators (I assume this will make them "current driven"?), and perhaps with some hole-filling.

I'll update once I have some results.
  Reply With Quote
Old 12th November 2008, 08:47 AM   #9
diyAudio Member
 
chinsettawong's Avatar
 
Join Date: Jun 2006
Hi Julian,

I just made a set of ELS headphone. I can tell you a few things.

1. The reason why the tension of your diaphragm comes back down after you heat shrink it could be the glue you used. When I tried epoxy glue, I had the same problem. Try some other glues.

2. The impedance of ELS speaker is very low. I learned it the hard way. You need to put a resister in series with the stator.

3. I use 4 ohm tabs with the ELS headphone and the amplifier. I found out that the bigger the amplifying factor the easier it is to drive the ELS.

I hope it helps.

Wachara C.
  Reply With Quote
Old 13th November 2008, 08:56 AM   #10
Calvin is offline Calvin  Germany
diyAudio Member
 
Calvin's Avatar
 
Join Date: Nov 2004
Location: close to Basel
Hi,

regarding 1)
The loss of tension could only be a matter with ´soft´ glues like certain double sided tapes. Use good tapes or ´hard glues´ and loss of tension is no issue.
The real problem with decreasing tension over time is a diaphragm material unsuitable properties. Only very few materials have proven stable over time. Polyester is not polyester is not polyester.

regarding 2)
The impedance of an ESL can be low, but usually its a high impedance device within audio freq-range. Its the coupling via a transformer that creates impedance problems in first place

regarding 3)
As in every other example from audio technology it´s always good to keep amplification factors as low as possible. Higher amplification factors only rises levels of voltage or SPL, but lowers all other performance aspects! If You experienced something different, the real problem is hidden elsewhere

jauu
Calvin
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
The loudness war.... Empee Everything Else 81 19th November 2008 01:37 PM
How much loudness is enough for HT? Phil Olson Multi-Way 19 29th March 2008 12:14 AM
three tap loudness pot adason Parts 8 26th January 2006 08:53 AM
Potentiometer loudness tap? paulb Solid State 3 13th May 2002 07:54 PM


New To Site? Need Help?

All times are GMT. The time now is 03:53 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright ©1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2