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Electrostatic speaker myths

I just did an exhaustive search of the web for any sort of hard science on electrostatic speakers, and except for a few blogs of variable quality, the only thing I consistently hit was an advertising blurb from Martin-Logan.  For obvious reasons, I don't think their take on the subject is unbiased.

What I'm curious about is whether there has been a decent write-up anywhere talking about some of the claims made about electrostatics, and any basis in fact associated with those claims.  One thing which comes up with maddening regularity is the so-called "massless" nature of the driver.  Obviously, this is pure hype because even the thinnest, most flimsy LDPE or Mylar films have a density of tens to hundreds of times that of normal atmospheric air.  Combined with the fact that the force behind the ESL "motor" is lower than that of a typical electrodynamic driver, some of the claims about the system seem questionable.  There's no doubt that a good speaker can be made with the technology, but the hype seems to over-reach the actuality.

It seems that so many people repeat the myths, any research, design or evaluation information has been buried under them.  Does anyone know of a good unbiased treatise on electrostatics which addresses some of the pros as well as cons of the type, and backs them up with measurements and logic?

Re: Electrostatic speaker myths

Reply #1
A good start:
https://en.wikipedia.org/wiki/Electrostatic_loudspeaker

It's as valid a speaker design as any.  My experience is that they need a big room and lots of space around them.  They don't sound good against a wall or in a corner.  Often best served by a high-current amp.  Probably need a sub to get that bottom octave, though M-L does make some nice hybrids.

Check other manufacturers (JansZen, Sound Lab) and read some reviews.  No substitute for hearing a pair yourself.

Re: Electrostatic speaker myths

Reply #2
I haven't ever heard electrostatics, but I did once get to hear some ribbon loudspeakers, which have some aspects in common as regards how you set them up in a room. They were Apogees (either Centaurs or Centaur Minors, I forget), and made some very nice sounds. However, like (most) Martin-Logans, their cone woofer/sub was part of the same cabinet as the mid/tweeter, which doesn't necessarily always make for perfect placement of either element.

If you want proper massless drivers, look up ionophone/plasma speakers... and all the problems that come with trying to operate them effectively while staying sane and un-electrocuted.

Re: Electrostatic speaker myths

Reply #3
There is some good info in the Wiki article; I was just hoping for more direct details and numbers.  What it has is fairly vague, though at least it sort of addresses a few things like air load.  I have heard a fair number of electrostatics in my travels, and the main problem seems to be building large panels in such a manner that you don't get excessive amounts of comb filtering (which is a characteristic of the Martin-Logans, among others).  The low mass is definitely good in some ways, although no one seems to talk about the limitation - which is that they can easily be over-damped if the electrostatic force generated is insufficient.  But numbers are hard to find, and I was hoping someone had done the math already and shared it.

Actually, you want a high voltage amp for electrostatics, since current demands by the driver are (or should be) minimal; if a transformer is used, in fact, stepping up the voltage to the level required by the diaphragms reduces the current delivered to the speaker by a comparable amount.  The problem is usually the reactive load causing heating in the output devices, and a higher current amp is more likely to be designed to tolerate that or at least have better protection against reactive loads, such as a Zobel network.

And it's not so much that I'm looking for a massless speaker as hoping to cut through all the audiophool and marketing gibberish about electrostatics.  They definitely have advantages, but also sonic disadvantages, and there don't seem to be many people willing to call them out on that.  Mostly it's manufacturers trying to sell them.

 

Re: Electrostatic speaker myths

Reply #4
ESLs are tricky.

In most cases, the impedance varies from a high in the bass to a low in the highs by about a factor of 9 or 10 to 1.  They are fundamentally different from a speaker in a box, whose impedance curve is a pretty good map of its efficiency (hence in many cases best driven by a voltage source for flat frequency response).  The impedance curve of an ESL is not based on the resonance of a driver in a box; essentially its based on a capacitor; so if one is to put out a given x sound pressure at 30Hz, the speaker needs the same power to do that as it does at 10KHz.

This means that ESL manufacturers have to resort to some tricks depending on what part of the market they cater to. Martin Logan for example keeps their impedances low- about 4 ohms in the bass, resulting in about 0.5 ohms at 15 or 20KHz. The latter impedance is so low that the speaker cable often becomes a factor in the driving impedance, which I suspect is by intention, in order to limit the current that the amp makes so as not to be as bright. If the amp driving the speaker is a voltage source and is able to act that way when driving a Martin Logan, you can see that there will be too much energy at high frequencies. Since a lot of solid state amps run into troubles with these impedances they get away with it; its helpful that there is not a lot of energy up there.

Sound Labs, at the other extreme (and capable of true 20-20KHz operation) are as much as 30 ohms in the bass, and to drive them with solid state takes a rather large amp despite the speaker being relatively efficient (you can see that a mic placed at 1 meter from the speaker will not pick up all that the driver radiates; for such speakers a more accurate value will be the measured sensitivity + 6db, which is experienced at a greater distance from the speaker). For example a 600 watt amp can only make about 150 watts in the bass on a Sound Lab- and not surprisingly, a lot of solid state amps tend to be bass shy on this speaker. Tube amps on the other hand can make power into loads like that so its not uncommon to see a 150 or 200 watt tube amp keep up easily with a 600 watt amp in this case.

This is not the case with ML and a number of others as their impedance does not favor tube amps at all. With the higher impedance ESLs it is common knowledge that you need an amp that can make voltage. At this link you can view the impedance curve of the old ESL57 (2nd post):

http://audiokarma.org/forums/index.php?threads/quad-esl-57s-destroyed-a-carver-tfm-55x-amplifier.785429/

With most speakers employing a permanent magnet, the magnetic field can sag a bit as current is put through the voice coil. According to a friend in the industry (who makes drivers for a lot of OEMs in the US) Alnico, while the weakest magnet used, also sags the least so has a certain following amongst audiophiles. An ESL's motive force is unaffected by the amplifier's current to move the diaphragm (since it derives from a high voltage power supply), and so the argument is that the driver is thus faster and lower distortion. Its certainly not about the diaphragm being as light as air, although because of their size, they don't need much excursion, like any planar.

The old Quads made by Peter Walker were reputed to be one of the lowest distortion speakers made for many years.

Some ESLs are quite fragile- the old Quad ESL57 could only handle about 35 watts and if you drove it too hard it would arc and damage the panel. Modern Quads have protection circuits. Overall ESLs can enjoy quite a loyal following so its common to hear about their owners going through quite a bit of hell to keep them alive and playing.

Re: Electrostatic speaker myths

Reply #5
I got a pair of the less expensive ML's that I got a good deal for years ago. The thing that most jumped out at me is not exactly the sound per se, which is just fine, but they have very narrow dispersion, so much so that moving your head just above the speaker panel's height, at fairly far away, will drop off treble noticeably. Just moving your head a few inches up and down you can easily tell the difference. The base even is angled like a wedge, and you can rotate it 180° to angle the speaker upwards a bit because of this. So, if I'm walking around the apartment, I don't use those speakers for music, they're my home theater mains.

Also the sound doesn't come from a point like regular tweeter speakers, it "shoots" from the whole surface of the panel, so I think (and others can correct me if I'm wrong) that volume should fall off with distance differently than a regular speaker.


Re: Electrostatic speaker myths

Reply #7
Placement in the corner? My Staxes do not sound too impressive when I put them i the corner, no. (To those who didn't catch it: they are headphones ;-) )
So obviously I am not going to fill this post with the hard science or measurements the OP wanted (and by the way I think the Wikipedia article suffers from even more [citation needed]'s than is even indicated in the text). But this part:

One thing which comes up with maddening regularity is the so-called "massless" nature of the driver.  Obviously, this is pure hype because even the thinnest, most flimsy LDPE or Mylar films have a density of tens to hundreds of times that of normal atmospheric air.  Combined with the fact that the force behind the ESL "motor" is lower than that of a typical electrodynamic driver, some of the claims about the system seem questionable. 

Is it really so relevant to directly compare mass density to that of air? One thing is "density" vs "total mass", I mean, if you had the same mass but just made it thinner, you would inflate density, and ... that would not matter to the argument, ceteris paribus.
And it is not the heaviness of mass either - you wish to accelerate the driver [film|cone], so it would be inertia? But not directly compared to air; if one attempted to ask whether the inertia of cone / inertia of air relation is troublesome, and the inertia of film / inertia of air is small enough for that problem to be practically solved - then one would make the error of presuming that we want to move a lot of air as if it were a rigid body. (Gases are not - air waves are generating by attempting to compress it, right?)

Connected to that: a cone is "driven" where it is glued to the voice coil, and relies on sufficient rigidity for the surface. Electrostatic film is certainly not much rigid, and is driven everywhere.
But of course, this consideration is a matter magnitude that should be measured: if cone speakers manufacturers have solved the rigidity/inertia trade-off to an insignificant order of magnitude, then the return on removing the issue would not be too great.

Re: Electrostatic speaker myths

Reply #8
As a Quad 57 owner, I found these speakers to be the best I've heard for acoustic music. They lack power and do better in smaller rooms and have a small "sweet spot". If the felt from the rear of the panels is removed, they are louder, but they then need to be placed further from walls or corners to maintain the bass. The midrange seems lifelike, subjectivily speaking. As for the mass, my unprofessional understanding is that when the panel is electrically charged, the mass / force ratio is the best of any driver, however, I have no reference for this info.

As well, I have had to replace arced tweeter panels on several occasions.
Quis custodiet ipsos custodes?  ;~)

Re: Electrostatic speaker myths

Reply #9
I got a pair of the less expensive ML's that I got a good deal for years ago. The thing that most jumped out at me is not exactly the sound per se, which is just fine, but they have very narrow dispersion, so much so that moving your head just above the speaker panel's height, at fairly far away, will drop off treble noticeably. Just moving your head a few inches up and down you can easily tell the difference. The base even is angled like a wedge, and you can rotate it 180° to angle the speaker upwards a bit because of this. So, if I'm walking around the apartment, I don't use those speakers for music, they're my home theater mains.

Also the sound doesn't come from a point like regular tweeter speakers, it "shoots" from the whole surface of the panel, so I think (and others can correct me if I'm wrong) that volume should fall off with distance differently than a regular speaker.

The sound pressure falls off slower.

Some ESLs have curved panels (like the Quads) and so have less of that 'head inna vice' thing going on. The Sound Labs are so tall its a non issue; they are curved though so the radiation pattern is fairly wide.

Re: Electrostatic speaker myths

Reply #10
"Why the Quad ESL is able to do so is simply because of the moving part of the Quad ESL, the electrostatic diaphragm, which is a thin layer of stretched Mylar, ten times thinner than a human hair and so light that it approaches the mass of air to which it is coupled." - http://quad-hifi.co.uk/product.php?cid=5
Quis custodiet ipsos custodes?  ;~)

Re: Electrostatic speaker myths

Reply #11
Also the sound doesn't come from a point like regular tweeter speakers, it "shoots" from the whole surface of the panel, so I think (and others can correct me if I'm wrong) that volume should fall off with distance differently than a regular speaker.
I'd guess that a speaker of the shape of a Martin-Logan might have the same properties as a more conventionally-driven "vertical line array" speaker like the ones made by Townshend, Scaena. or even the Grateful Dead's legendarily humungous 'Wall of Sound'. At least when it comes to volume/distance matters.

Re: Electrostatic speaker myths

Reply #12
Placement in the corner? My Staxes do not sound too impressive when I put them i the corner, no. (To those who didn't catch it: they are headphones ;-) )
So obviously I am not going to fill this post with the hard science or measurements the OP wanted (and by the way I think the Wikipedia article suffers from even more [citation needed]'s than is even indicated in the text).


Yeah, it reads like sort of vague advertising copy - very few of the assertions made in the article are backed up with facts.

Quote from: Porcus
One thing which comes up with maddening regularity is the so-called "massless" nature of the driver.  Obviously, this is pure hype because even the thinnest, most flimsy LDPE or Mylar films have a density of tens to hundreds of times that of normal atmospheric air.  Combined with the fact that the force behind the ESL "motor" is lower than that of a typical electrodynamic driver, some of the claims about the system seem questionable. 

Is it really so relevant to directly compare mass density to that of air? One thing is "density" vs "total mass", I mean, if you had the same mass but just made it thinner, you would inflate density, and ... that would not matter to the argument, ceteris paribus.
And it is not the heaviness of mass either - you wish to accelerate the driver [film|cone], so it would be inertia? But not directly compared to air; if one attempted to ask whether the inertia of cone / inertia of air relation is troublesome, and the inertia of film / inertia of air is small enough for that problem to be practically solved - then one would make the error of presuming that we want to move a lot of air as if it were a rigid body. (Gases are not - air waves are generating by attempting to compress it, right?)

Connected to that: a cone is "driven" where it is glued to the voice coil, and relies on sufficient rigidity for the surface. Electrostatic film is certainly not much rigid, and is driven everywhere.
But of course, this consideration is a matter magnitude that should be measured: if cone speakers manufacturers have solved the rigidity/inertia trade-off to an insignificant order of magnitude, then the return on removing the issue would not be too great.

Right; that's part of what I'm trying to determine.  You're moving a larger area but with (I suspect) much less force, and linearly over the vast majority of the surface but with a fraction of the excursion.  With a typical cone, the mass ratio is higher and the force much greater, but you have to have a greater excursion to achieve the same output level.  I was hoping to find some numbers to put that into perspective, so I could see whether the lower mass was a significant advantage or whether the ratios still came out similar.

I've heard some cone-type speakers which do a great job of reproducing transients and have the same sort of "hear-through" illusion that an electrostatic provides, so I have long felt it's not the transducer per se, but a combination of good drivers and crossover design (of course) plus other factors: room interaction, reflection/diffraction, and radiation pattern - all of which have common progenitors in the mechanical layout of a speaker but are usually not all well-behaved.

You can see in AJ's graph that the speaker represented has a lot of comb filtering at higher frequencies, and I'll bet there's a sort of "venetian-blind" effect from moving the listening position - plus probably timbral changes since the different frequencies don't change similarly at the different angles.  OTOH, they indicate a dipolar pattern which could be beneficial to eliminating room modes if positioned carefully.

Re: Electrostatic speaker myths

Reply #13
http://www.princeton.edu/3D3A/Directivity/Essence%20Electrostatic%20Model%201600/index_H.html



What spoke to me is this plot for the same speaker:



IME this is not atypical.

What I see is a speaker that is the opposite of something that approximates constant directivity. It's beamy at high frequencies and compounds that failing with a lot of side lobes.

Re: Electrostatic speaker myths

Reply #14
You can see in AJ's graph that the speaker represented has a lot of comb filtering at higher frequencies, and I'll bet there's a sort of "venetian-blind" effect from moving the listening position - plus probably timbral changes since the different frequencies don't change similarly at the different angles.  OTOH, they indicate a dipolar pattern which could be beneficial to eliminating room modes if positioned carefully.
Well, that's a good start, as I saw folks "guessing" and posting conjecture even after I posted that link. If you go to it, you'll find far more data (there is a <> nav bar above the graphs) on this and a couple other panel/stat type speakers.
There are no mysteries for non-audiophile folks like me. What you see is what you get. The polar pattern is indeed chaotic at best and quite different across the spectrum. Many, with rare exceptions like the huge Soundlabs, are actually dipolar panels atop monopole boxes. There is no question there are alluring aspects to the kind of soundfield they generate, especially to untrained ears, hence their popularity amongst a subset of audiophiles. The chaotic nature of the HF is actually quite ideal for the rear radiation. That part it good! (even if listeners have no clue why).
An ideal speaker for generating "realistic" soundfields would have indirect radiation that is highly diffuse, i.e lacking "leading edge" wavefront info for the ears to have directional cues to lock onto.
Unfortunately, stats also radiate this way forwards, with the direct field.
Loudspeaker manufacturer

Re: Electrostatic speaker myths

Reply #15
"Why the Quad ESL is able to do so is simply because of the moving part of the Quad ESL, the electrostatic diaphragm, which is a thin layer of stretched Mylar, ten times thinner than a human hair and so light that it approaches the mass of air to which it is coupled." - http://quad-hifi.co.uk/product.php?cid=5

From a scientific veiewpoint this is meaningless, until the repeal of the Law of Newton we often refer to as F = MA.  This means that you can manage high mass quite effectively if you can come up with a correspondingly high force.

Of all common formats of loudspeaker, electrostats have a track record for having problematically limited sources of force.

The simple fact is that obtainin lots of dynamic range is no problem for conventional designs. but are mission impossible for 'stat's.  If diaphragm acceleration were the most important thing, compression drivers would rule.

 
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