Sonic differences Isodynamics vs Electrostatics - theorethical thoughts.

[Nevod]

In the "Measurements" thread, everyone could see that top-tier electrostatic and isodynamic headphones are more similar than different, having similar FR, CSD, square wave and impulse responces, isodynamics sometimes having better measured parameters. However, their sound is not the same.

I have thought of 3 moments that seem to make difference between them. I must warn everyone from the start that I'm no specialist in these areas and everything below is wild speculation.

1) Air resistance and open area.

ES drivers use very thin stator plates with high open area - more than 50%, it seems. There is minimum amount of trapped air and minimum stored energy. Isodynamics have thicker stators due to thick magnets, and open area varies heavily based on design - Fostex T50RP have very little open area, while Hifiman HE-6 drivers seem to have at least 30% open area. The gaps between magnets also seem to be about equal to magnets' thickness, so the effect of stator thickness should be reduced to a minimum. Electrostatics have edge, but not by much.

2) Motive force and displacement.

There are some opinions that motive force produced in isodynamic designs is much higher than motive force produced in electrostatic designs. Higher motive force means higher acceleration - better treble extension, and higher attainable pressure - more power in bass region. Both in electrostatics and isodynamics, specific motive force is reversely proportional to the membrane - stator gap. Electrostatics actually have inverse square relationship - Fsig = e0*Vpol*Vsig/d^2, where e0 is permittivity, d is stator to diaphragm distance. Actually, Normal bias Stax models seem to have higher specific motive force compared to Pro bias, though not by much. But they do have less excursion, and..

And it's the excursion that allows deep bass and high dynamic range - and hence, deep bass dynamic range. Of two drivers with same motive force, the driver with more excursion would have more dynamic range and deeper bass. Of two drivers with same excursion, one with higher motive force would be able to provide deeper bass(provided that both wouldn't reach excursion limits) and more treble extension.

Isodynamic headphones have much higher excursion in general over electrostatics, so their bass and dynamic range capabilities are understandable. Isodynamics have edge, but not by much.

However, both above points regard large-signal characteristics, and maximum attainable parameters. Motive force directly relates to SPL, and we would anyway listen at about the same SPL, at about the same motive force value be it isodynamic or electrostatic.

And, real music is not 0dbFS sine wave, and contains very low level signals, which have to be considered.

3) Nonlinear effects on percieved detail.

This is even wilder speculation than the above. Isodynamics have much heavier diaphragm than electrostatics, mainly due to voice coil. While in electrostatics, membrane is many times lighter than coupled air, in an isodynamic driver membrane could weight more than coupled air. Heavy diaphragm eats up availible motive force, but its influence is linear and can be remedied by just more power. (Have to set a point to myself there, I'm not sure I'm right).

So, heavy diaphragm shouldn't influence detail level by itself. There must be some nonlinearity which is proportional to diaphragm weight/thickness that is not proportional to signal. Something like static friction force, which is constant, and completely counters any force not strong enough.

Such an effect would be more similar to crossover distortion though, and shouldn't influence small signals superimposed on peaks of larger signals. But that is already something.

Electrostatics seem to have edge, and it's not something isodynamics would be able to counter anytime soon, before some room-temperature superconductors appear.

Feel free to critic any way, as I don't pretend to be correct on any of these points.

[CherryBomb]

That is a long post.

Edited September 25, 2011 by CherryBomb

[mikeymad]

too long for me on a Sunday morning.. goes in the read it later bucket.

[deepak]

In the "Measurements" thread, everyone could see that top-tier electrostatic and isodynamic headphones are more similar than different, having similar FR, CSD, square wave and impulse responces, isodynamics sometimes having better measured parameters. However, their sound is not the same.

I have thought of 3 moments that seem to make difference between them. I must warn everyone from the start that I'm no specialist in these areas and everything below is wild speculation.

1) Air resistance and open area.

ES drivers use very thin stator plates with high open area - more than 50%, it seems. There is minimum amount of trapped air and minimum stored energy. Isodynamics have thicker stators due to thick magnets, and open area varies heavily based on design - Fostex T50RP have very little open area, while Hifiman HE-6 drivers seem to have at least 30% open area. The gaps between magnets also seem to be about equal to magnets' thickness, so the effect of stator thickness should be reduced to a minimum. Electrostatics have edge, but not by much.

2) Motive force and displacement.

There are some opinions that motive force produced in isodynamic designs is much higher than motive force produced in electrostatic designs. Higher motive force means higher acceleration - better treble extension, and higher attainable pressure - more power in bass region. Both in electrostatics and isodynamics, specific motive force is reversely proportional to the membrane - stator gap. Electrostatics actually have inverse square relationship - Fsig = e0*Vpol*Vsig/d^2, where e0 is permittivity, d is stator to diaphragm distance. Actually, Normal bias Stax models seem to have higher specific motive force compared to Pro bias, though not by much. But they do have less excursion, and..

And it's the excursion that allows deep bass and high dynamic range - and hence, deep bass dynamic range. Of two drivers with same motive force, the driver with more excursion would have more dynamic range and deeper bass. Of two drivers with same excursion, one with higher motive force would be able to provide deeper bass(provided that both wouldn't reach excursion limits) and more treble extension.

Isodynamic headphones have much higher excursion in general over electrostatics, so their bass and dynamic range capabilities are understandable. Isodynamics have edge, but not by much.

However, both above points regard large-signal characteristics, and maximum attainable parameters. Motive force directly relates to SPL, and we would anyway listen at about the same SPL, at about the same motive force value be it isodynamic or electrostatic.

And, real music is not 0dbFS sine wave, and contains very low level signals, which have to be considered.

3) Nonlinear effects on percieved detail.

This is even wilder speculation than the above. Isodynamics have much heavier diaphragm than electrostatics, mainly due to voice coil. While in electrostatics, membrane is many times lighter than coupled air, in an isodynamic driver membrane could weight more than coupled air. Heavy diaphragm eats up availible motive force, but its influence is linear and can be remedied by just more power. (Have to set a point to myself there, I'm not sure I'm right).

So, heavy diaphragm shouldn't influence detail level by itself. There must be some nonlinearity which is proportional to diaphragm weight/thickness that is not proportional to signal. Something like static friction force, which is constant, and completely counters any force not strong enough.

Such an effect would be more similar to crossover distortion though, and shouldn't influence small signals superimposed on peaks of larger signals. But that is already something.

Electrostatics seem to have edge, and it's not something isodynamics would be able to counter anytime soon, before some room-temperature superconductors appear.

Feel free to critic any way, as I don't pretend to be correct on any of these points.

None of the orthos I've heard can match the best electrostats in detail. They all have some slight fuzziness in the sound.

[arnaud]

I'll start with the first point for now: there is no such thing as energy storage in a thin airgap. Energy is stored in modes, therre are no modes through the thickness... What it does though is mass load the diaphragm and add damping if vsicous effects are significant (e.g thin layer, small holes in the stator...). I am in the process of looking into this for simulation model.

In summary, this stator to stator spacing (and stator perforation ratio) is part of estat design and has many function (controls the force, damping...) but it's not the source of estat advantage over different technology

[Nevod]

In summary, this stator to stator spacing (and stator perforation ratio) is part of estat design and has many function (controls the force, damping...) but it's not the source of estat advantage over different technology

Well, estats do have greater degree of freedom with regards to stator design, while others have more constraints. Other than that, I agree.

[livewire]

Now my head hurts...

Yes, stats are better. Much better.

And I'd like to add this:

I'd rather have a bottle in front of me than a frontal lobotomy. Nuff sed.