Asymmetric construction induced distortion in electrostatic speakers

[arend-jan]

In the past there has been some debate whether asymmetry in construction, e.g. the diaphragm not being equidistant to both stators, would result in increased distortion or not.
For example, Moray James ones proposed that second harmonic distortion could be raised by purposely adding asymmetry. thread on DIY

I thought it might be interesting to put this to the test in a real world ESL element: the Wachara headphone.

coating surface resistance: 1E7 ohms/square

First have a look at the second harmonic distortion:
blue trace: symmetric stators, d/s = 1mm.
yellow trace: asymmetric stators, d/s = 1 & 2mm

I compensated the output level for the decreased sensitivity of the asymmetric element (about 6dB).

[arend-jan]

For the same conditions, here is the result for third order distortion

Only a slight increase here, nothing spectacular except for the weird dip around 500Hz and peak around 1.5kHz. I would not pay too much attention to the raise above 2kHz, this is present in all my 3rd harmonic measurements and I suspect it's an artefact of the measurement setup (microphone?).

[arend-jan]

I'm sorry this thread was supposed to go into the Louspeakers > ESL/Planars section. Moderator, can you please move it? Thank you.

[SY]

Moved.

What does the acoustic loading look like on each side of the diaphragm? I would think that this contribution to asymmetry would be just as important as geometry.

[arend-jan]

Originally posted by SY
Moved.

Thanks!

What does the acoustic loading look like on each side of the diaphragm?

The element is suspended in air and during all measurements there was a thin felt mat on one side of the element (between the element and the microphone) to damp the main resonance.

I would think that this contribution to asymmetry would be just as important as geometry.

Interesting observation, but I'm not sure I agree with this line of thought. For example in this case there is a constructional asymmetry introduced by the felt, but would the element also experience this as asymmetric?

Acoustic loading on one side could make a difference if:

1. the diaphragm suffers from compression/expansion (in the 'thickness' dimension)
2. the loading itself is asymmetric, e.g. different if we 'push' or 'suck' air through it.

I assume that you would agree that point 1 is very doubtful. Point 2 sounds more plausible and would likely be a case if we had a woofer in an enclosure. But we are moving so little air here, that in this case I'm not so sure.

If you are interested I could do some measurements without damping, damping one one side and damping on both sides.

[SY]

If you are interested I could do some measurements without damping, damping one one side and damping on both sides.

Extremely! Many thanks, you know how highly I think of your work.

[arend-jan]

I had to do these measurements at a little higher output level (+9dB) because of background noise coming from outside of the house. So distortion levels may be a bit higher compared to previous measurements.


1. second harmonic, one side felt damping (blue) vs. no damping (yellow)

[arend-jan]

2. third harmonic, one side felt damping (red) vs no damping (grey)

[arend-jan]

3. second harmonic, one side felt damping (yellow) vs two sides (blue)

[arend-jan]

4. third harmonic, one side felt damping (grey) vs two sides (red)