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bentoronto 10th August 2012 12:03 PM

Ozone and ESLs
A question about ozone coming from ESLs came up in another thread and I wanted to crowd-source thoughts about it. A quick search here wasn't helpful.

I know ozone is truly dangerous when strong - destroys your lungs not to mention rubber and other stuff. But you can smell it it in concentrations 10% of the OSHA harmful level.

The once popular high voltage ion generators and electrostatic cleaners have lost their appeal (partly because of cheap HEPA-like filters being available).

Of course, that after-thunderstorm ozone smell is viewed as a nice. And ozone does have use as a disinfectant and water purifier.

If you smell some ozone, are you ESL panels doing something wrong or is that normal?

My 2-cents.


bolserst 10th August 2012 01:19 PM


Originally Posted by bentoronto (
If you smell some ozone, are you ESL panels doing something wrong or is that normal?

ESLs should NOT be generating ozone. Ozone is bad for you and the health of your ESL coatings and insulators.
If they generate ozone even when not playing music the bias voltage is set too high for the air gap size and stator construction type, or there is a leakage path.

Generally ozone production is accompanied by soft blue glow of corona visible in total darkness after waiting 15 minutes for your eyes to adjust. Alternatively, you can use a microphone/amplifier/headphones setup to listen for the tell tale wooshing sound of corona discharge.

bentoronto 10th August 2012 01:30 PM


Ozone is not produced unless there is voltage discharge? There are no discharges except when you blow moist breath at the ESL panel? No random discharges?

Here's a scary article on ozone from EPA:
Ozone Generators that are Sold as Air Cleaners | Indoor Air | US Environmental Protection Agency


bolserst 10th August 2012 01:50 PM

To the best of my knowledge, ozone is only generated when the voltage potential between diaphragm and stator is such that ionization in the airgap occurs and the air becomes conductive. It might be just slightly conductive, like when you blow moist air into the airgap and hear hissing. But, that is enough.

Often ionization only occurrs in the middle of the panel where the diaphragm is closer to one of the stators, or in a localized area where there are particularly sharp edges on the stators. In my experience, smooth wire stators are much less prone to ozone generation.

It can also be conduction through surface leakage paths along foam spacers or stator frames.

bentoronto 10th August 2012 02:04 PM

I'm not sure what comes first. But voltage flows through air when the voltage gradient is great enough to ionize the air molecules, esp. oxygen. That means it is making ozone. So there is no current flowing and no breakdown at lower voltages and no ozone?

But when you breathe moist air, the breakdown voltage becomes very low but I suppose it is still ionizing the molecules between the water droplets. So it is making a little bit of ozone.

I guess I am wondering if an ESL panel can be entirely free of ozone generation, even when mild bias voltage is used?


PChi 10th August 2012 02:37 PM

I used to have a pair of Quad ESL 57 and never smelt ozone in the 15 years of ownership.. I believe that the QUAD used covers to prevent dust getting in which would also prevent ozone getting out. I never had any problem or took them apart so I can't confirm that though.
I would have thought that any ozone production is low though because the resistive coating was such that the bias had to be on for many minutes before the speakers worked properly.

bear 10th August 2012 03:47 PM

nil ozone from properly operating ESLs, afaik.

You need a discharge for ozone... the whole idea behind ESLs is to provide a charge without arcing, thus the coatings on stators AND the very high Z diaphragm material, and the high Z (megohm) series resistor with the bias...


Andersonix 10th August 2012 11:26 PM

I can smell ozone after playing my ML SL3 very loudly (but you have to put your nose right up to them). There's no indication of any arcing or other problems....

bolserst 12th August 2012 06:48 PM

Out of curiosity, what is the output capability of your amplifier?
Did you have the MOSFET limiter hooked up? Or disconnected.

Andersonix 12th August 2012 08:34 PM

Voltage rails: +/-52VDC supplied through 3A SB fuses. (Stock Acurus amp came with 63V rails and 6A FB fuses.)
Limiter is connected. I am never anywhere near the panels' limits; it's always the woofers that surrender first...

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