Re: Question about electrical shielding

[Bob Masta <audio@xxxxxxxxxxx>]

On 14 Dec 2007 at 19:58, Katrin Krumbholz wrote:

> Dear All, 
> 
> we may be looking to buy a new sound-booth soon, which we want to use for
> EEG and which should thus have "electrical shielding". I have asked IAC and
> they said they build electrically shielded booths, but I am not sure whether
> they would be the best. Thus I wonder whether anybody could give me any
> advice on the following two questions:
> 
> (i) Which company would you recommend us to approach?
> (ii) What do you think are the requirements in terms of shielding for EEG? A
> norm published by the German Association for Electrical, Electronic &
> Information Technologies (VDE) says that noise levels should not be higher
> than Bpp (peak-to-peak amplitude of magnetic field) < 2e-7 T. However, some
> people seem to think that that limit is too lenient. 
> 

In addition to Ralph's suggestion about "rolling your own", 
I'd like to add that if the EEGs you will be recording are 
evoked potentials that involve lots of synchronous 
averaging (ABR, etc) then you may be able to get by with 
relaxed electrical noise specs.  Most electrical noise will 
be at power line frequencies or multiples thereof, and 
unless for some reason you go out of your way to make your 
stimulus synchronous with those, they will tend to average 
away with the rest of the noise.  In problem cases, you 
might actually want to make your stimulus in synchrony with 
the interference source, and sync half of the frames you 
average in opposite phase to provide near-perfect 
cancellation.

However, the electrical interference does set a limit to 
how much gain you can run on your preamps.  The noise
(and/or gain) must be low enough that the preamp is not
driven into clipping.  

Best regards,


Bob Masta
 
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