Subject: Re: Question about electrical shielding From: Bob Masta <audio@xxxxxxxx> Date: Sat, 15 Dec 2007 09:25:12 -0500 List-Archive:<http://lists.mcgill.ca/scripts/wa.exe?LIST=AUDITORY>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 D A Q A R T A Data AcQuisition And Real-Time Analysis www.daqarta.com Scope, Spectrum, Spectrogram, Signal Generator Science with your sound card!