Re: [AUDITORY] Sound proof booth and EEG recording [the revenge] ("Alain de Cheveigne'" )

Subject: Re: [AUDITORY] Sound proof booth and EEG recording [the revenge]
From:    "Alain de Cheveigne'"  <>
Date:    Fri, 7 Jun 2013 11:16:55 +0200

A few comments on suggestions so far. (1) Patch-panel vs holes. A patch panel is tidy, a hole is = future-proof. I'd go for the hole, as a sound booth is a long-term = investment. It shouldn't have too much impact on acoustic isolation, if = properly designed.=20 (2) The hole. Make it large enough to pass the largest expected = connector (e.g. a UK plug). For a double-walled booth there is little = benefit in offsetting the holes in the inner and outer shells, because = this makes little difference at low frequencies. It is important to = make the holes air-tight after passing the cables. A sand-box on each = side sounds like an effective solution, but probably a bit of a hassle = when passing cables. IAC suggests filling the space around the cables = in the hole with "pugging" (compressible foam or clay). For a set of = booths that we're ordering, I asked them to equip each hole with a box = with screw-on cover to allow the cables to be laid flat for a more = effective pugging. Someone suggested more than one hole to separate AC = cables from the rest : good idea. (3) AC power in the booth. If possible, ban it. Use filtered DC for = the lights. Don't install AC outlets in the booth (if you need AC at a = later time, you can pass an extension via the hole).=20 (4) Equipment. If possible, ban AC-powered stuff from inside the booth, = as well as things such as displays that can produce lots of = electromagnetic interference components. Equip your booth with a window = such that you can place a monitor outside visible to the subject. (5) Shielding. There are several things to consider. Major sources of = EM (electromagnetic) interference are AC power lines, electronic = equipment (computers, displays, switching power supplies), and radio = waves (e.g. cell phones and cell phone relays). The EM field includes = both an electric and a magnetic component. For low frequencies (power = lines) the E and M are independent, for higher frequencies they are not, = so shielding requirements vary according to the interference. For power line interference, the electric component can be blocked by = any metal shield, for example the steel skin of sound booth. The = electric component is usually the main culprit, and this may be quite = sufficient to block it. The magnetic component, if present, is much = harder to block. Blocking it requires mu-metal shielding as used in = MEG. In practice: (a) install the booth well away from large equipment = such as elevator motor or transformer station, and (b) reduce the = surface of any loops in your cable (e.g. run the electrode leads = together in a bunch). For power line interference, a Faraday cage = should not provide any advantage over a metal-skinned booth. I doubt = however that it could make things worse by "trapping interference". Radio-frequency EM interference is a bit trickier. You might think it's = not an issue, because EEG amplifiers are equiped with low-pass filters = with a much lower cutoff. The problem is that high-frequency power can = be demodulated by nonlinearities, for example at the electrode-to-skin = contact, or overloading of the electronics. It's hard to diagnose, as = the interference components in the EEG bear no obvious frequency = relation to the interference itself. You need specialized equipment to = sample the EM field itself. The Faraday cage should help shield from = this kind of intereference. (6) Choose your EEG equipment carefully. Some systems are inherently = less susceptible to interference, at least in principle. The Biosemi = website has pointers to some interesting documents that explain some of = the issues.=20 (7) A general recommendation: know your noise. Try to understand it, = before trying to remove it. That should be easier than discovering = what goes on within a listener's brain... Alain On 7 Jun 2013, at 09:24, Massimo Grassi <massimo.grassi@xxxxxxxx> wrote: > Dear list member, >=20 > the question I submitted the other day raised an unexpected response: = many of the people I came in contact with were actually *unhappy* of the = electrical shielding of the booth. >=20 > I quote here the words of Dave Hairston: "While having a fantastic = Faraday cage is theoretically great, in reality in can cause as many = problems as it solves. For instance we have discovered that in many = cases if you have ANY electrical devices inside, such as monitors, = keyboards, whatever, it traps and bounces the power line noise much = worse than just doing in a normal office room." >=20 > Here, we are definitely going to take some electrical device inside = the booth. Because the electrical shielding has a cost (about 5000 = euros) I'm now wandering whether it is worth to spend this money. >=20 > Here comes the question (addressed to those like me that must take = some electrical devices inside the booth): did the electrical shielding = of your booth improve substantially the quality of your recordings? >=20 > Thank you all in advance, > m >=20 > --=20 > > > >=20

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Electrical Engineering Dept., Columbia University