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Re: [AUDITORY] Sound proof booth and EEG recording [the revenge]
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.
(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).
(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.
(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...
On 7 Jun 2013, at 09:24, Massimo Grassi <massimo.grassi@xxxxxxxx> wrote:
> Dear list member,
> 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.
> 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."
> 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.
> 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?
> Thank you all in advance,