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Hi Massimo,
I worked with an ETS Lindgren room at UC Davis with serious shielding. It blocked mobile phone signals even in the control room about 4 m from the cage. This is overkill as the brain signals of interest are of a much lower frequency. There were some high frequency oscillation in the EEG with a fundamental around 1.6 kHz, which whilst not an issue for auditory long latency responses, became more problematic when recording at higher sampling rates, e.g. for ABR, as visible in: We also recorded "EEG" from a bucket of salt water and still found the oscillations were still there.Campbell, T.A., Kerlin, J.K., Bishop C.W., & Miller, L.M. (2012). Methods to eliminate stimulus transduction artifact from insert earphones during electroencephalography. Ear & Hearing, 33, 44-50. doi: 10.1097/AUD.0b013e3182280353 So we stripped the lab of equipment and went in with an oscilloscope and antenna. Whether using a power source for the oscilloscope inside or outside the cage, there it was, the high frequency oscillation. We contacted facilities to verify if there was a common ground in the control room, inside the cage and for the cage. Apparently there had been a recent change and the cage had been given a separate ground. Something to do with the building being struck by lightning and protecting equipment and people touching that equipment, such as an EEG participant. The head of the lab, a physicist, used the _expression_ of "chasing a phantom", at which point I visualised the omnipresent source of the high frequency oscillation haunting me day and night, and laughing at me. So the rooms can have problems and the extent of the problem can vary with what you do in the lab. We did not, however compare responses inside and outside the shielded room, as is a very interesting question. Shielding needs to be grounded for the cage to be effective. Also, it is worth emphasising, the ideal is that you have a common ground for all electrical equipment near or in the lab and this is the ground of the cage, assuming you never have a participant connected to electrical equipment with that ground in a thunderstorm. You may be able to improve the effectiveness of the cage by the removal of a green copper oxide that can appear on the fingers of the door of an ETS lindgren shielded room. This can be done by regular cleaning off with rubbing alcohol or with a severe build up sanding with emory paper. This might reduce the hysteresis problem of the charge carriers getting trapped that one of the other responses mentioned, as will be particularly severe if the cage is not properly grounded. There is a tendency now to do more EEG research outside of the shielded room with many different kinds of EEG system. I have heard a distributor of a high cost systems with active electrodes suggest that there were no appreciable signal:noise gain to be found with a shielded room and such a system. However, I know of no systematic independent study that compares auditory evoked potential signal:noise with such a system inside or outside well-maintained shielding with a common ground with electrical equipment. It would be of practical and financial significance to compare the signal-to-noise of evoked potentials recorded with different EEG systems with passive and active electrodes under these different conditions of no shielding versus well-maintained shielding with a common ground. I'd be very interested what you hear of people's experiences. Best regards, Tom. > Date: Sat, 8 Jun 2013 00:01:11 -0400 > From: LISTSERV@xxxxxxxxxxxxxxx > Subject: AUDITORY Digest - 7 Jun 2013 (#2013-135) > To: AUDITORY@xxxxxxxxxxxxxxx > > There are 2 messages totalling 153 lines in this issue. > > Topics of the day: > > 1. Sound proof booth and EEG recording [the revenge] (2) > > ---------------------------------------------------------------------- > > Date: Fri, 7 Jun 2013 09:24:56 +0200 > From: Massimo Grassi <massimo.grassi@xxxxxxxx> > Subject: Sound proof booth and EEG recording [the revenge] > > 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, > m > > -- > http://www.psy.unipd.it/~grassi/ > http://www.springer.com/978-1-4614-2196-2 > http://www.finveneto.it/nuoto_schedaatleta.php?id_atleta=73076 > > ------------------------------ > > Date: Fri, 7 Jun 2013 11:16:55 +0200 > From: Alain de Cheveigne' <alain.de.cheveigne@xxxxxx> > Subject: Re: 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.=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 > > http://www.psy.unipd.it/~grassi/ > > http://www.springer.com/978-1-4614-2196-2 > > http://www.finveneto.it/nuoto_schedaatleta.php?id_atleta=3D73076 > >=20 > > ------------------------------ > > End of AUDITORY Digest - 7 Jun 2013 (#2013-135) > *********************************************** |