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Re: How many aftermarket cords have I heard? One is way too many.

" Mu Metal is really expensive!"

Tell me about it. Custom built enclosures for CRT workstations from Sun Microsystems and others about 12 year ago were $1000 each. External magnetic field strength was 20 milligauss 60 hz due to overhead utility power lines. Estimated current required to produce that field strength given the height and distance was 100 amps.

I visited a facility of a large Pharmaceutical company in Northern NJ. They had measured a couple of hundred milligauss in an office area directly above a substation. CRT images were unstable and the workers were anxious. Facilities management had a custom built mu metal enclosure built around a 4000 amp bus duct, the main source of the field. Cost was $100,000. It was not an entirely satisfactory shield. When I asked the engineer (a recent graduate of my alma mata) if he hadn't considered that the bus duct was no longer UL listed and would have to be derated, he had an "oh shit" day.

RF is relatively easy to shield although it can get expensive (built several RF shielded rooms for EMI emissions testing to verify FCC emissions radiation compliance.) Magnetic shielding is another matter altogether. You cannot shield a magnetic field with a filter. And as some others in my company learned the hard way, you can't interpose a sheet of mu metal either. It must be an enclosure which creates a magnetic shunt. This corresponds to Gauss' Law. Most times when people talk about noise from electromagnetic fields, they are really talking only about problems due to electrical fields.


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  • Re: How many aftermarket cords have I heard? One is way too many. - Soundmind 19:28:33 11/05/06 (0)


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