You can leave plasma flashing away while you move about the house, and it will have some effect, but not anywhere near the same as if you're sitting or lying right next. If you're planning to buy it, then be prepared to get the best out of it by using it in the near field (within 6 inches), otherwise, you're really not getting value for money.
With regard to noise, we use plasma out of the Spooky Central unit. There is a fan "whir" which is undisturbing to us, but I believe the Spooky Plasma unit's fan is quieter, or so it's said. As for frequency noise, it depends on the frequency. Occasionally we hear it, but never to an irritating degree.
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