The following is from Thorsten, one of our designers. And from an objective, electrical point of view. So please bear with us!
https://www.technologyfactory.eu/en/blue-horizon/blue-horizon-mains-noise-analyser/a-5717-320
This does not strictly measure mains noise, but distortion on the mains sine wave in the audio band, which is dominated by low order harmonics. It actually says so in the small print on the case.
This kind of measurement device is particularly useless, as the normal rectifier inside equipment will create exactly the same kind of distortion so it must be well filtered anyway to not affect the device’s performance. What is more, the “meter” reading is dominated by low frequencies, so it cannot show improvements at higher frequencies!
In other words, it does not detect “any change” but on “certain specific changes”, which do not apply to the iFi product in question.
The AC iPurifier/PowerStation operates only for high frequencies above the audio range, primarily for the noise generated by the now ubiquitous switching power supplies above 10kHz to 10’s of MHz and up.
For testing, we use a so-called LISN (Line Impedance Stabilisation Network – e.g.: https://www.com-power.com/lisns.html ) and a 5GHz spectrum analyser with tracking generator (e.g. similar to this:
https://www.rohde-schwarz.com/sg/product/fsl-productstartpage_63493-8042.html).
Past that the design of an actual electrical distribution system (solid bus bars) is based on minimising added impedance in line with mains and earth connections. Doing so avoids problems caused by circulating ground leakage currents.
