surely, very high frequency noise could manifest itself in way that looks like lower frequency noise?

Question

say you have a noisy, very high frequency, electronic environment

that noise could bunch together / overlap to look like lower frequency noise - a kind of frequency modulation perhaps

so taking measures to reduce the very high frequency noise may be useful if that noise is appearing like lower frequency noise - and consequently having the same detrimental effects as lower frequency noise?!?

know what I mean???

Answer 1

It's called intermodulation and it manifests itself as the sum and difference of two frequencies so 16khz and 15.5khz would produce 500HZ and 31.5Khz .

Answer 2

There are two things you could be talking about.



One is that when you add two frequencies together, you get beats or the appearance of another frequency that is lower than the original frequencies. This is not a true frequency component unless you are working with a nonlinear system such as an unbiased diode. Thus, unless you have diode mixing or another nonlinear process going on, elimination of the high frequencies will also remove the apparent low frequency.



The other thing you could be talking about could be aliasing -- if you sample at a rate lower than twice the highest frequency component in the incoming signal, then any frequency components higher than half of the sampling rate will show up as lower frequencies in your sampled signal. The ways to get rid of that are to increase your sampling rate, filter your incoming signal so it has no frequency components greater than half the sampling rate, or assume that the signal is periodic and you don't care about phase and just vary the sampling rate and any frequency components that change when you change the sampling rate must be higher than your sampling rate cutoff.

Answer 3

I know watcha mean mate. However some years ago I did a study on computer fans (sad I know) and even though they were chucking the same airflow through per minute fans with a lower centre frequency (about 256Hz) sounded quieter than fans that had a centre frequency of 400Hz. I used an audio spectrum analyser and the noise power envelope for both fans was approx the same it was just the "apparent" noise to the lughole that made the 400Hz fan sound louder.



I tried all sort of garbage like blinds, filters and funnels but only reducing the frequency of the noise (slower bigger fan and wider venturi) worked. Having some sort of frequency changing lump of plastic on the back of the unit would have been undesirable and expensive.



Thermal control of fans works well for noise reduction and there used to be a good market for add in thermally controlled fan controllers but this seems to have gone by the wayside for equipment which is stuck in a computer room environment and in general for motor driven stuff in personal computers. Friend of mine has a tower PC with drives that would wake the dead.

Answer 4

That sounds like the diffraction interference patterns that you get with slightly inclined gratings. Seem logical that reducing either of the high frequency sources will impact the pseudo low frequency. I seem to remember that the usual way to deal with noise in amplifiers is the take a sample of the output, invert it and feed it back to the input. Something like that. More of a sticky bun bear really. Built my own torch once out of lemons & copper wire. Learning to live in darkness.