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Rigol DHO804 Test and Compare Thread

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Performa01:

--- Quote from: ebastler on December 17, 2023, 09:38:56 am ---I think that the noise spectra are hard to interpret and compare in general. Predicting from the spectra how annoying a particular fan sound will be perceived will only be possible with a lot of experience, at best.

--- End quote ---
Why do you think so? I happen to have some experience in the audio field, but even without that the fundamentals of human hearing together with a look at the Fletcher-Munson curves would be enough to interpret an audio spectrum correctly:

https://en.wikipedia.org/wiki/Equal-loudness_contour

We are talking about SPL (sound pressure levels) between 30 and 40 dB here. The threshold of human hearing at 50 Hz is at about 42 dB.

So why should I not be able to safely state that 32 dB @ 49 Hz is inaudible for the mere mortal?

34 dB @ 590 Hz on the other hand is equivalent to at least 30 phon – quite audible, independent from the age of a person.

ebastler:

--- Quote from: Performa01 on December 17, 2023, 10:16:00 am ---
--- Quote from: ebastler on December 17, 2023, 09:38:56 am ---I think that the noise spectra are hard to interpret and compare in general. Predicting from the spectra how annoying a particular fan sound will be perceived will only be possible with a lot of experience, at best.

--- End quote ---
Why do you think so? I happen to have some experience in the audio field, but even without that the fundamentals of human hearing together with a look at the Fletcher-Munson curves would be enough to interpret an audio spectrum correctly:

https://en.wikipedia.org/wiki/Equal-loudness_contour

We are talking about SPL (sound pressure levels) between 30 and 40 dB here. The threshold of human hearing at 50 Hz is at about 42 dB.
So why should I not be able to safely state that 32 dB @ 49 Hz is inaudible for the mere mortal?
34 dB @ 590 Hz on the other hand is equivalent to at least 30 phon – quite audible, independent from the age of a person.

--- End quote ---

"Inaudible" is fine, of course, and knowing or looking up a threshold for that is no problem. Having a good feel for how much more noticeable +3dB are (at various absolute levels) may already be less clear.

But what I was mainly thinking about are deviations from white or pink noise. We frequently read descriptions of (and complaints about) noise that is perceived as "whiny", "high pitched", "with a clicking noise" etc. To recognize the signatures of such noise components in the spectra is where I think a lot of experience is needed. And to assess from looking at the spectrum how obnoxious the noise will sound is probably impossible, since it is a rather subjective perception anyway.

Performa01:

--- Quote from: ebastler on December 17, 2023, 12:03:20 pm ---"Inaudible" is fine, of course, and knowing or looking up a threshold for that is no problem. Having a good feel for how much more noticeable +3dB are (at various absolute levels) may already be less clear.

--- End quote ---
Quite clear if one sticks to the fundamentals: an increase in 10 phone (above some 40 phone) is equivalent to a perceived doubling of the sound volume. Once again, the Fletcher-Munson curves help us to find the correlation of SPL and the loudness measured in phon. There’s usually no scaling, i.e. a 10 dB increase in the phon measure will correspond to 10 dB more SPL, but there is a frequency dependent shift in sensitivity.



--- Quote from: ebastler on December 17, 2023, 12:03:20 pm ---But what I was mainly thinking about are deviations from white or pink noise. We frequently read descriptions of (and complaints about) noise that is perceived as "whiny", "high pitched", "with a clicking noise" etc. To recognize the signatures of such noise components in the spectra is where I think a lot of experience is needed. And to assess from looking at the spectrum how obnoxious the noise will sound is probably impossible, since it is a rather subjective perception anyway.

--- End quote ---
I don’t even believe that it’s that subjective. Yes, white (and even more so pink) noise should be quite unobtrusive – and it shows, since it’s used for medical purposes in noise masker systems for tinnitus treatment.

What is more annoying are dominant frequencies, and these in turn all the more so as closer they are to the maximum sensitivity of the human ear, i.e. 3 kHz.

Even more annoying are low frequencies (which would be inaudible themselves) with very high harmonic content that is audible (“ticking sounds”).
Or any form of frequency modulated dominant tones (“whining noise”). Here it might be a little subjective indeed, which one are perceived as more annoying.

And for all types of unwanted sounds and noises it is clear that they are all the more annoying the louder they are.

core:
The noise spectrum plots are very interesting. Since local conditions cannot be easily compared, measurements taken very close to the fan may be most useful.

Personally, I would not change a fan without knowing that the new one does not have at least the same airflow or without comparing the measured temperature with a thermal imaging camera.

Also, the average temperature measured by a speed controller added later to a default installed fan doesn't tell much about certain hot spots like local power supplies, which can get too hot without a minimal airflow.

Fungus:

--- Quote from: core on December 17, 2023, 07:58:10 pm ---Personally, I would not change a fan without knowing that the new one does not have at least the same airflow or without comparing the measured temperature with a thermal imaging camera.

--- End quote ---

These have a single heatsink and can show their internal system temperature on screen. Thermal camera isn't really needed.

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