Finding out fan RPM even with one wire or even w/o any? Yep, also possible
. . . For the first you need a properly tuned at least one (1) guitar chord/string, for the second a piano or an accordion ! There would be a need also of a small thin plastic tube too ! Some basic natural musical talent is involved & required also .

The procedure: first you have to count how many rotor blades the fan has & remember it. Than you start the fan to spin at desired speed & thru that mentioned thin plastic tube blow the air thru fans rotor blades spinning. Now you should hear a "windy" tone of certain height & remember it to compare it with some musical instrument tones like a guitar ones. When you find out to which musical note that fan tone corresponds, check the frequency of it on the lower partial chart !

[Another similar way is to take one meter of flexible 0,5cm diameter plastic tube & one end stick it in your ear & another end put it as close as possible to a rotating fans rotor blades.

  You should hear a weak tone; if two, remember a higher one for same procedure of finding out a frequency of tone! (if the tone is strong, could be that your head is resonating ... )]

If you are not certain if the note is correct, repeat the procedure (some people have difficulties to recognize proper tone hight because of tone harmonics - octave or third & fifth of it) !

The frequency you got you have to divide it with a number of fan rotor blades to get a proper number of fan rotor revolutions per second, which you than multiply with 60 to get a RPM number. Computer fans usually have 5,7,9 or 11 rotor blades ...

Example: 7-rotor blades TTvolcano-II fan at 12V gives close to tone Cis 2 (Des, C#), which is an octave higher (double frequency) than Cis 1 (277,2 Hz). So: [554,4 / 7] x 60 = 4.752 Rpm. Simple as that . . . . .


....Clicking the speaker icon, DownLoads & plays a tone (with a bit of delay, on Dial-Up few seconds )

. . . Tone & frequency, higher to lower . . . piano key color, guitar tone & string remarks
.E 2 .659,3 Hz .w. - (guitar 1st string = thinest, on 12th field pressed, octave up)
.A 1.....(standard !) .440,0 Hz .w -. ... .... ....... ..(guitar 1st string = on fifth field pressed)
.Gis /As (G#) .415,4 .b
.G .392,0 .w... - . (guitar 1st string = on third field pressed)
.Fis /Ges (F#) .370,0 .b
.F .349,2 .w
.E .329,7 .w - ....................(guitar 1st string = thinest empty)
.Dis /Es (D#) .311,2 .b
.D .293,8 .w... - ....(guitar 2nd string - on third field pressed)
.Cis /Des (C#) .277,2 .b
.C 1 .261,6 .w
.H .246,9 .w - ....................(guitar 2nd string = second thinest empty)
.B /Ais (A#) .233,1 .b .....
.A 0 .220,0 Hz .w
.Gis /As (G#) .207,7 .b
.G .196,0 .w - ....................(guitar 3th string = third thinest empty)
.Fis /Ges (F#) .185,0 .b
.F .174,6 .w -..
.E .164,8 .w
.Dis /Es (D#) .155,6 .b
.D .146,9 .w - ....................(guitar 4th string = third thickest empty)
.Cis /Des (C#) .138,6 .b
.C 0 .130,8 .w -. - ......IMHO third C from left on concert piano ...
.H .122,5 .w
.B /Ais (A#) .116,6 .b
.A -1 .110,0 Hz .w - ................. .(guitar 5th string = second thickest empty)
.E -1 ...82,4 Hz .w - .................... (guitar 6th string = thickest "empty" one)
As you can see, this upper chart only partially covers the audio/music spectrum, but with knowing that a tone difference of an octave up/down (once higher/lower tone) covers double/half of determined frequency, is no problem to calculate the rest of them ! This was discovered practically more than 2.000 years ago by famous old-Greek mathematician Pitagora, who found out, that at the same string tension if the string is shortened to half, it resonates at double vibration speed (frequency) = same tone but octave (once) higher! No, it was not the same man that found out that famous Pi number ( P = 3,14xx...) useful in geometry & more /that one was Archimed/ ... ... Similar is with lowering tones!

If you make some calculations, you can find out by yourself, that the two neighbor half-tones are approx. for factor 1,05922x...in frequency away from each other. And if you look at the chart with a head turned left closer to the left shoulder, you can realize, that piano key colors are in fact its keyboard (except first & last tone non correspondent) for better recognition of chart tones ...
 
This chart is also very useful to find out a frequency of a tone produced by some electronic audio oscillator if fiddling with that & not having some kind of frequency-meter ; hey & if having it, can be used also to tune a guitar !

 

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Zdenko Jerman-Spajky