If we new the number of padels then it would be possible to calculate the rpm from the frequency of the sound.
Bill
There are sixteen (16) vanes. Working out that calculation would be wayyyyy above my pay grade.
OK . Assume 10,000 RPM
Then the frequency is 10000 / 60 or 167 Hz ( Ie Revs. per SECOND )
But we have 16 vanes so the freq. then becomes 16 x 1667 = 2667 Hz
From this we can derive ( Freq. / # Vanes ) = ( RPM / 60 )
OR ( pertaining to this case )
RPM = (2667 x 60) / 16
Check:
2667 x 60 = 160,020 ..... Divide by 16 ....
10,001 ... which is where we started ..
Usual truncation errors
Dave
Addendum ... Don't the aero geezers use / did use acoustic tachometers ?