can we calculate velocity of a spinning apparatus by just analysing the frequency of sound emitted by it I have used a new term   spinning apparatus as I was unable to name it.
I have tied a thread to a stone and was spinning it and I heard a sound something like that of a rotating propellor of a helicopter and then this idea stuck my mind 
can i calculate it? if yes then how can any one suggest me something and also if any extra information any one needs regarding that apparatus feel free to comment
 A: If you frequency analyse the sound from your equipment then the fundamental frequency will the same as the rotational frequency. You could record the sound and use some software like Audacity to do the frequency analysis. This is exactly what alemi did in his reply to Can I compute the mass of a coin based on the sound of its fall?. Alternatively, and inevitably, there is an app for that.
A: looking to the doppler effect: frequency equals (the speed of sound in what ever temperature air you are in: $c$)+(Velocity of you relative to the measured object: so if you are stationary this value is zero: $V(r)$)/(speed of sound in what ever temperature air you are in: $c$)+(Velocity of object relative to you $V(s)$: essentially the speed of the object you are measuring)=frequency: $f$
$f=(c+V(r))/(c+V(s))$; this rewrites to: $V(s)=(V(r)+c(1-f))/f$; if $V(r)=0$, then $V(s)=(c/f)-1$; resolving for f: $f=c/(V(s)+1)$
your $V(s)$ should equal: $(2πL_{rope})/(t_{2π})=V(s): (2πr)/t=V(s)$
Next comes the experimental set up. Your observer should be placed so that it forms a square with side lengths equaling the radius of your rope. This is so that when the object is coming directly towards the observer, and after π/2 interval of it's rotation is going directly away from you. With these two points of data we can extrapolate: $f=c/(c-V(s))$ when it is coming at you, and $f=c/(c-V(s))$ when going away from you. If this does not give the same V(s) after input of the observed frequency, then your object probably can't be solved for.
if you observe all the frequency over all time it should be semetric. Spendinding equal times in red/blue shift. Solve for the time when your object is at both points so you can know where to look in your data.
c in this: $c=331.4+.6(T°C)$
