How can sound be used to calculate the RPM of a yarn bubble formed by rotating or spinning yarn at high speed? I need to know how to use sound to find the rpm of a "yarn bubble."  A yarn bubble is formed as yarn is twisted together at high speeds.  The yarn is then "heat set" and made into carpet.  A yarn bubble is formed as one strand of yarn is twisted onto another strand of yarn.  I need to find out how to detect the rpm of one yarn as it spins around another yarn.   
 A: Find a frequency of sound that, when it encounters the yarn it will bounce off and the reflection can be captured. In other words, a frequency where you can capture an echo from the yarn. 
Then, set up a speaker to transmit that known frequency in pulses. When the speaker is not transmitting (i.e. Between pulses) listen for the echo.  Aim the speaker such that the yarn that is rotating gets closer and further from the speaker as it rotates.  Set the yarn in motion and start transmitting the pulses of sound at least 10+ times as fast as the time it takes the yarn to rotate around once. 
As the yarn rotates and pulses of sound and echoes are collected, observe the difference between the transmitted frequency and the echo received. When the rotating yarn is coming towards the echo receiver, the apparent frequency should be higher than the transmitted frequency. When the yarn is moving away, the echo frequency should be lower. This is the Doppler effect. 
Based on the amount of time between pulses and the times when you detect a "high" echo, you can count the number of "high" echos compared to "low" echos per second and therefore calculate RPM. 
Note that if your pulses are too slow compared to the spinning yarn it is possible to arrive at multiple solutions. To rule out the false solutions you could vary the pulse rate in a controlled way.
