The discussion revolves around determining the RPM of a CD through audio analysis, focusing on the relationship between sound frequency and the rotational speed of the CD. Participants explore various aspects of audio signals produced by the CD player, including mechanical noise and the implications of frequency analysis.
Participants express differing views on the relationship between sound frequency and RPM, with no consensus reached on whether one revolution corresponds to one cycle of sound frequency. The discussion remains unresolved regarding the implications of mechanical noise and the nature of disalignments affecting sound production.
Participants highlight potential limitations in the assumptions made about the relationship between sound frequency and RPM, as well as the influence of mechanical factors that may introduce additional frequencies.
Plotted a small segment of audio just before pop in Audacity http://i.imgur.com/R4nXf87.png
Peak at 456Hz, multiplying by 60 gives 27360RPM
Plotted a small segment of audio just before pop in Audacity
jtbell said:I can't find the comment [on YouTube] that this comes from, but the audio must refer to the mechanical noise made by the device that's rotating it, not to the sound encoded on the CD.
jtbell said:I'd expect that sound to have a strong component corresponding to the rotational speed, from vibration caused by a slightly non-uniform mass distribution.
Generally speaking, one revolution is one cycle in the frequency - generally because disalignments in the rotating mass will force the mechanism to viberate. However, there can be gear mechanisms, other mechanical things that might have viberations, gear noise, etc which generates other frequencies. Multi cylinder engines might generate frequencies higher than what the revolutions is as the pistons combined fires several times pr. revolution.InvalidID said:Why is one revolution one cycle in the sound frequency?
Low-Q said:disalignments in the rotating mass will force the mechanism to viberate.
1Hz will be in the frequency spectrum, and probably 2Hz and some higher frequencies are there as well.InvalidID said:What exactly determines the frequency of sound? If I have a mass on a spring that oscillates from equilibrium from one maximum to the other and back once every second, will the frequency of the sound be 1 Hz?
The center of the CD is not exactly the center of the rotation, and the CD itself is not a perfect disk as well.What kind of disalignments are we speaking about here?
Just small disalignments. Even a flywheel which seams to be perfectly round, perfectly centered on the axis, will make a audioable viberation when the RPM gets high enough. And this small inaccuracies will force the rotating mass to wobble a tiny bit, and make a sound with a frequency (keynote) that is exactly the same as the RPM/sec.InvalidID said:What kind of disalignments are we speaking about here?