Thread: How to determine RPM of small rotating drum

Been a while since I studied physics at uni.
What I need to know is,'is the frequency produced by a small rotating drum (up to 20000rpms) in hz, the same as it's rotations per second?

EDIT : Or to be more precise is it's predominant frequency in hz a fundamental or harmonic of its revs/sec?

frequency of what- the noise, reflection,etc

Frequency or pitch of the noise produced. More specifically this kind of noise



http://www.youtube.com/watch?v=7cr4DSE55Uc

How the hell do you work that out? Ok, maybe I should have put it in the Biology section.
Is it because my post isn't as advanced as you are used to, or isn't post-Newtonian enough for you?
Your post has put me right off this forum and I've only just joined it!
Maybe if you are such an expert you could have answered the question instead of resorting to that kind of nonsense, or kindly suggested it gets moved to another section.

'The science of matter and energy and of interactions between the two, grouped in traditional fields such as Acoustics, Optics, Mechanics, Thermodynamics, and Electromagnetism, as well as in modern extensions including Atomic and Nuclear Physics, Cryogenics, Solid-state Physics, Particle Physics, and Plasma Physics'

How does my question not qualify?

You don't seem to grasp that it actually is a physics related question.

Maybe the moderator on this forum can determine if it belongs here. If he or she thinks not then that's fine, I will go to another forum where people are more helpful.

By the way, most people would say, 'most stupid', not stupidest. While it is a word, it isn't the best use of the english language.

Originally Posted by MeteorWayne

I see you edited you original post to make it less stupider

What do you mean by the "frequency" of the drum. The sound frequency, the light frequency, the electromagnetic spectrum frequency (other than light)?

Wayne, if you're going to be so aggressive toward someone and accuse them of being stupid, you should really try to avoid making yourself look stupid when doing so. That's just a nickel's worth of free advise for you. Your question was answered already above when he said:

Frequency or pitch of the noise produced.

So, how about you either help him answer the question, or go away until someone who can decides to post?

Originally Posted by MeteorWayne

But what would produce the noise? If it's a perfectly smooth drum, it wouldn't by itself create any noise at all. The only source of noise would be surface imperfections, bearings, or something else that would regularly create sound waves.

And BTW, the youtube link was when I called it stupid

It is a fishing reel -- full of gears, bearings and a drag mechanism. The rotational speed of the spool will increase as line is removed and the diameter of the wound line decreases. Casting Reels scream like crazy when a big fish is stripping line. There will be no single sound "frequency, but a complex waveform.

Ok cheers for the replies. Basically it is to do with long distance tournament casting. When the cast is made the spool is in free rotation mounted on 2 ball races. So yeah it may be a complex waveform. And because the spool is filled with line by hand it is hard to balance the spool perfectly. I just want to analyse the speed of roation of the spool by recording the audio of the noise it produces and put it thru a frequency analyser.
As the lead slows down due to air resistance the spool is braked by the induction of a magnetic field which is adjustable via a knob on the end of the reel. Other wise the spool intertia continues to spill off line at a rate faster than it being taken off by the lead and it overuns and tangles up.

If you could manage to cast your lure through a chronograph you could get the speed, then calculate the frequency from the diameter of the outer wrap of line on the spool. You could compare that to the frequency spectrum and calibrate it in that way. If it's too hard to use the chronograph, you could measure the distance of the cast, divide by the time of travel, and get the average speed. Then you could assume a constant acceleration to calculate the speed of the lure as it comes off the rod. This would give you a ballpark estimate for speed which you could then used to calculate the rotational speed and compare to the frequency spectrum to see what kind of harmonics there are.

When you say chronograph do you mean a stopwatch? It is generally a 150g lead weight we cast up to 250,260 metres. Takes about 8 secs so the average horizontal velocity is 30 odd m/s. Like you say I know the circumference of the outer wrap of line which is 12.5cm. So ultimately I want to know the actual velocity the lead leaves the rod tip not just the horizontal component. It is for when I'm practicing when there is no way of measuring the distance travelled to indicate whether I am improving or not. For instance practicing casting out to sea. Usually we do it over grass with a measure 30 degree V court similar to javelin.
Atmospheric conditions play a huge role in the distance achieved. A still day with high air pressure I might only achieve 220m. On a very breezy day I might achieve 250m. Therefore distance cast is not an accurate indicator of good technique/power.
Thats why I am trying to work out for every cast via some kind of pickup what the spool revs are to work out launch velocity. It is a more accurate way of determining how well you are casting.
Any way I was down the beach there armed with a digital camera and am going to analyse the audio later.

Originally Posted by Stevier

When you say chronograph do you mean a stopwatch? It is generally a 150g lead weight we cast up to 250,260 metres. Takes about 8 secs so the average horizontal velocity is 30 odd m/s. Like you say I know the circumference of the outer wrap of line which is 12.5cm. So ultimately I want to know the actual velocity the lead leaves the rod tip not just the horizontal component. It is for when I'm practicing when there is no way of measuring the distance travelled to indicate whether I am improving or not. For instance practicing casting out to sea. Usually we do it over grass with a measure 30 degree V court similar to javelin.
Atmospheric conditions play a huge role in the distance achieved. A still day with high air pressure I might only achieve 220m. On a very breezy day I might achieve 250m. Therefore distance cast is not an accurate indicator of good technique/power.
Thats why I am trying to work out for every cast via some kind of pickup what the spool revs are to work out launch velocity. It is a more accurate way of determining how well you are casting.
Any way I was down the beach there armed with a digital camera and am going to analyse the audio later.

It would be difficult to use chronograph in that application.

http://www.oehler-research.com/

What would work is a hand-held radar gun.

http://www.opticsplanet.net/radar-gu...FRtKgwod4DRCSQ

Originally Posted by MeteorWayne

A stroboscope is designed for that specific purpose But it only works well for an object rotating at a constant speed.

It also requires features on the rotating object that are amenable to being "frozen" by the strobe, and time to dial in the strobe frequency. It helps if the experiment is not carried out in bright sunlight.

A hand held radar gun is the standard piece of equipment for setting target speeds in trap and skeet. They are effective and affordable.