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Who says it doesn't? I suppose ideally a tuning fork describes simple harmonic motion, but I'm not sure that's true in practice.
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I always thought they didn't except the 6th partial you can hear from a distance as the "ping" when you strike the fork. However, recently, I proved to myself that it does have a weak second partial that has no measurable inharmonicity. As for why that is, I don't know the physics of it. However, for all practical purposes, the tone you hear as you listen close to your ear is about as "pure" (fundamental only) as you can get.
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The tuning fork can have many different modes of vibration. Some higher partials can be amplified a bit by striking one of the tines at an angle rather than in line with the other tine. You can also amplify the fork by placing a dividing plate between the tines to prevent cancelling effect. You can also attach it to a string by the handle and dangle it freely to get more higher partials. Check out this video and you will see how the dampening effect of your fingers effectively cuts out most of the partials other than the fundamental: http://www.youtube.com/watch?v=m7xUtR2qevA
Piano Technician George Brown College /85 Niagara Region
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A tuning fork is constructed so that only one mode of vibration occurs with great efficiency. A fork is very close to the ideal mass/spring resonant system. However there is usually one or more very high-pitched pings, as Bill described. The pitch of those pings is different from one fork to another, and is related to different modes of vibration. The strength of those pings depends on the stiffness of the object it strikes and the place on the fork where it strikes. If you strike a fork against an object with just the right amount of "give" and at just the right place on the fork, this ping will be totally silent. (I use my knee, but I don't want to encourage anyone to hurt themselves!) As for 2nd and 3rd partials, these are not the result of different modes of vibration. They are artifacts of distortion that occurs in whatever is holding the fork, and even in your ears. Any distortion from a perfect sine wave adds pure harmonics.
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Wow, I just love these kinds of things. This one is now on my facebook page. Thanks!
Don Mannino RPT, MPA
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YVW Don. Incidentally, if I analyze the spectrum reading of my A440 tuning fork with RCT I get a reading on what would be the second partial for a sustain that dies out very quickly, and at about 1/10th of the amplitude of the fundamental. Although it shows a tiny bit of a reading of what would be the 3rd partial it is so insignificant there is no data related to it.
I did notice I get a slight difference in the pitch depending on which tine of the fork I hold towards the mike. Could this be a slight imbalance in the lengths of the tines? It is a very small difference that my ear does not hear as a beating, but the ETD does show them as slightly off from each other by an amount of .1-.15 Hz (using averaging).
Piano Technician George Brown College /85 Niagara Region
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Interesting that your fingers can damp unwanted partials. Conversely, I've heard that holding the fork against another object (like the underside of the keybed) can amplify those partials.
--Cy--
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Another interesting thing to note about the fork's partials is that they are not even close to being whole number nultiples of the fundamental frequency. "Musical" vibrators, or those that do produce whole-number multiples, are really a subclass of all the things that can vibrate. As an exmaple, almost any type of beam will produce non integral partials.
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Piano
by Gino2 - 04/17/24 02:34 PM
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Piano
by Gino2 - 04/17/24 02:23 PM
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