Shift in vibrating resonance with decreasing amplitude. - Page 1
Forum section: electronics / general

author: Circlotron
author_rank: Super Contributor
timestamp: April 11, 2023, 12:20:02 am
content: Plugged my guitar into one channel of the scope, as you do, and the sig gen into the other channel. Synced to the sig gen and adjusted the pitch of the strings so that the plucked note trace drifted neither to the right or left. What I noticed is that the frequency of the plucked note would initially be dead on, then after maybe half a second it would drift to the right because it's frequency had dropped slightly. Why would this happen? I would have expected it to stay constant.

author: ejeffrey
author_rank: Super Contributor
timestamp: April 11, 2023, 12:48:05 am
content: Because the frequency depends on tension and the tension depends on amplitude. The larger the amplitude the longer the string and the higher the tension.

author: Circlotron
author_rank: Super Contributor
timestamp: April 11, 2023, 01:38:16 am
content: Okay. I note that the frequency of oscillation of a pendulum is somewhat amplitude dependent as well, but for different reasons. But physics being what it is, maybe there is a relationship.https://en.wikipedia.org/wiki/Pendulum#Period_of_oscillationIf this is the case then, this begs the question - why doesn't an LC tuned circuit behave this way?

author: Tomorokoshi
author_rank: Super Contributor
timestamp: April 11, 2023, 01:53:43 am
content: Is it a coiled steel string, a steel wire string, or a nylon string?

author: twospoons
author_rank: Frequent Contributor
timestamp: April 11, 2023, 02:30:55 am
content: Quote from: Circlotron on April 11, 2023, 01:38:16 amIf L and/or C are non-linear then this would happen to LC tuned circuits too. e.g inductor core heading towards saturation at peak current. One outcome of this is the oscillation is no-longer purely sinusoidal.

author: Circlotron
author_rank: Super Contributor
timestamp: April 11, 2023, 02:32:14 am
content: Quote from: Tomorokoshi on April 11, 2023, 01:53:43 amBoth wound and plain steel wire strings did it.

author: Circlotron
author_rank: Super Contributor
timestamp: April 11, 2023, 02:35:33 am
content: Quote from: twospoons on April 11, 2023, 02:30:55 amYes. I have noticed this while trying to measure the inductance of a powdered iron toroid by parallel resonating it with a cap and feeding it with a sine wave in series with a high resistance. Resonant frequency varied from 25kHz to 33kHz depending on amplitude, but with relatively low drive. Didn't noticeably vary from sinusoidal though. May have done if I pumped it hard enough.

author: porter
author_rank: Regular Contributor
timestamp: April 11, 2023, 03:54:42 am
content: I've always wondered about guitar string dynamics. Some heavy duty scholars are doing research.Looking around the internet for a bit I came across thishttps://www.researchgate.net/publication/263471653_The_plucked_string_An_example_of_non-normal_dynamicsThat begins with this paragraph:The plucked string: an example of non-normal dynamicsDavid Politzer∗California Institute of Technology, 452-48 Caltech, Pasadena, CA 91125(Received June 26, 2014; Accepted)AbstractThe motion of a single Fourier mode of the plucked string is an example of transient, free decayof linear, coupled, damped oscillators. It shares the rarely discussed features of the generic case,e.g., possessing a complete set of non-orthogonal eigenvectors and no normal modes, but it canbe analyzed and solved analytically by hand in an approximation that is appropriate to musicalinstruments’ plucked strings.

author: ejeffrey
author_rank: Super Contributor
timestamp: April 11, 2023, 05:22:47 am
content: Quote from: Circlotron on April 11, 2023, 01:38:16 amBoth are non-linear at large amplitude but it's a different relationship and the opposite sign.QuoteIf you use a varactor or an inductor with a core that saturates you will definitely see frequency shift with amplitude. Of course you might design your circuit to minimize this effect, but it's still there.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 11, 2023, 07:13:15 am
content: Indeed if you excite a nonlinear resonator, you'll find a hysteresis loop as it follows resonance as you sweep up (to a point), but jumps through with much less of a peak as you sweep down.Tim