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Old 01-25-2021, 12:56 PM
Tigro Tigro is offline
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Default Why are my strings sometimes vibrating slowly and sometimes not?

I suppose this is a weird question as I'm a newbie and can't really phrase it accurately I think but what I mean is, sometimes the strings on my guitar (the two thickest ones most notably, don't think I've noticed this phenomenon with others) have this nice, slow vibration (ie. the frequency of vibration slows down to a point where you can see the strings rocking back and forth, like in this video from John Fogerty for example) while other times, it just looks like a really quickly vibrating wire (which I get that the strings are but you know what I mean ). One day I can pluck the A and D strings together and the A would be vibrating with this nice, long, slow strokes while the D will just look like it's kind of "trembling" in place. A day after that when I pluck them, they may very well both have this high frequency vibration suddenly as if I flipped a switch.

I always tune my guitar before every session so it's not like some of the strings are off pitch anytime. And yet, sometimes when I play, this phenomenon does happen on the E and A strings, and sometimes it does not. If it does, I did not figure out a way to get these back to the regular vibration - if it does not, i did not find a way to make them

What's the reason behind this? I could not find anything as literally one day the strings behave this way, and the other day - the other way, despite me not having changed a thing in between and the guitar hanging on the wall untouched for that whole time.
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Old 01-25-2021, 01:05 PM
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srick srick is offline
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Tigro - What type of lighting do you have? LED bulbs and to some extent fluorescents strobe at 60 cycles. Does this happen in sunlight? How about with incandescent lights?

IOW - most of us don't notice this effect because of 'persistence of vision'. The strobing of the aformentioned bulbs would make it noticeable to the naked eye. Just a guess on my part.


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Rick
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Old 01-25-2021, 01:35 PM
Goat Mick Goat Mick is offline
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Also when you fret a string it shortens the length and the oscillations will be smaller. Also if your strings all moved at the same frequency of oscillation the notes would be all the same.
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Old 01-25-2021, 01:46 PM
Tigro Tigro is offline
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Quote:
Originally Posted by srick View Post
Tigro - What type of lighting do you have? LED bulbs and to some extent fluorescents strobe at 60 cycles. Does this happen in sunlight? How about with incandescent lights?

IOW - most of us don't notice this effect because of 'persistence of vision'. The strobing of the aformentioned bulbs would make it noticeable to the naked eye. Just a guess on my part.
Oh my, you're absolutely right! Thanks to your comment, I realised that every time I saw those slow vibrations was when I was playing in the living room - while all the others were in my bedroom. And they DO use different kind of lighting! Just went to the living room, plucked the E - slow vibration. Came back to the bedroom, plucked the E - quick vibration! Problem solved, haha Thanks a lot for the reply, never would have thought lighting would have anything to do here, was sure it was something regarding the guitar itself!
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Old 01-25-2021, 01:50 PM
Fatfinger McGee Fatfinger McGee is offline
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Persistence of vision is also why spoked wheels sometimes look like they're rotating backwards (stroposcopic effect).

Anyway, question already answered but it reminded me of this neat video:

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Old 01-25-2021, 01:57 PM
Earl49 Earl49 is offline
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The phenomena is called a "beat frequency". When two tones, or two vibrations, differ by just a few Hertz you will see the relative motion as if it were a strobe effect. Seeing it it depends on the lighting source, or the scan rate on a video. In 60 Hz electrical countries, the pulsation will be relative to 60 Hz or 120 Hz. In 50 Hz countries, it will be 50 Hz or 100 Hz.

You can hear this when tuning two strings. A guitar fifth string A vibrates at 110 Hz, two octaves below A=440 Hz. If the harmonic of the 6th string is 111 Hz, you here a pulsation or "beat" happening once per second. When adjusting so that the audible pulsation goes away, the two strings are at the same frequency.
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Old 01-25-2021, 02:06 PM
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KevinH KevinH is offline
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Quote:
Originally Posted by Fatfinger McGee View Post
Persistence of vision is also why spoked wheels sometimes look like they're rotating backwards (stroposcopic effect).

Anyway, question already answered but it reminded me of this neat video:
Yes, interesting video. But to avoid confusion, the effect in the video is likely due to a rolling shutter - assuming it was recorded with a phone camera.
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Old 01-25-2021, 02:54 PM
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Quote:
Originally Posted by Tigro View Post
Oh my, you're absolutely right! Thanks to your comment, I realised that every time I saw those slow vibrations was when I was playing in the living room - while all the others were in my bedroom. And they DO use different kind of lighting! Just went to the living room, plucked the E - slow vibration. Came back to the bedroom, plucked the E - quick vibration! Problem solved, haha Thanks a lot for the reply, never would have thought lighting would have anything to do here, was sure it was something regarding the guitar itself!

winner winner chicken dinner! Woot!
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Old 01-25-2021, 03:03 PM
LakewoodM32Fan LakewoodM32Fan is offline
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Quote:
Originally Posted by Earl49 View Post
The phenomena is called a "beat frequency". When two tones, or two vibrations, differ by just a few Hertz you will see the relative motion as if it were a strobe effect. Seeing it it depends on the lighting source, or the scan rate on a video. In 60 Hz electrical countries, the pulsation will be relative to 60 Hz or 120 Hz. In 50 Hz countries, it will be 50 Hz or 100 Hz.

You can hear this when tuning two strings. A guitar fifth string A vibrates at 110 Hz, two octaves below A=440 Hz. If the harmonic of the 6th string is 111 Hz, you here a pulsation or "beat" happening once per second. When adjusting so that the audible pulsation goes away, the two strings are at the same frequency.
This. I have a friend whose LED lighting is "just the right frequency" that some of my strings exhibit that cool effect. Never does it in my house.
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