Harmonics demonstrated

[walrus]

This is a very cool video, coutesy of Guitar Squid. Can anyone explain the "science" behind this, in (hopefully) simple terms?

http://www.guitarsquid.com/Video/harmon ... rated.html

walrus

 

[adorshki]

Ok, I'll take a whack at it.
Vibrations are introduced into the test area, in this case a thin flat sheet.
The frequency of the vibrations can be varied.
As the vibrations hit the edges of the sheet they "reflect" back into it. When they meet with vibrations coming from a different direction, "interference patterns" are created.
Those are the patterns you see. Their shape can be altered by flexing the sheet or changing the frequency or both. The salt tends to gather in the "dead zones", the clear areas are still vibrating.
"Phase shifting" uses this principle, of frequencies cancelling each other out when they're perfectly out of phase, or of modifying the waveform when they're not perfectly in phase..
You can also see it in ultrasonic cleaning tanks.
In fact you can even see it in coffee in a styrofoam cup if you slide it across a desktop at just the right speed to make the styrofoam squeak.
NOW, anybody want to learn how to make a light show?
(Hint: Shine your Heathkit home laser into the ultrasound cleaner tank and reflect that through the overhead projector) :lol:

 

[fronobulax]

Something similar discussed here.

 

[12 string]

The sand collects in the nodal points. The nodes are the places that don't move much and they change with each harmonic mode of vibration. The math for a vibrating plate is more complex than a string because width is added. I've forgotten almost all of it but I seem to remember it coming in to play when studying the working of the bells in brass instruments an undergraduate music major and taking the required coursework in the physics of music and speech. There was a prof who produced similar results with sand and a mounted steel plate which he excited with an old cello or bass bow strung up with some kind of rubber or plastic. He did not acheive so many different harmonic modes.

Question: Does the video play in real time? I seem to remember that professor sawing away for quite a while before discernable patterns appeared on the plate.

If you play a 12th fret (midpoint of the string) harmonic you create a node there and equal segments from the nut to the 12th and from the 12 to the saddle vibrate at twice the original frequency (1 octave). A 7th (or 19th) fret harmonic sets up nodes at both locations and produces a tone 3 times the original (an octave plus a 5th). A 5th fret harmonic creates nodes dividing the string into four equal segments and produces a tone 4 times the original frequency (another octave). A 4th fret harmonic divides the string in to 5 equal segments and sounds 2 octaves and a 3rd higher. And so on.

Dang, I've forgotten more about this crap than I can remember!

' Strang

 

[12 string]

Ok, I'll take a whack at it.
Vibrations are introduced into the test area, in this case a thin flat sheet.
The frequency of the vibrations can be varied.
As the vibrations hit the edges of the sheet they "reflect" back into it. When they meet with vibrations coming from a different direction, "interference patterns" are created.
Those are the patterns you see. Their shape can be altered by flexing the sheet or changing the frequency or both. The salt tends to gather in the "dead zones", the clear areas are still vibrating.

I think we were doing "Carmina Burana", but whatever, I was in the bass section of the choir standing on the back row of an impossibly tall riser with the timpani directly below me and with the glare of a bright overhead light being reflected from the drumheads directly into my eyes. As the timpanist would play the interaction of the lights and vibrations on the drumheads disclosed wave patterns radiating away from the point where the mallets were hitting. They traversed the skins and reflected back creating crosshatch patterns. In the glare of the lights those kettledrums looked liked giant bowls of water. Dangdest thing I've ever seen.

' Strang

 

[walrus]

Thanks for the info...

12 string, I had that happen to me too, years ago, but I'm pretty sure it was from drug intake...

walrus

 

[CA-35]

Walrus great find! Phenomenal demonstration of harmonics and it's effect on tangible items.

Not unlike a similar demonstration of "flutter", in which the Tacoma Narrows bridge collapsed when a 42 mph wind gust made it vibrate beyond control.
("Flutter is a self-feeding and potentially destructive vibration where aerodynamic forces on an object couple with a structure's natural mode of vibration to produce rapid periodic motion")
Great video watch it here;http://youtu.be/j-zczJXSxnw

 

[Ridgemont]

Ok, I'll take a whack at it.
Vibrations are introduced into the test area, in this case a thin flat sheet.
The frequency of the vibrations can be varied.
As the vibrations hit the edges of the sheet they "reflect" back into it. When they meet with vibrations coming from a different direction, "interference patterns" are created.
Those are the patterns you see. Their shape can be altered by flexing the sheet or changing the frequency or both. The salt tends to gather in the "dead zones", the clear areas are still vibrating.
"Phase shifting" uses this principle, of frequencies cancelling each other out when they're perfectly out of phase, or of modifying the waveform when they're not perfectly in phase..
You can also see it in ultrasonic cleaning tanks.
In fact you can even see it in coffee in a styrofoam cup if you slide it across a desktop at just the right speed to make the styrofoam squeak.
NOW, anybody want to learn how to make a light show?
(Hint: Shine your Heathkit home laser into the ultrasound cleaner tank and reflect that through the overhead projector) :lol:

Here is an illustration I stole from Wiki.

As the pitch becomes higher, so do the frequency of the nodes. The sand collects in the nodes because those are points of no movement in the flat sheet. My guess is that the point of origin for the vibration is in the center of the flat sheet, thus the vibrations being multidirectional.