Give it a chance, you won’t be disappointed. While the first 30 seconds of the video above may resemble an amateur iPhone prank, it soon becomes something unexpectedly enchanting—a visualization of the physics of music in real-time. The Youtuber places his phone inside an acoustic guitar, then plays Metallica’s “Nothing Else Matters" against a backdrop of clouds and blue sky. Due to what Twisted Sifter identifies as the phone camera’s rolling shutter effect, the actual waves of the vibrating guitar strings are as clearly visible as if they were on an oscilloscope.
The comparison is an apt one, since we might use exactly such a device to measure and visualize the acoustic properties of stringed instruments. “A guitar string”---writes physicist and musician Sam Hokin in his short explanation---is a common example of a string fixed at both ends which is elastic and can vibrate. The vibrations of such a string are called standing waves, and they satisfy the relationship between wavelength and frequency that comes from the definition of waves.”
Those with a physics background might appreciate The Physics Classroom's technical description of guitar string vibration, with several technical diagrams. For others, the video above by Youtube physics teacher Doc Shuster may be a better format. Shuster explains such entities as nodes and antinodes (you’ll have to tell me if you get any of his jokes). And at about 2:25, he digresses from his musings on these phenomena to talk about guitar strings specifically, which “make one note for a given tightness of the string, a given weight of the string, and a given length of the string.”
This is, of course, why changing the length of the string by pressing down on it changes the note the string produces, and it applies to all stringed instruments and the piano. Other factors, says Shuster, like the body of the guitar, use of pickups, etc., have a much smaller effect on the frequency of a guitar string than tightness, weight, and length. We see how the complexity of different standing wave forms relates to harmonics (or overtones). And when we return to the Metallica video at the top, we'll have a better understanding of how the strings vibrate differently as they produce different frequencies at different harmonics.
Shuster’s video quickly lapses into calculus, and you may or may not be lost by his explanations. The Physics Classroom has some excellent, free tutorials on various types of waves, pitch frequency, vibration, and resonance. Perhaps all we need to keep in mind to understand the very basics of the science is this, from their introduction: “As a guitar string vibrates, it sets surrounding air molecules into vibrational motion. The frequency at which these air molecules vibrate is equal to the frequency of vibration of the guitar string.” The action of the string produces an equal and opposite reaction in the air, which then creates “a pressure wave which travels outward from its source.” The pressure waves strike our eardrums, our brains interpret sound, and there you have it.
via Twisted Sifter