Pluck a rubber band stretched between two fixed points, and it doesn't vibrate randomly — it settles into one particular shape, bouncing back and forth between the two ends. Blow across a tube, and the air inside does the same thing, bouncing between the tube's ends.
Because the ends are fixed (or nearly so), only certain wavelengths actually "fit" neatly between them and reinforce themselves trip after trip. This reinforced, self-sustaining pattern is called a standing wave, and it's why a string or tube rings out specific pitches instead of a random buzz.
A string fixed at both ends (a rubber band, a guitar string, a wire) only fits certain wavelengths between its ends — shorter string, shorter wavelength, and by v = fλ, a shorter wavelength means a higher frequency.
Tension changes things too: a tighter string carries waves faster (higher wave speed v along the string), which — again by v = fλ — raises the frequency for the same length. This is exactly what you're doing when you tighten a guitar string to tune it up.
Blowing across a tube or a bottle opening sets the air column inside vibrating as a standing wave, the same way a string does. A longer air column fits a longer wavelength — lower pitch. A shorter air column fits a shorter wavelength — higher pitch.
Try both build types below and get a feel for direction and rough size of pitch change before you touch real materials.
Pick a build type, then drag length (and tension, for strings) to predict how the pitch shifts relative to a baseline setup. This shows direction and rough size, not an exact frequency — real scrap materials are never perfectly predictable.
| Rubber-band string | Shorten the vibrating length or stretch it tighter to raise pitch. |
| Straw or tube | Cut it shorter to raise pitch; leave it longer for a lower pitch. |
| Bottle with water | Add water to raise pitch (shortens the air column); remove water to lower it. |
| Drum or membrane | Stretch the membrane tighter to raise pitch, same idea as a string. |
You now have everything you need to build and tune a real instrument on purpose. Head to the Project page to get started — and keep this cheat sheet open while you build.