how does pipe length of a wind instrument change the frequency of the vibration, especially in brass and reed instruments where you have a physical object vibrating, rather than just the air?

Question

I get that the length of the pipe affects the wavelength of the sound wave, but I’m curious how that forces the player’s lips, or reed to vibrate at different rates

In: 4

Answer 1

Air is elastic, and rebounds like a rubber ball when compressed at resonance. Air that is excited at lower or higher frequencies than the container’s resonant frequency simply dissipates.

The length of the instrument’s tube determines the size of the compressed air ball (or balls/nodes, or sausage/nodes) that forms in the tube. A smaller ball means a higher frequency which means higher pitch.

Compression forces act within air along the entire length of the instrument. If one end terminates at a reed, then these compression forces also act on the reed and reinforce only reed vibrations that match instrument’s resonant frequency. You can force a reed to change pitch a little (to bend a note) but this takes extra energy to sustain.