PURPOSE: To determine the frequencies and harmonic numbers of the resonances in a plastic tube blown like a trumpet.
DESCRIPTION: Blowing into the end of the tube as if it were a trumpet creates the odd harmonics. The lip end acts acoustically like a closed end, so in the fundamental mode the tube is one-quarter wavelength long. For this tube, the length is about six feet, so the wavelength of the fundamental is about 24 feet, and its frequency is about 50 Hz. The harmonics are then 150, 250, 350, 450, 550, etc. It is virtually impossible to sound the fundamental, but someone with a bit of finnesse with brass instruments can easily demonstrate five or six overtones.
SUGGESTIONS: Use the Fourier synthesizer (Demonstration H4-01) to sound the harmonics, tuned to the fundamental of this tube, as a reference for the harmonics.
REFERENCES: (PIRA 3D32.27)
EQUIPMENT: Six-foot plastic tube.
SETUP TIME: None.
Go back to Lecture-Demonstration Home Page.