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.

Using a tube about 130cm long, seen in the photograph at the right above, the harmonics are odd multiples of 66Hz; this can be heard in an mpeg video with comparison to the frequencies of the overtone series using a Fourier synthesizer by clicking here.

Putting a mouthpiece on one end and a metal funnel "bell" on the other end of a 130cm long flexible tube adjusts the harmonics of the tube so that they become the harmonics of a trumpet with no valves, as can be seen in Demonstrations H4-37: FLEXIBLE TRUMPET and H4-38: CORNET.

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.