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Resonance

  • H3-62: TEACUP STANDING WAVES

    H3-62
    Demonstrate circular standing waves in an interesting way.
    A teacup can be tapped with a spoon to excite standing waves around its rim, exactly like the standing waves in a glass beaker. The standing wave consists of four alternating nodes and antinodes spaced at 90 degrees around the teacup. If the handle is at an antinode, the resonant frequency is lower than if the handle is at a nodal point, because the vibrating mass is greater but the restoring force is the same. Tap the rim of the teacup moving around the rim at intervals of 45 degrees to get alternating higher and lower frequencies This can be used in conjunction with H3-61 to illustrate the effects of the beaker's spout.
    H3

  • H4-22: BOTTLE BAND

    H4-22
    Demonstrate how edge tones and Helmholtz resonators can be used to create a bottle band.
    This demonstration requires a bass bottle player, three alto bottle players, and a recorder player, but can be rehearsed for a polished performance in a few minutes. The bass bottle player has three notes, while each of the alto bottle players is limited to two notes, so major musical experience is not necessary. The recorder player plays the melody of the song ("Home, Home on the Range" or "Ach du lieber Augustine") while the bottle band vamps.

    Consider inviting students to volunteer in the class period before, then come early to class to try it out. Note that the recorder is also available separately as H4-42.

    H4

  • H4-33: ORGAN PIPE

    H4-33
    Illustrate how an organ pipe works.
    The pipe can be activated by blowing in the end tube, and the pitch varied by sliding the "closed" end in and out. The end of the shorter tube is open, so it produces a fixed pitch. Pipes of several lengths can be provided.
    H4

  • H4-36: LIP-BLOWN TUBE

    H4-36
    Determine the frequencies and harmonic numbers of the resonances in a plastic tube blown like a trumpet.
    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 finesse 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 the link below.

    Tubes of different lengths can be made available upon request.
    OS0