Follow

Wave Motion

  • H4-11 SAVART'S DISCS

    H4-11
    Demonstrates the relationship of pitch and musical intervals to mechanical vibration frequency
    A set of four toothed wheels is mounted on a fast rotator, where the ratio of number of teeth on the four wheels is 4:5:6:8. Tones are produced by holding a piece of cardboard or plastic against the spinning teeth. The resulting notes are the harmonics 4, 5, 6, and 8 of the overtone series, which form a major triad with the octave. A second set of wheels contains a different set of tooth ratios and therefore creates a different (minor) chord.
    H4

    h4-11a

  • H4-12: LISSAJOUS FREQUENCY MEASUREMENT

    H4-12
    Show how frequencies can be matched using Lissajous figures.
    Obtain a Lissajous figure using a sine wave oscillator (on the horizontal axis) and a musical sound such as a voice or instrument (on the vertical axis). In the photograph above, a soft falsetto voice (approximately a sine wave) was used on the vertical axis to produce the Lissajous figure, then grabbed by the scope for a stable display. The frequency of the oscillator is displayed at the upper right of the oscilloscope tube. Using the Lissajous figure and the frequency of the oscillator the frequency of the musical note can be determined.
    H4, ME2, ME3

  • H4-18: LISSAJOUS FIGURES WITH SOUND

    H4-18
    Demonstrate the relation between "consonance" (musical intervals) and frequency ratios.
    Two identical oscillator/loudspeaker setups are sounded and simultaneously input into the two axes of an oscilloscope to produce a Lissajous figure. Two pitches which are related by simple rational whole numbers such as 3:2 or 5:4 are musical intervals and produce stationary figures. Beats between two close frequencies are also visibly evident by this technique. Try two close frequencies and two related by a small whole number interval for contrast. Let your students judge their consonance.
    H4, ME2, ME3

  • H4-34: GUITAR AND OSCILLOSCOPE

    H4-34
    Illustrate how a guitar works
    Play notes or chords on the guitar to see their wave shapes on the oscilloscope. Notice that as the notes decay their wave shapes change, a result of different decay times for different harmonics.
    OS5, ME2, ME3

  • 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

  • H4-41: DRUM

    H4-41
    Demonstrate a pretty drum.
    This is an Ashiko drum, apparently similar to that pictured with Feynman. Unfortunately or fortunately, as the case may be, this one has a synthetic head. This drumhead is nicely tuned to its resonant cavity. For comparison, we have a beat-up steam-damaged older drum with a compromised resonant cavity, so it does not have as nice a tone.
    OS3

  • H4-51: MODULATION - AM AND FM

    H4-51
    Demonstrate AM and FM signal modulation as an introduction to vibrato and tremolo.
    The Pasco Dual Function Generator is used to produce either amplitude modulation or frequency modulation using various combinations of sine, triangular, and square waves. Frequency modulation is pure vibrato and amplitude modulation is pure tremolo; actual vocal vibrato is a combination of pure vibrato and pure tremolo.
    H4, ME2

    h4-51ah4-51b

  • H4-52: SPECTRUM ANALYSIS OF MODULATION

    H4-52
    Compare and analyze the frequency spectra of various modulated sounds such as tremolo, vibrato, and beats.
    Using the Pasco Dual Function Generator, a 1000 Hz sine wave is modulated by a 100 Hz sine wave and the spectrum of the modulated signal displayed using the spectrum analyzer. The photograph at the center shows the original 1000 Hz sine wave and the photograph at the right shows the case where that wave is amplitude modulated by a 100 Hz sine wave, producing a beat-like wave and a spectrum that has two sidebands around the 1000 Hz carrier. Amplitude modulation, frequency modulation, or double sideband modulation (sometimes called balanced modulation, or ring modulation with synthesizers) can be used. Two sine waves can be added together using the Dual Function Generator to produce beats, and the spectrum of the beats obtained and compared with that of double sideband modulation. The waveform is displayed on one trace and the spectrum on the other.

    Try out some frequency combinations ahead of time, then have students predict the results.
    H4, ME2, ME3

    h4-52ah4-52b

  • H4-58: MODULATION - AM RADIO

    H4-58
    Show amplitude modulation in in AM radio signals.
    The amplitude modulated signal from an intermediate stage in the portable radio is viewed using an oscilloscope. You may leave it freely running or you may freeze the trace using the oscilloscope stop button in order to obtain the clearest picture for your particular need. Please turn off radio when done to save battery.

    Please handle with care, as discrete-component radios are hard to come by!
    J2B, ME2, ME3