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Acoustics

  • G2-06: FRAHM'S FREQUENCY METER

    G2-06
    Check the 60 Hz line voltage frequency.
    A set of reeds, all the same length, are weighted to resonate near 60 Hz. The reeds are driven by an electromagnet. Just plug the meter leads into the end of a power cord to check the line frequency. Use an oscillator for driving the meter at frequencies other than 60 Hz.

  • G4-21: CHLADNI FIGURES - BOWED

    G4-21
    Show two-dimensional standing waves in a metal plate

    Sand is sprinkled onto a circular or square thin metal plate, which is then stroked along the edge using a violin bow. The sand moves to nodal lines of the standing wave pattern. Stroking at one point while holding your finger at another point to forces a node at that point, if desired, creating a variety of standing wave patterns.

    A nice article describing the Chladni plates at the Whipple Collection wil be found by clicking http://www.hps.cam.ac.uk/whipple/explore/acoustics/ernstchladni/chladniplates/.

    G4

  • G4-22: CHLADNI FIGURES - OSCILLATOR DRIVEN

    G4-22
    Show two-dimensional standing waves in a metal plate
    The Chladni plate is a system for creating and illustrating two-dimensional standing waves in a surface. A variety of flat plates can be mounted on the oscillator (including square, circular, and violin-shaped plates). As the plate vibrates, fine white sand is shaken about and traces out the nodal lines of the vibrations of the plate. The system operates by means of magnetostriction. A thin-walled annealed nickel tube is used to drive various Chladni plates. The nickel tube is threaded into the center of the plate, and inserted through a coil under the plate, which rests on a thick felt surface. An oscillator in the 10-30 kHz frequency range drives a 20-Watt audio amplifier to provide the current creating the magnetic field. The field is biased by a small horseshoe magnet to avoid frequency doubling in the tube. A mirror allows larger groups to view the plate easily.
    FS1

  • H1-11: MICROPHONE AND OSCILLOSCOPE

    H1-11
    Show the wave shape of various sounds.
    This setup can be used to look at the wave shape of the speaking voice, singing, whistling, playing musical instruments, musical synthesizers, oscillators with speakers, etc. The oscilloscope trace shown is that of a baritone voice singing the vowel "ee" with a frequency of about 160 Hz. The photographs from the oscilloscope above compare the sounds of a clarinet (top, resembling a square wave), a crumhorn (center, resembling a sawtooth wave), and a recorder (bottom, resembling a triangular wave).

    A variety of sound sources can be requested separately.
    H1, ME2, ME3

  • H1-13 WAVEFORM GENERATOR, SPEAKER AND OSCILLOSCOPE

    H1-13
    Demonstrates waveform and sound of standard waves
    A function generator is used to produce a variety of waveforms in the audible range, to be both played through a speaker and displayed on an oscilloscope. The wave generator is fed simultaneously into the audio amplifier/loudspeaker and the oscilloscope, to prevent loading of the generator by the speaker and the concomitant distortion. The sound and wave shape can then be observed simultaneously. Sine waves, square waves, and sawtooth waves are readily available. The effect of changes in the frequency as well as the wave shape can also be observed.
    ME3, ME2

  • H1-32: WAVETEK AND AUDIO CART - EQUAL SOUND LEVEL STEPS

    H1-32
    Illustrate the effect on the ear of successive changes of exactly 10 dB.
    Setting the generator to a sine wave in the 100-1000 Hz region, the intensity can be changed up and down by 10 dB steps, covering about a 50dB range. Be careful not to exceed the maximum of either the loudspeakers or your ears.

    Invite students to compare their experience of different levels to phenomena they are familiar with, such as conversations and concerts.

  • H1-43: ULTRASONICS AND HEARING

    H1-43
    Demonstrate that there exist ultrasonic waves with frequencies above the range of human hearing.
    The oscillator drives a supertweeter at frequencies from 500 Hz to over 25 kHz. The microphone signal on the oscilloscope provides evidence that the ultrasonic waves really are present, even when inaudible.

    Invite students to raise their hands when they can or cannot hear the sound, and discuss natural variation in hearing, and how hearing can change with age and environment.

    Caution: Be careful with volume levels if you have any students with service dogs.
    H1, ME2, ME3

  • H1-51: AUDIOTAPE 42 MIN - SCIENCE OF SOUND - SHORT VERSION

    H1-51
    Illustrate various audio phenomena.
    This is a copy of selected audio demonstrations from the Bell Telephone Laboratories recording "The Science of Sound." (H1-52) Topics include: Side 1: How we hear, Frequency, Pitch, Intensity, The Doppler Effect. Side 2: Echo and Reverberation, Delay Distortion, Fundamentals and Overtones, Quality, Filtered Music and Speech.

  • H1-52: AUDIOTAPE 82 MIN - SCIENCE OF SOUND - LONG VERSION

    H1-52
    Illustrate various audio phenomena.
    This is a classic, comprehensive set of audio demonstrations from the Bell Telephone Laboratories recording "The Science of Sound." Topics include: Side 1: How we hear, Frequency, Pitch, Vibration and Resonance, Intensity, Loudness, Noise Measurement, Masking, Echo and Reverberation, Delay Distortion. Side 2: Fundamentals and Overtones, Quality, Subjective Tones, Music or Noise, Filtered Music and Speech, Dissonance and Consonance, Music Scales, Vibrato and Tremolo, Doppler Effect.

  • H2-28: FOURIER SYNTHESIZER - ADDITION OF WAVES

    H2-28
    Demonstrate addition of two sine waves with variable phase difference.
    Two identical sine waves from the University of Maryland Fourier Synthesizer are added together and the sum is viewed along with each component using a three-trace oscilloscope. The sum can be studied as the phases of one or both of the component waves are varied. This demonstration can be used as an aid in the study of beats or interference of sound waves. Invite students to make predictions about the effects of changing phase and amplitude of components.
    H2, ME2

  • H4-01: FOURIER SYNTHESIS

    H4-01
    Demonstrate Fourier synthesis of complex wave shapes.
    Complex waves may be formed using up to twelve harmonics with independently variable amplitudes and phases. Any individual harmonic, including the fundamental, can be shown on one trace of the oscilloscope, while the sum is shown on another trace. The wave can be simultaneously seen on the oscilloscope and heard using a loudspeaker with a separate volume control. Digital phase locking of all harmonics allows the frequency to be varied from below 100 Hz to above 1000 Hz while the wave shapes remain fixed, to show that timbre is primarily dependent on harmonic structure, and not on frequency or intensity. Some easily produceable wave shapes are square wave, sawtooth wave, triangular wave, and pulse train.
    H4, ME2, ME3

    h4-01ah4-01ch4-01dh4-01pulseh4-01squareh4-01triangleh4-01ch4-01trih4-01sawtoothh4-01saw

  • H4-04 FOURIER ANALYSIS - DIGITAL OSCILLOSCOPE

    H4-04
    Demonstrates the Fourier spectrum of complex waves
    This experiment uses a digital oscilloscope with a fast Fourier transform module to determine the Fourier spectrum, simultaneously displaying the wave shape and the Fourier spectrum on its monitor. Any periodic wave from a wave generator or sound, such as a musical instrument or the singing voice, can be analyzed. A variety of waves can be input from wave generators, such as the standard wave shapes, and a microphone, such as steady-state instrumental or vocal sounds.

    Invite student musicians to bring in their instruments for analysis.
    H4, ME2, ME3

  • 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

  • H4-13: PLAYING IN JUST INTONATION

    H4-13
    Show musical intervals in just intonation.
    Musical intervals between the oscillator and an instrument or voice are played so as to create stable Lissajous patterns on the oscilloscope. These stable patterns result from frequencies in the ratio of whole numbers, corresponding to "just" or beatless musical intervals. This can require practice; consider inviting musically inclined students to try it out ahead of time.
    H4, ME2, ME3

  • H4-14: LISSAJOUS FIGURES - VOICE

    H4-14
    Provide a Lissajous type display from a single vocal source.
    The signal from a microphone is amplified and fed into a circuit which produces two signals which are different in phase, and these two signals are fed into the X and Y coordinates of the oscilloscope to create a type of Lissajous pattern. The circuit makes the two signals exactly 90 degrees apart at 300 Hz, producing a circle. For other frequencies the phase shift is different and various figures are formed.
    H4, ME2, ME3

  • H4-16: TUNER - KORG

    H4-16
    Demonstrate use of an electronic tuner.
    The tuner can be used to produce notes in tune in the equal tempered scale or to compare the frequency of notes from an external source with a standard stored in the instrument. It displays errors on an analog meter
    H4

  • H4-17: AUDIOTAPE 16 MIN - MUSICAL TEMPERAMENT

    H4-17
    Compare and contrast several historically important musical scale temperaments.
    This is a recording of audio examples. Intervals illustrating properties and problems are given for the following temperaments: (1) Pythagorean, (2) Just, (3) Mean Tone, (4) Werckmeister, and (5) Equal Temperament. To illustrate the sounds of some temperaments in different keys, a short segment of a four-voice Bach chorale is played on an electronic organ, first in the key of C, then in the key of C#, in the following temperaments: (1) Just, (2) Mean Tone, (3) Werckmeister, (4) Equal Temperament. Detailed description of the various demonstrations is included.
    H4, FS1

    h4-17a

  • 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

  • H4-21: EDGE TONES

    H4-21
    Illustrate edge tones and the factors that effect their sound.
    Compressed air is blown out a slit onto the point of a wedge-shaped barrier. The wedge can be positioned with respect to the slit and the air flow can be regulated to produce controllable edge tones.

    This demonstration can be finicky, and is sensitive to both careful positioning and to environmental conditions. Try it out before hand.
    H4

    h4-21ch4-21ah4-21b