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Acoustical & Musical Instruments

  • G4-13: DRUM HEAD STANDING WAVES

    G4-13
    Demonstrate standing waves in a circular membrane.
    A ripple tank vibrator is used to create standing waves in a circular rubber membrane. Adjusting the frequency changes the standing wave pattern.
    P1, G4

    g4-13a

  • G4-14: SPOUTING BOWL

    G4-14
    Demonstrate resonance in a dramatic way

    Fill the bowl half-way with distilled water only. Make sure your hands are very clean, and wet them slightly before rubbing.

    Rhythmically rubbing the handles of this ornate Chinese bowl sets up an energetic standing wave that will propel the water in the bowl up to a half-meter into the air! Click the link below to see a video of the spouting bowl in action.
    G4

  • 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

  • H1-03: BELLS

    H1-03
    Use bells as a sound source.
    This is simply a couple of fixed brass bells. Strike the bell with the hammer and listen to the interesting sounds. Talk about vibrations of the bell being transferred to the air and then to your ear. This can be a fun way to get the class's attention for the beginning of a lecture on sound propagation.
    H1

  • H2-42 DOPPLER EFFECT - TUNING FORK ON STRING

    H2-42
    Demonstrates Doppler effect

    A tuning fork is struck to activate the "clang tone" and whirled about the instructor's head on a string. The Doppler effect can easily be heard in a small classroom or a reasonably quiet lecture hall.
    Engagement Suggestion
    • Encourage students to listen closely to how the pitch changes, and compare it to other similar sounds. Where else do they experience this effect?
    Background
    As the source of the sound waves moves through the air, the wavefronts in the direction of motion are compressed, while the wavefronts in the opposite direction are extended, changing the pitch we hear. Because the fork is rotating, this causes a repeating pattern as the pitch is first higher, then lower, than the natural pitch of the tuning fork.
    H2a

  • H2-52: BEATS AND RESONANCE - TUNING BARS

    H2-52
    To demonstrate beats, and to demonstrate resonance between two identical tuning bar resonators.
    Two identical tuning bars are mounted atop resonators. Adding a small clamp onto one of the tuning bars reduces its frequency. Striking two tuning bars, one with a weight, then produces beats. The frequency of the beats can be adjusted by varying the position of the weight on the bar. Without weights on either bar, strike one of the tuning bars, then hold the other adjacent to the struck bar for a few seconds. If the struck bar is then damped, the sound continues. The second bar is in resonance with the struck bar, and some energy is transferred if they are physically near each other.
    H2

  • H3-04: KUNDT'S TUBE - STROKED ROD

    H3-04
    Demonstrate standing waves in an air column.
    This is the classical Kundt's tube standing wave apparatus. A rag covered with violin rosin is used to stroke an aluminum rod which excites standing waves in the air tube. Cork dust in the tube is agitated by the standing wave and deposited on the bottom of the tube such that it shows the basic form of the pattern of air motion. For this system the wavelength is about 11 cm, or one loop about 5.5 cm.

    Note: The modern, oscillator-driven version of this experiment, H3-05, is a more effective demonstration of standing waves for classroom use. We recommend using it primarily, with this traditional version alongside to illustrate the geometry of the original experiment.
    OS5

  • H3-11: TUNING FORKS AND RESONANT TUBE

    H3-11
    Illustrate resonance in an air column.

    This demonstration includes a clear plastic tube and two tuning forks, of slightly different frequencies.

    Strike either tuning fork and hold it to the end of the tube. The sound intensity of the fork at the resonant frequency (480Hz) of the tube increases dramatically, as the second harmonic of the tube is excited; whereas the fork with the non-resonant frequency (384Hz) does not become significantly louder.

    Background

    This illustrates the principle of resonance. One tuning fork's frequency is a multiple of the natural frequency of the air column in the tube, while the other is not.
    H3

  • H3-12: ROARING TUBE - 4 FT

    H3-12
    Demonstrate standing sound waves in air excited by convection currents.
    A switch is held closed, activating a nichrome wire coil in a vertical glass tube, leading to a very loud roar at about 130 Hz, the fundamental frequency of a four-foot air tube. This is the classic Rijke tube demonstration with an electrical heater replacing a gas burner and screen as the source of the convection currents.

    Consider combing this with H3-13, and invite students to make predictions about the differences in pitch and volume.
    FS1

  • H3-13: ROARING TUBE - 8 FT

    H3-13
    Demonstrate standing sound waves in air excited by convection currents.
    A switch is held closed, heating a nichrome wire coil in a vertical four-inch diameter galvanized steel downspout tube, leading to a very loud roar at about 65 Hz, the fundamental frequency of an eight-foot air tube. This is the classic Rijke tube demonstration with an electrical heater replacing a gas burner and screen as the source of the convection currents.

    Consider combing this with H3-12, and invite students to make predictions about the differences in pitch and volume.

    h3-13coilh3-13drawing

  • H3-14 TWIRL-A-TUNE

    H3-14
    Demonstrates standing wave resonances in an open tube
    This popular toy is available in many stores and students may have seen it before, but this is an opportunity for them to explore how it works. To produce resonant frequencies of the tube, hold the tube by one end, keeping that end free for flow of air, and swing it around your head. Increasing the speed of the rotation raises the harmonic produced. Up to seven harmonics can be produced, illustrating the notes of the overtone series. The fundamental can only be produced by blowing gently into one end. SUGGESTIONS: Read Invited talk : Sounds Like Fun, presented by Paul Doherty of the Exploratorium at the 2004 meeting of the AAPT at Sacramento, CA, discussing how the twirl-a-tune works.
    H3

  • H3-15: TWIRL-A-TUNE AND VACUUM CLEANER

    H3-15
    Demonstrate standing wave resonances in an open tube.
    To produce resonant frequencies of the tube, hold the end with the cork up to the input of the vacuum cleaner. As you cover the vacuum input more and more with the cork, more air will be pulled through the Twirl-a-Tune, exciting higher harmonics. Up to around 16 harmonics can be obtained.

    Note that this demonstration is very loud, and should not be used for very long or in a small, enclosed space. For smaller classes or for extended analysis and discussion, consider other demonstrations from this section.
    OS1

  • H3-16: SINGING PIPES

    H3-16
    Show sound resonance created by convection currents in a tube.
    A gas flame is inserted into one end of the tube, heating the wire mesh which has been pre-positioned in the lower half of the tube using the plunger. Holding the tube vertical after the mesh has been heated red hot creates convection currents which enable the tube to resonate. Tilting the tube nearly horizontal limits the convection currents and the sound ceases.

    This can be an exciting demonstration, but requires careful handling for safety. Also consider H3-12 and H3-13.

  • H3-24 OPEN AND CLOSED PIPES

    H3-24
    Demonstrates open and closed tube standing resonances
    Blow across the open end of the open and closed tubes. The frequency of the closed tube is approximately half that of the open tube, or about one octave lower. (Actually, due to the end correction, which applies to the open end of the closed tube but both ends of the open tube, the frequency ratio is slightly less than one octave to the trained musical ear.)

    For comparison, a half-length tube is also available. Invite students to predict how this one will compare to the open and closed tubes of twice its length
    H3

  • H3-25: GALTON'S WHISTLE

    H3-25
    Produce high-frequency tones.
    This device consists of a very short tube which can be activated by blowing air past an edge using the bulb. The tube is calibrated (in mm of increased tube length), and can produce both high-frequency audible pitches and ultrasonic frequencies. (The manufacturer's instructions recommend calculating frequency as of a 1/4 wavelength organ pipe, with the internal plunger moving from 0 to 9mm from the opening.)

    Consider polling students to see how many can hear the various frequencies.
    H3

  • H3-31: DUCK CALL

    H3-31
    Quack like a duck
    Just quack You might want to use this with a spectrum analyzer. Despite legend, it will echo under appropriate circumstances, but none of our lecture halls are that large.
    H3

  • H3-51: SONOMETER WITH TUNING PEGS

    H3-51
    Demonstrate standing waves in a stretched wire and to demonstrate Mersenne's first and second laws.
    The tensions in two stretched wires can be separately adjusted, creating different fundamental frequencies. Stops can be inserted along the wire to observe the frequency as a function of length, or to show the frequency ratio between two similar wires with the same tension but of different lengths. Note: this is easier to use than the sonometer with weights, but the tension cannot be measured quantitatively.
    OS0

  • H3-52: SONOMETER WITH WEIGHTS

    H3-52
    Demonstrate standing waves in a stretched wire and to demonstrate Mersenne's laws.
    This device can be used to demonstrate Mersenne's three laws for stretched strings.Keeping two of the three variables constant:

    (1) the fundamental frequency is inversely proportional to the length of the string. A stop is inserted under a point on the string, dividing the string into two segments.

    (2) the fundamental frequency is directly proportional to the square root of the tension. Note the frequency of the thinner string with two kilograms of weight. Quadrupling the weight doubles the frequency, raising it one octave.

    (3) the fundamental frequency is inversely proportional to the square root of the mass per unit length. The thicker string is about twice the diameter or four times the mass per unit length of the thinner string. With the same weight the pitch of the thicker string is about one octave lower. The thicker string must have about four times the tension (hanging mass) of the thinner string to make their fundamental frequencies the same.
    OS0

  • H3-71 STROKED ALUMINUM ROD

    H3-71
    Illustrates longitudinal standing waves in an aluminum rod.
    Apply powdered violin rosin to your fingers or wear a rosined glove and stroke the aluminum rod firmly while holding it at a nodal point. Holding it in the center produces the fundamental, holding at 1/4 of the way from one end produces the second harmonic, holding at 1/6 of the way from one end produces the third harmonic, etc. The rod is about 6 ft long, and the speed of sound in aluminum is about 16,700 ft/sec, so the frequency of the fundamental is about 1400 Hz. The sound is very loud and lasts a long time; the Q for this system is around 100,000!
    Alternatively, request an (optional) mallet to use with the rod. Use the mallet to strike the rod on one end; by holding the rod at a node or antinode, all or some modes can be excited or damped.