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PHYS260

  • G4-31: MOIRE PATTERNS

    G4-31
    Show a type of interference pattern.
    A set of matched patterns can be positioned on the overhead projector such that they create a type of interference pattern, as seen in the photographs. Try to figure it out, or just enjoy.

    g4-31a

  • H1-01 BELL IN VACUUM

    H1-01
    Demonstrates sound wave requirement for a medium

    An alarm-style electric bell is mounted inside a large glass bell jar, with external switches to control both the bell and the pump. This enables the instructor to compare the propagation of sound and light.

    Start the bell, then pump the air out of the jar. Air pressure in the jar is read by the large gauge. As the air is removed, the sound intensity decreases, ultimately to nearly zero. Turn off the vacuum pump when the jar is evacuated and crack the valve open, allowing air to re-enter the jar. As the pressure increases the sound of the bell comes back, but without the noise of the pump.

    Engagement Suggestion
    • Consider asking the students to make predictions before each step - how will removing the air change what they hear? What they see? What will happen as the air returns?
    • Compare this to videos the see of people working in the vacuum of space, in real life and in the movies. What do you see and hear in real life? How is this presented in fiction, and why?
    Background
    There are subtleties to this effect. The pump is not creating a true vacuum within the chamber. The vast majority of the air has been removed, reducing the environment’s ability to transmit sound; but the other (perhaps more important) effect in play is the difference in density between the interior of the chamber and the glass and the external atmosphere; this creates a major change in impedance, causing what little sound can be transmitted within the chamber to reflect back. Also, off course, the bell is not floating in free space, and some vibrations can always be transmitted through the supports and wires.

    For small groups, also consider H1-04, a more portable version of this demonstration.
    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

  • 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

  • H2-21 AUDIBLE YOUNG'S EXPERIMENT - GROUP LISTENING

    H2-21
    Demonstrates interference of sound waves with two coherent sources
    The oscillator-amplifier is set to approximately 3000 Hz, with identical signals being applied to both loudspeakers. Rotating the loudspeakers past the listeners allows them to observe the interference pattern by hearing the alternating maxima and minima in the intensity pattern.
    OS2

  • H2-32: SPEAKER WITH BAFFLE

    H2-32
    Demonstrates diffraction and interference of sound waves

    A small loudspeaker plays music with lots of bass, but the bass is not very loud. When the speaker is held up behind a hole the size of the speaker in a board about two feet square, the sound becomes much louder to the audience; this is particularly noticeable in the lower (bass) frequencies.
    Background
    A loudspeaker produces two distinct sound waves: one from the front and one from the back, which are out of phase with respect to each other. In the absence of the baffle, these sounds both diffract in all directions, and, because they are exactly out of phase they interfere destructively, especially the bass. The baffle forestalls the diffraction and thus reduces the magnitude of the interference. This effect is used in constructing speakers and their enclosures, to ensure that the maximum of output energy is passed to the listener. It can also be observed in nature, as some insects have been noted to use such surfaces to effectively amplify their calls in the wild (see references below).
    H2

  • H2-41 DOPPLER BALL

    H2-41
    Demonstrates Doppler effect

    An electronic device making a loud squeal is turned on and placed inside a foam ball. The ball is then zipped inside a cloth cover hooked to the end of a cord, and whirled about the instructor's head or carefully tossed from person to person. The Doppler effect can easily be heard throughout even a large room.
    Engagement Suggestion:
    • Challenge students to describe other circumstances where they have heard this phenomenon
    Background:

    This is a classic illustration of the Doppler Effect. When a wave source is in motion, the wavelength of the emitted waves is observed to change by an observer along its direction of motion.

    It can be useful to present this in conjunction with an animation or simulation, to illustrate the effect visually; see the relevant page of our Directory of Simulations.
    H2

  • 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

  • H2-51 BEATS - AUDIO OSCILLATORS, SPEAKER & OSCILLOSCOPE

    H2-51
    Hear audio beats and see the wave forms on an oscilloscope
    Two oscillators (in a Pasco Dual Function Generator) create sine waves which are displayed on the oscilloscope. The sum is also created by the Dual Function Generator and output to the third trace of the oscilloscope as well as to a loudspeaker. The beats can be seen and heard as the frequency of one of the sine waves is varied, producing slow changes in amplitude (small frequency differences) or the wave packet effect (larger frequency differences).
    H2, ME2, ME3

  • 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

  • H2-53 BEATS - AUDIO OSCILLATORS AND SPEAKERS

    H2-53
    Hear beats
    To obtain beats, set the two oscillators to the same amplitude at very nearly the same frequency. Adjust the frequency of one oscillator to change the beats.
    ME3

  • H2-54: BEATS - MOIRE PATTERN MODEL

    H2-54
    Model for beats between mistuned audio oscillators.
    Two transparencies of equally-spaced lines and spaces are used to model beats. One transparency has been reduced about ten percent so the two transparencies represent two sound waves of different frequencies, where (for example) the dark lines represent the compressions and the white lines represent the rarefactions. Aligning the two transparencies on top of each other on the overhead projector creates waves which go in and out of phase as time progresses: when they are in phase the sound is loud, but when they are out of phase the sound is soft, creating beats.
    h2

    h2-54a

  • 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

  • H3-17 FLAME TUBE

    H3-17
    Demonstrates standing waves in a tube
    A loudspeaker in one end of a four-inch diameter galvanized iron tube creates standing waves in propane gas in the tube. The gas emerges out of a series of small holes in the top of the tube, forming a long line of flames when lit. Any sound resonant with the length of the tube can create standing waves in the gas which are readily visible as a pattern in the height of the flames. Both rhythm and frequency response can be seen nicely in music. An oscillator and a cassette deck are provided with the demonstration to be used as simple sources for the loudspeaker. Or, a voice or other music or audio can introduced using a microphone and amplifier or external input jacks, available upon request.
    FS1

  • 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

  • H3-61 BEAKER BREAKER

    H3-61
    Breaks a glass beaker with sound

    An audio oscillator and 100 Watt power amplifier are used to drive a heavy-duty horn driver which is mounted in the back of the plastic beaker cavity with the sound emerging through a hole, which can be seen in the photograph. The beaker is positioned on a foam pedestal in front of the speaker hole. A microphone is mounted at 90 degrees from the position of the speaker.

    The beaker is marked with its primary resonant frequency, found in advance using digital spectrum analysis of a recording of the beaker ringing after being tapped. Most beakers have two possible resonant modes 45 degrees apart, due to the weight of the spout; the most effective technique is to drive the resonance with the spout facing directly away from the speaker. Set the frequency of the oscillator as shown on the beaker, with an amplitude of around 140mVpp. The oscilloscope will show two waveforms, the input signal and the signal picked up by the microphone. You may need to adjust the frequency slightly to account for changes in temperature or age since the beaker was tested; slowly shift the frequency by tenths or hundredths of a Hertz to find the amplitude peak (do not try to tune by watching for a displacement in the phase relationship, as there is a time delay between the signals introduced by the hardware). This done, set the strobe around 3000 cycles per minute, and adjust it until you can see the sides of the beaker flexing.

    This can be used to show the resonance of the beaker. You can also, optionally, shatter it, by increasing the input voltage at resonance. Be careful not to exceed 1Vpp.

    After the resonant frequency is found and the amplitude turned up, the oscillation of the beaker can be caused to exceed its elastic limit and thus to shatter. See the video links below to view a slow-motion video of the beaker at the moment it breaks.

    Engagement Suggestion
    • Show the students that there are two different resonant frequencies, and challenge them to develop theories of why this is.
    • Consider using this in conjunction with H3-62 to illustrate the effects of the beaker's spout in a more obvious (and quieter) manner.
    Background
    This process of driven resonance potentially leading to mechanical failure can be related to many engineering problems. This is an excellent opportunity to discuss how physics applies to real-world problems, like the Tacoma Narrows Bridge collapse.
    Also, be sure to explore our directory of oscillations and waves simulations to show other examples of complex mechanical oscillations.
    FS1, LS2, SU5

  • 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.

  • H4-42 RECORDER

    H2-42
    Can hear the sounds of a recorder
    This is a soprano recorder that uses German fingering. Show what a recorder looks like, how it sounds, and use it in other demonstrations illustrating the wave shape or the spectrum.
    H4

  • I1-01: THERMOMETERS

    I1-01
    Show several types of thermometers.
    Several thermometers, as photographed above, just lie there reading the temperature. You must plug in the electronic one and turn it on.
    I1, I0

  • I1-11 THERMAL EXPANSION - BALL AND HOLE

    I1-11
    Illustrates thermal expansion
    At room temperature the ball will not fit through the hole in the metal plate. When the plate is heated by a burner for about 30 seconds, the ball easily fits through the hole
    I1, I0