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PHYS102

  • 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

  • H2-52: 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

  • H3-11: 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

  • H3-17 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

  • H3-21: SOUND RESONANCE IN WATER TUBE

    H3-21
    Demonstrate standing waves in a closed tube.
    A tuning fork mounted over the top of the tube is activated by striking it with a rubber hammer. Raising and lowering the reservoir varies the water level in the tube to change the length of the air column. Because the air column is closed on one end (the surface of the water) resonances occur when the length of the tube is approximately 1/4, 3/4 or 5/4 of a wavelength, neglecting the end correction at the top of the tube. Using this apparatus standing waves can be demonstrated and the speed of sound determined to within about one percent.

    h3-21a

  • 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

  • H3-41: RESONANCE CURVE - HELMHOLTZ RESONATOR

    H3-41
    Demonstrate the resonance behavior of a Helmholtz resonator.
    The Helmholtz resonator is excited by an oscillator driving a small loudspeaker at about 250 Hz. Resonances in the system are sensed using a sound probe inserted into a small, rubber-padded opening on the resonator, and displayed on the oscilloscope.

    These sorts of globular resonators were used by Helmholtz in the nineteenth century, in the early days of acoustics experimentation. Before the development of spectrum analyzers and similar tools, he developed techniques to analyze the structure of sounds simply by holding a succession of resonators of different frequencies to his ear to pick out the components of complex sounds. The principle behind this is little different from twentieth century analog electronic frequency analyzers, which feed a signal from a microphone into a series of resonant circuits analogous to Herlmholtz's glass globes.
    H3, ME2, ME3, OM1, OM2

  • 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

  • H3-61 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

  • H3-62: 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

  • H4-01: 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

  • H4-22: 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