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Wave Motion

  • H2-01: FOCUSING OF SOUND WITH CONCAVE REFLECTORS

    H2-01
    Demonstrate how sound waves can be focused by concave reflectors.
    Set the oscillator at about 3000 Hz for best results. Install speaker at the focus of one reflector and microphone at focus of second reflector; the oscilloscope views the microphone output. With the positions of all elements optimized the sound from the speaker reflects off its mirror to create a beam of sound, which is focused by the second mirror onto the microphone. All elements may be adjusted to verify the existence of foci.

    When discussing wave phenomena, this can be usefully compared to optical focusing demonstrations in section L.
    H2, OM1, ME3

  • H2-02: PARABOLIC MICROPHONE

    H2-02
    Show what a parabolic microphone is and to demonstrate its sensitivity individually.
    Sound waves from a distant source are focused by the parabolic concave reflector onto the microphone transducer element. The device is directional and extremely sensitive. The output may be heard directly using headphones or input into a tape recorder. Discuss its operation using the device as a prop, then let students listen individually.

    This is similar to the devices used at sporting events.
    H2

  • H2-03: ACOUSTIC RADAR

    H2-03
    Illustrate how RADAR and SONAR work
    A low-frequency square wave input to a speaker emits a short pulse at the leading edge of the square wave, seen on the lower trace. The microphone picks up this pulse, displaying it on the upper trace; the initial pulse is the direct sound, while the second pulse is the reflection of the receding pulse off the screen. Moving the screen varies the time the reflected pulse returns to the microphone, producing acoustic radar.
    H2, ME3, OM1

  • H2-11: SOUND LENS

    H2-11
    Demonstrate focusing of sound by refraction in a sound lens
    A balloon filled with carbon dioxide acts as a focusing sound lens, due to its convex shape and the smaller velocity of sound in the carbon dioxide. When the lens is inserted between the loudspeaker and the microphone, the sound wave is focused, increasing the sound level at the microphone, as seen on the oscilloscope. The source is either a small chunk of dry ice in a flask or a cylinder of carbon dioxide.

    For comparison, air (very little focusing) and helium (defocusing) balloons can also be provided upon request.

    For good results, position the microphone and the loudspeaker about 40 cm apart, inflate the balloon to about 20 cm diameter, and use a frequency of about 2-4 kilohertz.

    If you use additional balloons of different gases, as mentioned above, have students make predictions about what effect density will have before showing the result.
    H2, OM1, ME2. ME3, I0, FS1

  • 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

  • H2-22: INTERFERENCE - TRANSPARENCIES

    H2-22
    An optical analog to the interference pattern from two identical sound sources.
    Transparencies consisting of a series of equally-spaced concentric rings are superimposed on each other. The interference pattern from these two identical sources can be observed as the distance between the two sources is varied. Two sizes of circle spacings (corresponding to two wavelengths) are supplied, as shown in the photographs.
    H2

    h2-22a

  • H2-24: AUDIBLE YOUNG'S EXPERIMENT - MIC AND SCOPE

    H2-24
    Demonstrate interference of sound with two coherent sound sources in a quantitative way.
    The oscillator is set to approximately 3000 Hz, with identical signals being applied to both loudspeakers and displayed on the lower oscilloscope trace. The microphone, with its signal displayed on the oscilloscope upper trace, can be moved around to observe the interference pattern by displaying the alternating maxima and minima in the intensity pattern. Nodal and antinodal lines can be observed and measurements made to show the relationships between the wavelength, source separation, and the nodal/antinodal lines. Invite students in the audience to volunteer what they hear at different points, and compare it to what the microphone picks up.
    H2, ME2, ME3, OM1

  • H2-25: QUINCKE'S INTERFERENCE TUBES

    H2-25
    Demonstrate interference of sound waves in a perhaps surprising way.
    An oscillator, with its output displayed on the lower trace of the oscilloscope, is attached to a speaker such that the sound is introduced into one end of a tube through a funnel. A microphone is inserted into the other end of the tube with its output shown on the upper trace of the oscilloscope. The tube between the speaker and the microphone splits into two paths, one being about 50 cm longer than the other. At a frequency of about 350 Hz, the waves from the two separate paths are out of phase when they recombine, so the signal reaching the microphone is a minimum. Pinching the longer tube removes one-half of the signal, yet the amplitude at the microphone increases. This may seem to be a surprising result! As the frequency is increased, values will be found where the waves from the two paths are alternately in and out of phase, yielding a series of maxima and minima in the recombined signal. Pre-set this device at a nodal point, so that when you stop the longer tube by squeezing, the signal at the microphone increases in amplitude, and let your students try to explain it.
    H2, ME2, ME3, OM1

  • H2-26: PHASE REVERSAL BETWEEN STEREO SPEAKERS - MUSIC

    H2-26
    Demonstrate interference of sound in a dramatic way.
    Two loudspeakers are connected in the monaural mode to the power amplifier and positioned close together as shown in the photograph at the left above. A switch box in the leads of one of the speakers allows reversal of the phase of that speaker. When music with lots of bass is played, flipping the phase reversal switch causes huge reduction in the amplitude of the bass frequencies. This is a very dramatic effect.

    A nice experiment shows the relation of phase to physical position. Play an 80 Hz tone into the two speakers, then reverse the phase to reduce the sound to virtually nothing. Uncoil the wire from the back of one speaker and move the speaker 12 or 15 feet across the front of the room; the loud bass tone returns! The waves from the two speakers are no longer out of phase. Can easily be combined with H2-27.
    FS1

    h2-26a

  • H2-27: PHASE REVERSAL BETWEEN STEREO SPEAKERS - OSCILLATOR

    H2-27
    Dramatically demonstrate interference between two identical sources.
    Two loudspeakers are connected in the monaural mode to the power amplifier. A switch box in the leads of one of the speakers allows reversal of the phase of that speaker.

    A nice experiment shows the relation of phase to physical position. Set the speakers to monaural and play an 80 Hz tone into the two speakers, then reverse the phase to reduce the sound to virtually nothing. Uncoil the wire from the back of one speaker and move the speaker 12 or 15 feet across the front of the room; the loud bass tone returns! The waves from the two speakers are no longer out of phase. Can easily be combined with H2-26.
    FS1

    h2-27a

  • 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

  • H2-31: ACOUSTIC COLLIMATOR

    H2-31
    Demonstrate the effect of frequency on diffraction of sound.
    The collimator is aimed and rotated through the class. For low frequencies (long wavelengths) there is lots of diffraction, and very little collimation is observable. For high frequencies (short wavelengths) there is substantially less diffraction, and the sound is significantly louder when the collimator is aimed at the observer. Good frequencies are about 120Hz and 4000 Hz.
    H2, ME2

  • 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

  • H2-33: SPEAKER AND EXPONENTIAL HORN

    H2-33
    Demonstrate the effect of an exponential horn enclosure.
    A small loudspeaker is held up behind the opening of an exponential horn. The sound becomes much louder, especially in the bass. A horn enclosure has the effect of taking an extended source such as a loudspeaker and creating the best impedance match with the outside world, providing the most coherent plane wave. Compare this to H2-32, which uses the same speaker with a flat baffle. Invite students to speculate about what the effects the different shapes have.
    H2, OS5

  • 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