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Wave Motion

  • 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

  • G4-32: MOIRE PATTERNS - COLOR

    G4-32
    Show color "interference" effects.
    Pattern transparencies of different color and physical character can be combined in various ways to obtain a variety of exotic effects. Several patterns obtained using various sets of colored transparencies are shown below. The moire patterns between one transparency and the lines of the video are as interesting as the lines created by the superposition of the two colored transparencies!
    G4

    g4-32a g4-32b g4-32c

  • G4-33: GROUP VELOCITY - TRANSPARENCIES

    G4-33
    Demonstrate the relationship between the wave velocity and the group velocity
    A set of two 8"x11" transparencies with closely-spaced parallel lines, are slowly slid across each other. One has a spacing about ten percent smaller than the other, so the difference in spacing creates a Moire interference pattern. Wave groups move rapidly across the picture while the wave velocity (motion of the transparency) is much smaller. Note that the scale of the photo at the bottom, containing the superposition of the two transparencies, is greater than that of the top photograph containing the component transparencies.

    g4-33a

  • 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

  • H1-02 SPEAKER AND CANDLE

    H1-02
    Demontrates longitudinal behavior of sound waves
    A lighted candle is placed directly in front of the center of a large loudspeaker, which is operating in the 10 Hertz range. The motion of the candle flame is longitudinal, following the motion of the air, illustrating the longitudinal nature of sound waves.

    With a bit of exploration, one can find resonances in the system that produce the most dramatic flame displacement. Consider having students make predictions about how different waveforms will make the flame respond differently
    OS5, ME2

  • H1-04: BELL IN VACUUM - PORTABLE

    H1-04
    Small version of the standard bell in vacuum demonstration.
    This is a more portable version of the classic demonstration H1-01, suitable for small classes. A small battery-powered bell hangs inside a glass jar. The jar has a snug-fitting lid with a gasket to give a reasonably airtight seal.

    Turn the bell on by connecting it to the battery with banana plug wires. Connect the portable pump to pump air from jar. The sound of the bell will quickly diminish as the air is removed.
    H1, I0

  • 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

  • H1-12: VISIBLE WAVEFORMS ON LARGE SPEAKER

    H1-12
    Show the loudspeaker motion produced by waves of various shapes.
    Using waves of a few Hertz, the shape, amplitude and symmetry of the pulses can, to a limited extent, be observed by watching the movement of the speaker cone. The oscillator must have a reasonably large amplitude.

    This can be a good demonstration for student interactivity; ask them to predict how what they see will change with different wave shapes, frequencies, and amplitudes. A video camera can be provided upon request to make the demonstration more visible in large classrooms.

  • 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

  • H1-21: SPEED OF SOUND - PHASE CHANGE

    H1-21
    Determine the speed of sound.
    A sine wave from a standard function generator is sounded by a small loudspeaker and picked up by a microphone. The original sine wave triggers the oscilloscope, which displays the signal from the microphone. Motion of the microphone away from the loudspeaker, measured by a ruler or the optical rail scale, is accompanied by a phase or time delay, measured by motion of the oscilloscope trace against the underlain gridlines. The speed of sound is the distance the microphone is moved divided by the additional time lag. This works best at frequencies of at least 3000 Hz.

    This is one of our most reliable demonstrations for showing an accurate measurement of the speed of sound in class, but does require a bit of explanation. This can also be an opportunity to discuss measurement equipment and the mathematics behind the process.
    H1, ME2, ME3, OM1, OM2

  • H1-22: SPEED OF SOUND - USING PULSES

    H1-22
    Determine the speed of sound in air.
    A standard bench oscillator is set up to produce a square wave that is narrowed into a series of pulses. This series of pulses from the oscillator is input into a small loudspeaker and picked up by a microphone. The oscilloscope is triggered by the pulses to the loudspeaker, but also displays the signal from the microphone. If the microphone is moved away from the loudspeaker, the signal picked up by the microphone will be delayed in time, shifting to the right on the scope. The speed of sound is the distance the microphone is moved divided by the time shift of a pulse on the oscilloscope, as measured by the oscilloscope reticule gridlines. Start with narrow pulses around 500 Hz and a scope time scale of about 500 microseconds per division.

  • H1-23: SPEED OF SOUND IN ALUMINUM

    H1-23
    Compare the measured and the theoretical values of the speed of sound in aluminum.
    An aluminum rod is stroked (See Demonstration H3-71: STROKED ALUMINUM ROD.), setting up longitudinal standing waves in the rod. The frequency f is determined using a frequency meter, with or without the aid of an audio oscillator. The length L of the rod, one-half wavelength for the fundamental, is measured using a two-meter rule. The speed of sound in aluminum is then S = 2fL. The theoretical value is obtained by using the Young's modulus Y and the mass density d: S = SQRT(Y/d), where the Young's modulus Y=7.0x10^+10 Pa and density of aluminum d=2.699x10^+3 kg/m^3. Putting in numbers, S = SQRT(Y/d) = 5,093 m/s. For the first mode of the stroked rod, the wavelength is twice the length of the rod, so measuring the length of the rod L = 1.83m, and the frequency of the first mode f = 1370 Hz, the speed of sound in aluminum is S = 2fL = 5,014 m/s.

  • H1-24: SPEED OF SOUND IN HELIUM

    H1-24
    Determine the speed of sound as a function of gas density.
    A section of garden hose, coiled around a metal drum, is filled with helium. Two microphones at the ends of the hose are connected together electrically, with their output fed into an oscilloscope. Tapping one of the microphones with a small hammer produces the pulse at the left in the photograph above; the second pulse is that tap after traveling through the tube of helium to the other microphone. The length of the hose is about 14.4 m. Approximate values for the speed of sound obtained using this apparatus are: 340 m/s for air, 900 m/s for helium.

    Compare H1-26, which uses the same setup with air as the medium.
    OS7, ME2, FS1

  • H1-25: SPEED OF SOUND BETWEEN TWO MICROPHONES

    H1-25
    Measure the speed of sound by determining the travel time of a pulse between two microphones.
    A loudspeaker sends pulses past the two microphones, and the oscilloscope displays the pulses on its time axis. The oscillator is set for 2 Hertz square waves to produce well-separated pulses. The speed of sound is the distance between the two microphones divided by the time delay read from the oscilloscope. In the photo above the microphones are 34.5 cm apart and the scope is set at 200 microseconds/div.

    For a simpler calculation, you can preselect the spacing of the microphones to give a convenient time delay, such as 1ms.

    Note that this demonstration does not reliably give results as accurate as demonstrations such as H1-21, but the simpler setup can be useful in some classes. One option can be to use multiple methods, and invite students to discuss the differences between results.

  • H1-26: SPEED OF SOUND IN GARDEN HOSE

    H1-26
    Direct measurement of the speed of sound in air.
    Two microphones at the ends of a section of garden hose (wound around a metal drum for convenience) are connected together electrically with their output fed into an oscilloscope. Tapping one of the microphones with a small hammer produces the pulse at the left in the photograph above; the second pulse is that tap after traveling through the air to the other microphone. The length of the hose is about 14.4 m, and the time interval measured from the scope trace is about 40 ms.

    Note: This is related to H1-24: Speed of Sound in Helium. These can both be used in a single class, but can require some time to switch over. This can be a good project for in-class discussion, giving students the opportunity to make predictions and discuss while the gas is changed. This can also be an opportunity to introduce concepts related to measurement uncertainty and propagation of error. For a shorter demonstration, use H1-26 alone to make a single measurement.
    OS7, ME2

  • H1-27: SPEED OF SOUND - LISSAJOUS FIGURES

    H1-27
    Measurement of the speed of sound in air using Lissajous figures.
    The signal to the loudspeaker is used as the horizontal input of an oscilloscope, and the signal picked up by the microphone is used as the vertical input, forming Lissajous figures. When they are in phase a diagonal line is produced, running from the lower left to the upper right of the oscilloscope screen. This situation is seen in the photograph above.

    As the microphone is moved away from the loudspeaker the vertical signal falls 90 degrees behind in phase, causing the Lissajous figure to form an ellipse. When the two signals are out of phase (180 degrees phase difference) the pattern is a line along the opposite diagonal. As the microphone is withdrawn further, the microphone signal becomes 270 degrees behind in phase and the pattern again becomes an ellipse. One important difference between the two ellipses is that they are rotating in opposite directions, but this is not observable on the oscilloscope. Withdrawal of one full wavelength, when the signal from the microphone lags a full period (360 degrees) behind the original condition, creates a pattern similar to the original pattern. In this case the signal picked up by the microphone is reduced in amplitude due to the inverse square law, reducing the slope of the line.

    For the most accurate measurement a frequency meter is connected to the trigger output of the oscillator. In the case shown below:

    S = 3385Hz x 104mm = 352 m/s.

    The photographs above show the Lissajous patterns at 90 degree intervals as the microphone is withdrawn.

                       

  • H1-31: SOUND LEVEL METER

    H1-31
    Demonstrate use of a sound level meter.
    Several loud sources can provided upon request, including musical instruments, noisy laboratory apparatus, and a portable audiotape machine with earphones. You can also invite students to bring up their own devices to test. It is surprisingly easy to get over 100dB in earphones. The sound level meter can viewed by a TV camera and displayed on the main screen.

  • 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.

  • H1-41: ULTRASONIC MOTION DETECTOR

    H1-41
    Demonstrate how an ultrasonic motion detector works.
    This device senses motion by comparing the Doppler shifted wave reflected by a moving object with the original wave created by the device. Any frequency change is accompanied by a continuous phase change between the two waves, which is sensed and ultimately turns on a light bulb on top of the unit, corresponding to the alarm.

    (1) Moving some object toward or away from the device sets off the alarm light.

    (2) Moving an object across the beam does not cause a Doppler shift, and therefore does not set off the alarm.

    (3) The frequency of the emitted wave is about 40 kHz. With a wave generator and supertweeter, one can shine waves of about 40 kHz on the receiver transducer, fooling the device into thinking that a moving object is in the vicinity (as seen in the photograph).
    H1, ME3

  • H1-44: ULTRASONIC MOTION DETECTOR WAVE FORM

    H1-44
    Show the wave form of the ultrasonic signal created by the ultrasonic motion detector.
    A microphone is used to pick up the signal from the ultrasonic motion detector; that signal is then amplified and viewed using an oscilloscope. The wave form consists of bursts of 45 kHz ultrasound at intervals of about 25 milliseconds. Above is the signal seen by the oscilloscope with the horizontal scale at 10 ms/division (left) and at 100 microseconds per division (right).

    A question for students: Why do motion detectors use such high frequencies?