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  • G3-52: ROPE WAVE GENERATOR - ROPE TENSION VS WAVELENGTH

    G3-52
    Observe the change in wavelength of a vibrating rope as the tension is varied.
    Set the generator frequency using about 250g to produce 2 loops. Quadrupling the weight increases the wave speed (and thus the wavelength) by about a factor of two, creating one loop, while using about 110 grams creates three loops, two-thirds of the original wavelength. This is Mersenne's second law for stretched strings.
    FS1

  • G3-53 STANDING WAVES IN A STRING

    G3-53
    Demonstrates standing waves on a thin string
    The string is driven by a 60 Hz vibrator with the number of standing wave loops determined by adjusting the tension. Black painted channel is the background for improved visibility.
    OS0, LS1

  • G4-12: STANDING WAVES ON A SOAP FILM

    G4-12
    Demonstrate standing waves in a circular membrane.
    A circular wire loop (from M4) holds a soap film which is positioned in front of a loudspeaker (the rectangular box to the left of the soap film). It is illuminated by a bright point source which casts a reflected pattern on the screen as shown in the photograph. A sine-wave oscillator attached to the speaker can be tuned to obtain various standing wave patterns, which are defined by the bright lines in the reflected light. Some standing wave patterns are shown above.
    M4, ME3, LS1

    g4-12a g4-12b

  • 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

  • H1-04: 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.

  • H1-21: 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

  • H1-23: 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.

  • H1-44: 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?

  • 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

  • H2-03: 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

  • H2-24: 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

  • H2-28: 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