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  • H3-01: STANDING SOUND WAVES - TWO SOURCES

    H3-01
    Demonstrate the origin of standing waves from two identical sources.
    Both speakers are driven by the same 3000 Hz sine wave from the audio oscillator, which is shown on the top trace of the oscilloscope. The standing wave pattern, observed by sliding a microphone along the optical rail between the speakers, is displayed on the lower trace.

    Consider showing both this and H3-02, and invite students to speculate about the differences.

  • H3-02: STANDING SOUND WAVES - REFLECTION

    H3-02
    Demonstrate the origin of standing waves from one source and its reflection.
    A loudspeaker is driven at about 3000 Hz by a sine wave oscillation which is displayed on the top trace of the oscilloscope. The standing wave pattern, created by the sound from the speaker and the wave reflected off the flat metal plate, is picked up by a microphone sliding along the optical rail and displayed on the lower trace of the oscilloscope.

    Consider showing both this and H3-01, and invite students to speculate about the differences.

  • H3-03: REFLECTION OF PULSES IN TUBE

    H3-03
    Demonstrate the phase of pulses reflected from open ends and closed ends of a tube.
    A pulse is created by the leading edge of a low-frequency square wave input into a loudspeaker at one end of a long, rigid plastic tube. This pulse travels along the tube and is reflected back and forth at the ends. The phase of the pulse as it reflects back and forth is observed using a microphone inserted into the center of the tube. Several reflections can be observed. It is easy to see the difference between reflection at the open end and reflection at a closed end, obtained by placing a metal plate against the end of the tube opposite the loudspeaker.
    H2, ME2, ME3, OM1, OS0

    h3-03b

  • H3-05: KUNDT'S TUBE - OSCILLATOR DRIVEN

    H3-05
    Demonstrate standing waves in an air column.
    An oscillator in the 1000-5000 Hz frequency range drives a loudspeaker at one end of a clear glass tube, with the other end stopped by a moveable plunger. Varying the frequency of the oscillator or the position of the plunger, one can obtain a series of standing wave patterns, which are made visual by the motion of cork dust in the bottom of the tube. The standing wave pattern is shown to large groups by placing the device on an overhead projector. This is a very dramatic demonstration, and is very effective in providing an introduction to standing sound waves. Examples of standing waves as seen using the overhead projector are shown below.

    Be aware that the tube is glass, and must be handled carefully.
    H2, ME3

    h3-05ah3-05b

  • 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

  • H3-15: 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

  • H3-21: 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

  • H3-23: RESONANCE TUBE - OSCILLATOR, PLUNGER AND MICROPHONE

    H3-23
    Demonstrate standing sound waves in a closed tube.
    An oscillator drives a small loudspeaker which is mounted at one end of a tube, with the other end stopped by a moveable plunger. A microphone adjacent to the loudspeaker at the open end of the tube is connected to the oscilloscope. When the frequency is varied or the position of the plunger in the tube is changed, sound resonances can be created in the tube and are displayed on the oscilloscope as an increase in amplitude. Resonances occur when the length of the tube is equal to any odd multiple of one-quarter wavelength of the sound wave.

    This is a variation on H3-22, and can be combined with it.

  • H3-32: RESONANCE IN TUBE - POURING WATER

    H3-32
    Demonstrate standing wave resonances in an acoustical closed tube.
    Water is poured into a tall graduated cylinder, creating a gurgling sound as the water level in the tube rises. Because the air column on top of the water is becoming shorter, the frequencies of the resonances rise, which can be easily observed. Compare H3-33.
    h3, OF2

  • 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

  • H3-42: RESONANCE CURVES - OPEN AND CLOSED TUBES

    H3-42
    Demonstrate the resonance behavior of open and closed tubes.
    A tube is excited by an oscillator driving a small loudspeaker. The sound is picked up by a sound probe adjacent to the driven end of the tube, and displayed on the oscilloscope. Resonances are clearly observable on the oscilloscope. The closed tube is obtained by placing a cap on the left end of the open tube shown in the photograph.

    These are the same tubes used in H3-24. Consider showing them to students first and having them make predictions, then using this apparatus to analyze the results.

    H3, ME2, ME3, OM1

  • H3-51: SONOMETER WITH TUNING PEGS

    H3-51
    Demonstrate standing waves in a stretched wire and to demonstrate Mersenne's first and second laws.
    The tensions in two stretched wires can be separately adjusted, creating different fundamental frequencies. Stops can be inserted along the wire to observe the frequency as a function of length, or to show the frequency ratio between two similar wires with the same tension but of different lengths. Note: this is easier to use than the sonometer with weights, but the tension cannot be measured quantitatively.
    OS0

  • 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

  • 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

  • H4-43: UKELELE

    H4-43
    Music?
    A simple Rogue baritone ukelele, useful as an example of smaller stringed instruments.
    OS5

  • H4-52: SPECTRUM ANALYSIS OF MODULATION

    H4-52
    Compare and analyze the frequency spectra of various modulated sounds such as tremolo, vibrato, and beats.
    Using the Pasco Dual Function Generator, a 1000 Hz sine wave is modulated by a 100 Hz sine wave and the spectrum of the modulated signal displayed using the spectrum analyzer. The photograph at the center shows the original 1000 Hz sine wave and the photograph at the right shows the case where that wave is amplitude modulated by a 100 Hz sine wave, producing a beat-like wave and a spectrum that has two sidebands around the 1000 Hz carrier. Amplitude modulation, frequency modulation, or double sideband modulation (sometimes called balanced modulation, or ring modulation with synthesizers) can be used. Two sine waves can be added together using the Dual Function Generator to produce beats, and the spectrum of the beats obtained and compared with that of double sideband modulation. The waveform is displayed on one trace and the spectrum on the other.

    Try out some frequency combinations ahead of time, then have students predict the results.
    H4, ME2, ME3

    h4-52ah4-52b

  • H4-55: YAMAHA DX7S DIGITAL SYNTHESIZER

    H4-55
    Demonstrate features of a modern digital synthesizer.
    This device is a modern digital synthesizer. An enormous number of functions and effects can be illustrated using this instrument. Please see Demonstration Reference File for manuals on its operation and features.
    OS5, ME3

  • H5-11: WAVETEK AND AUDIOCART - FREQUENCY RANGE OF HEARING

    H5-11
    Demonstrate the approximate frequency range of human hearing.
    The audio system has a useful range from below 20 Hz through well above 20 kHz, although it doesn't do well below about 40Hz. Have people raise their hands when they hear the tone to see the hearing range of the group. Note also that, keeping the intensity constant while sweeping from 1 kHz to 10 kHz, people hear the tone as louder around 3-5 kHz because the ear is most sensitive in that frequency range. Invite student discussion of why different people may have slightly different hearing ranges, and how that affects us in everyday life. How can this information be used to improve accessibility and inclusivity?
    FS1

  • H5-12: WAVETEKS AND AUDIO CART - CRITICAL BAND

    H5-12
    Demonstrate the effect of the critical band on the sound of two simultaneous sine waves.
    Set the two oscillators to equal amplitudes and the amplifier in monaural, with one at 500 Hz. Starting at less than 100 Hz, sweep the frequency of the second oscillator slowly past that of the first oscillator. When the two oscillators come within about 20 percent in frequency, a coarseness can be heard. This coarseness arises from the overlap of the critical bands of the two tones. Moving the two frequencies closer creates beats.

    Note that these frequencies are chosen to provide an easily audible demonstration; critical bandwith varies significantly with frequency.