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Sound Reproduction

  • G3-31: Spring and Horns

    G3-31
    To audibly illustrate transmission of energy in a mechanical wave
    This long spring has a horn mounted on each end, which amplifies the vibrations from the spring as they pass into the air. Invite a student volunteer to hold each end, and show how energy is transferred through a wave and produces sound at the far end of the spring.

    This demonstration was donated by Prof. William Dorland.

    G3

  • 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

  • H1-32: 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.

  • H1-53: AUDIOTAPE 8 MIN - WOMB SOUNDS

    H1-53
    Hear heartbeats.
    This is an audiocassette recording made in an obstertric lab. The parent's heartbeat is clearly audible. The fetal heartbeat is barely discernable. Primarily of interest for medical students.

  • 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

  • H2-31: 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

  • 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

  • H4-34: GUITAR AND OSCILLOSCOPE

    H4-34
    Illustrate how a guitar works
    Play notes or chords on the guitar to see their wave shapes on the oscilloscope. Notice that as the notes decay their wave shapes change, a result of different decay times for different harmonics.
    OS5, ME2, ME3

  • H4-51: MODULATION - AM AND FM

    H4-51
    Demonstrate AM and FM signal modulation as an introduction to vibrato and tremolo.
    The Pasco Dual Function Generator is used to produce either amplitude modulation or frequency modulation using various combinations of sine, triangular, and square waves. Frequency modulation is pure vibrato and amplitude modulation is pure tremolo; actual vocal vibrato is a combination of pure vibrato and pure tremolo.
    H4, ME2

    h4-51ah4-51b

  • 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

  • H4-56: SYNTHESIZER INTRODUCTION - VOLTAGE CONTROL OF FREQUENCY

    H4-56
    Illustrate a voltage-controlled frequency device.
    The oscillator at the left produces a slowly varying voltage which is input into the VCG (voltage-controlled generator) input of the oscillator on the right. The frequency of the second generator is controlled by the VCG signal. The loudspeaker is driven directly by the output of the second generator. This is one method of control in analog synthesizers.
    ME3

  • H4-58: MODULATION - AM RADIO

    H4-58
    Show amplitude modulation in in AM radio signals.
    The amplitude modulated signal from an intermediate stage in the portable radio is viewed using an oscilloscope. You may leave it freely running or you may freeze the trace using the oscilloscope stop button in order to obtain the clearest picture for your particular need. Please turn off radio when done to save battery.

    Please handle with care, as discrete-component radios are hard to come by!
    J2B, ME2, ME3

  • H4-61: HARMONIC DISTORTION OF TAPE RECORDER

    H4-61
    Demonstrate harmonic distortion in an audiotape.
    Set up to record the wave generator square wave on tape. By switching the monitor on the tape deck to "SOURCE" you see the square wave after it passes the input and output pre-amps in the tape recorder. By switching the monitor to "TAPE" you see the square wave after it has been recorded and played back. The sound of either wave is heard using the power amplifier and loudspeakers. Changes of phase of the various harmonics during the recording and playback process results in distortion, or changing of the final wave shape. By expanding the horizontal (time) and vertical (voltage) scales on the oscilloscope, you can view the (approximately) 100 kilohertz "bias" voltage that is applied to the incoming wave before it is recorded.

  • H4-64: DISTORTION IN AUDIO AMPLIFIER

    H4-64
    Demonstrate distortion due to over-driving an audio ampiflier.
    Raising the input signal level past its linear range creates distortion in the output signal, which can be seen on the upper trace of the scope, and additional harmonics in its spectrum, which is shown on the lower trace. The change in sound, resulting from the creation of additional harmonics, can be easily heard.
    ME2, ME3

    h4-64ah4-64b

  • H4-65: CROSSOVER NETWORK FOR SPEAKERS

    H4-65
    Demonstrate the operation af a crossover network.
    With white noise playing through the system, the fact that high frequencies come through the tweeter, while the lower frequencies come more through the woofer, can easily be demonstrated. A microphone is held in front of either the woofer or the tweeter, with the microphone signal input into an oscilloscope displaying both the wave shape and the frequency spectrum. (See Demonstration H4-02.)
    H4, ME2, ME3, FS1

    h4-65a