DEMONSTRATIONS IN ACOUSTICS
DEMONSTRATIONS IN ACOUSTICS
Dr. Richard E. Berg
Dr. David G. Stork
Department of Physics
University of Maryland
College Park, MD 20742
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Demonstrations in Acoustics
TABLE OF CONTENTS
- Demonstration 1 (DVD 1): Frequency, Amplitude and Tone Quality (9 minutes)
- A waveform generator, a loudspeaker, and an oscilloscope are used to demonstrate the effects of varying the period, the
amplitude and the wave shape of a sound wave. A recorder, a violin, a crumhorn, and a clarinet are played and their wave
shapes are displayed on an oscilloscope.
- Demonstration 2 (DVD 1): Damped and Driven Oscillations - Resonance (9 minutes)
- Mass-on-spring systems are used to analyze and graph the phenomena of damped and driven simple harmonic motion. The
condition for resonance is discussed and examples shown.
- Demonstration 3 (DVD 1) : Coupled Oscillations (6 minutes)
- Coupled resonances are studied using several mechanical systems. A demonstration of coupled tuning bars, the sound
analog to one of these demonstrations, is shown.
- Demonstration 4 (DVD 1): Glass Breaking with Sound Resonance (4 minutes)
- A glass beaker is broken using sound waves at the lowest resonant frequency of the beaker.
- Demonstration 5 (DVD 1): Bell in Vacuum (5 minutes)
- The sound of a bell in a jar becomes inaudible as the air is pumped from the jar. A gauge shows the air pressure in the
- Demonstration 6 (DVD 1): Speed of Sound (4 minutes)
- The speed of sound is determined using an electronic wave generator, a loudspeaker, a microphone and an
oscilloscope. The microphone is moved away from the speaker by the measured distance along an optical rail, causing a time
delay in the signal received by the microphone as determined from the calibrated oscilloscope time scale.
- Demonstration 7 (DVD 1): Reflection from Concave Surfaces (5 minutes)
- Light rays are focused by a concave mirror to a point. The audio analog experiment is performed using a concave
parabolic reflector. Finally, a "beam" of sound is produced using a speaker at the focal point of one reflector
and is detected by a microphone placed at the focus of a second reflector.
- Demonstration 8 (DVD 1): Refraction - Sound Lens (3 minutes)
- Light rays are focused to a point by an optical lens. Acoustic lenses are produced by filling balloons with various
gases, both heavy (CO2) and light (He). Focusing and defocusing of sound waves is illustrated.
- Demonstration 9 (DVD 1): Interference (11 minutes)
- Interference of sound is illustrated using a Quincke's Interference Tube. Young's two- source interference is
illustrated visually with Moire patterns, and then acoustically, using two loudspeakers. This experiment is first performed
in the studio, and then outside, to eliminate effects of reflection off the walls, floor and ceiling. An experiment is
presented illustrating the effects of reversing the phase between two stereo loudspeakers.
- Demonstration 10 (DVD 1): Diffraction (5 minutes)
- Using a sound collimator constructed from a speaker and a sound absorbent tube, diffraction effects are shown to be
greater at low frequencies than at higher frequencies. The diffraction of sound waves from a small loudspeaker, which leads
to interference and a marked decrease in sound intensity, can be largely eliminated, as demonstrated using a simple baffle
constructed from a 2 foot square piece of plywood with a small hole in the center, behind which the speaker is placed.
- Demonstration 11 (DVD 1): Doppler Effect (3 minutes)
- The Doppler Effect is illustrated by throwing a ball in which a 3000 Hz electronic whistle is sounding.
- Demonstration 12 (DVD 1): Beats (5 minutes)
- Using two audio oscillators, a mixer amplifier, a loudspeaker and a dual trace oscilloscope, various aspects of beats are
studied. The individual waves are shown, with the sum sound varying in amplitude as the phase of one wave shifts with respect
to the other. By changing the time scale on the oscilloscope, the envelope of the beat pattern can be observed, and the sum
wave can be correlated with the loudness of the sound.
- Demonstration 13 (DVD 1): Shive Wave Machine - Standing Waves
- Standing waves in a Shive Wave Machine are shown with both ends fixed, with both ends free, and with one end fixed and
one end free.
- Demonstration 14 (DVD 1): Standing Sound Waves (8 minutes)
- After a brief exposition on the origin of standing waves, a standing sound wave is created between two speakers about a
meter apart. One source is then replaced by a metal plate, and a standing wave is produced by the wave from the speaker with
the reflected wave. Some details of standing sound waves are shown using a Kundt's Tube driven by an audio
- Demonstration 15 (DVD 1): Reflection of Pulses (10 minutes)
- Reflection of pulses of a transverse wave from fixed and free ends is examined using a Shive Wave machine. A detailed
study is then presented showing how sound pulses reflect off open and closed ends of an air column.
- Demonstration 16 (DVD 1): The Overtone Series (10 minutes)
- The overtone series through the tenth harmonic is illustrated using a Fourier synthesizer. First, the individual tones
of the series are shown, and then the sum is obtained. The harmonics obtained in air columns are illustrated using a
corrugated plastic tube. Harmonics are excited in a violin string by bowing while touching it lightly at 1/N of its
length. The harmonics of an aluminum rod are excited by stroking the rod with resin on fingers. Chords are illustrated by the
Fourier synthesizer and a Savart's disk demonstration.
- Demonstration 17 (DVD 1): Standing Waves in a String (6 minutes)
- A large rope wave generator is used to illustrate standing waves. A tightly stretched string and a stroboscope are used
to study details of a standing wave.
- Demonstration 18 (DVD 2): Mersenne's Laws (6 minutes)
- A two-stringed sonometer is used to show the effect of changes in length, tension and mass per unit length on the
fundamental frequency of a wire under tension.
- Demonstration 19 (DVD 2): Standing Waves in Air Columns (11 minutes)
- A tuning fork is used to excite the fundamental mode in an air tube. The relationship between the fundamental modes of
open and closed tubes of the same length is shown. A nichrome heater inserted into a 4" diameter, ten foot long tube
mounted vertically creates convection currents which produce noise, driving the tube at its resonant frequency of about 55
Hz. A vacuum cleaner draws air rapidly through a Twirl-A-Tune, producing up to fifteen harmonics. A manometric flame tube,
in which a loudspeaker drives a six-foot long 4" diameter horizontal gas tube, illustrates standing waves in a very
- Demonstration 20 (DVD 2): Basic Trumpet Acoustics (6 minutes)
- Blowing a plastic tube at one end in the manner of a trumpet shows that the tube behaves acoustically like a closed tube,
with the lip end closed. The technique of obtaining the notes on a standard trumpet is demonstrated by inserting a trumpet
mouth piece into the end of a piece of flexible tube, showing that all harmonics can be obtained. Adding a bell made from a
metal funnel increases the loudness of the sound.
- Demonstration 21 (DVD 2): Chladni Plates (10 minutes)
- After a brief demonstration of the standard bowed Chladni plate, the use of magnetostriction in a thin-walled, annealed
nickel tube to drive the Chladni plates is explained and demonstrated. Figures are generated in square, circular,
violin-shaped and necktie-shaped metal plates.
- Demonstration 22 (DVD 2): Fourier Synthesis (9 minutes)
- Using a Fourier synthesizer constructed at the University of Maryland, triangular, square, sawtooth, and pulse train waves
are synthesized. The sum wave and the harmonic being added are shown simultaneously on a dual-trace oscilloscope, and the tone
is heard as the harmonics are added or removed. The frequency of the synthesized wave is varied while the phases of the
harmonics are locked to preserve the wave shape, thus illustrating the consistency of tone quality for a given wave shape
over a large frequency range.
- Demonstration 23 (DVD 2): Fourier Analysis (11 minutes)
- Wave shapes and Fourier spectra are displayed simultaneously for some standard waves: sine, square and sawtooth, as well
as for notes on a recorder, a violin, a crumhorn, and a clarinet.
- Demonstration 24 (DVD 2): Resonance Curves (12 minutes)
- Resonance curves are shown for a Helmholtz resonator, an open tube and a closed tube, and some applications are
- Demonstration 25 (DVD 2): Modulation (12 minutes)
- Frequency modulation, amplitude modulation and balanced (or ring) modulation are illustrated through appropriate
oscilloscope displays while the resulting waves are heard simultaneously.
- Demonstration 26 (DVD 2): Musical Synthesizer Fundamentals (12 minutes)
- An introduction to the analog musical synthesizer is given using an ARP 2600 synthesizer. The differences between control
and audio signals are discussed. The various synthesizer components, such as keyboard, VCO, VCA, envelope generator (or ADSR),
low frequency control oscillators, noise generators, filters, ring modulator, and sample and hold, are discussed and
illustrated individually and in various combinations.
- Demonstration 27 (DVD 2): Vocal Formants (8 minutes)
- Vocal formants are defined and then observed using a Fourier analyzer. Several brief demonstrations are performed which
illustrate the formant regions of certain vowel sounds as well as the effect of changing the fundamental frequency on the
- Demonstration 28 (DVD 2): Audio Spectrograms (10 minutes)
- Simple audio spectrograms for some basic electronic sounds are presented. Comparison and contrast of the four vocal
sounds "see", "soo", "nee" and "noo", are made with the use of the Fourier
spectrograms. The word "wow" is electronically synthesized and compared with the sound of the spoken word.
- Demonstration 29 (DVD 2): Effect of Gas on Voice (4 minutes)
- Formant frequencies change as a result of a change in the gas inhaled. This effect is illustrated by singing and
talking after inhaling a light gas (He), and then a heavy gas (SF6).
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