Transcript 01Intro_Acoustics_F09

Physics 1200: Behind the Music
Aka “Musical Acoustics” or “Science of Sound”
Cluster: “Beats, Physics and the Mind”
[ Physics 1200, Music 1085 and Philosophy 1303 ]
“Dr. Bill” Pezzaglia
Physics Dept
CSU East Bay
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Notes
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Lecture Notes can be downloaded at:
http://www.clifford.org/drbill/csueb/
TextBook:
– Donald E. Hall, Musical Acoustics
(3rd edition) [Brooks/Cole 2002]
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References (more advanced)
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Rossing, Moore & Wheeler, “The Science of Sound”
(3rd ed, Addison Wesley 2002)
Fletcher & Rossing, “The Physics of Musical instruments”
(2nd ed, Springer 1998)
Olson, “Music, Physics & Engineering” (2nd ed, Dover 1967)
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Physics 1200 (Musical Acoustics)
Topic 01
Introduction to Acoustics
Updated 2010Jan03
Dr. Bill Pezzaglia
Physics CSUEB
Outline
I.
Intro to Acoustics
A. What is “Acoustics”
B. Nature of Sound Waves
C. Speed of Sound
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A. What is “Acoustics
1. Root of the Word
2. Classes of Sound
3. What we study in this class
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1. The term: “Acoustics”
a) Joseph Sauveur (1653-1716) coined the term
acoustique, which he derived from the ancient Greek
word ακουστός, meaning "able to be heard".
b) Acoustics is the interdisciplinary science that deals
with the study of sound (all mechanical waves in gases,
liquids, and solids)
c) Musical acoustics or music acoustics is the branch of
acoustics concerned with researching and describing
the physics of music .
Reference: wikipedia
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2. Classes of Sound
a) Ultrasound: above hearing
(sonograms!)
b) Infrasound: below hearing
(earthquakes!)
c) Audible Sound
(between 20 and 20,000 oscillations per second)
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3. What we study
a) Production of sound
(instruments, speakers)
b) Propagation of sound
(waves)
c) Perception of sound
(Psychoacoustics: ear, harmony)
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B. The Nature of Sound
1. Wave Modes
2. Structure of a wave
3. Sound waves
1. Types of Waves
(a) Longitudinal (“p” waves) travel through gas, liquid and
solids. Particle motion in direction of wave
(b) Transverse (“s” waves) travel only in solids. Particle
motion perpendicular to direction of wave.
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(c) Surface Waves
• Ocean Waves are “Cycloid” Waves. Particles travel in
circles.
• Rayleigh Waves (“R” wave) are similar, but exist in solids
(more damaging in earthquakes than “p” or “s” waves)
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(c) Light Waves
• Light is a wave of electric and magnetic phenomena that can travel
through empty space (no medium!)
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(d) Gravity Waves?
• Not discovered yet, but we are looking for them.
They would distort space as they travel by!
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2. The Structure of a Wave
a) Nodes: no displacement
b) Antinode: maximum displacement
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Wave parameters
(c) Wavelength (measured in meters)
(d) Amplitude (loudness) is the maximum wave
displacement
3. What is the “wave” of sound?
(a) Aristotle: suggested that the movement of air
carries sound to our ears, and if there is no air,
there will be no sound.
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Sound is immaterial. For it is not air, but it is the
form about the air and the appearance after some
sort of percussion which becomes sound; and
every appearance is immaterial; for it moves with
bodies, but is itself absolutely immaterial; as in the
case of a bent rod the surface-appearance suffers
no change, but the matter is what is bent.
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…air motion is generated by a source, "thrusting
forward in like manner the adjoining air, to that the
sound travels unaltered in quality as far as the
disturbance of the air manages to reach."
Aristotle 384 BC-322 BC
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3b. Marcus Vitruvius Pollio
• Roman Engineer (ca. 80/70 BC- ca. 25 BC)
• Realizing that a vibrating string strikes the air
many times in a series of blows, not just once
• suggests that the air not only moved, but
vibrated in response to the vibrations of the
string.
• Proposes that what we hear as sound are
these air vibrations.
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3c. Sound is displacement of pressure
• Rarefaction: the “trough” of the wave where pressure (density) is low
• Compression: the “peak” of the wave where pressure (density) is high
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C. The Speed of Sound
1. Wave Speed vs Particle Speed
2. Speed and Density of Medium
3. Dispersion
1. Wave speed vs Displacement Speed
a)
b)
Wavespeed: is how fast the nodes (or antinodes) are moving. In air, all
sounds move the same speed whether loud, soft, high or low.
Particle (Displacement) Speed: is how fast the particles in the medium are
moving, which can be quite different! This is related to the intensity of the
sound (soft sound they move slowly and not very far)
c) In a “standing wave”, the particles are moving but the wave is not moving
at all!
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2a. Speed of sound is finite
• Leonardo da Vinci
(1452-1519)
• A bell far away, will be heard
to resonate in response to a
bell ringing, after a delay in
time.
• 1500 Did he measure the
speed of sound? (some
references say yes).
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2b Speed of sound depends upon medium!
• (1640) classic
experiment on the
sound radiation by a
ticking watch in a
partially evacuated
glass vessel provided
evidence that air is
necessary, either for
the production or
transmission of sound.
Robert Boyle (1627-1691 AD)
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2c Measurement of Speed of Sound
• William Derham (1657-1735)
First to accurately measure
speed of sound (in air)
(341 meters/second)
• Newton used his value in the
Principia (1686), although it
was 16% higher than the
value Newton theoretically
calculated.
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2d Speed of Sound in Water (1826)
Ion Lake Geneva, Switzerland, JeanDaniel Colladen, a physicist, and
Charles-Francois Sturm, a
mathematician, measured speed to
be 5x faster than in air.
In their experiment, the underwater
bell was struck simultaneously with
ignition of gunpowder on the first
boat. The sound of the bell and flash
from the gunpowder were observed
10 miles away on the second boat.
The time between the gunpowder
flash and the sound reaching the
second boat was used to calculate
the speed of sound in water.
Speed did NOT depend upon
frequency!
2e Ernst Chladni (1756—1827)
• First measurement of speed
of sound in solids (up to 40x
faster than in air!)
• Measures speed of sound in
different gases
(slower in heavier gases)
• 1787 “Chladni Plate” shows
vibration of sound using sand
on a plate.
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3. Dispersion
(a) Sound in air is “nondispersive”, meaning speed is
independent of the frequency
of the sound
• Gassendi demonstrated
speed of sound is
independent of pitch by
comparing measurements
from cannon and rifle (no
“dispersion”)
Pierre Gassendi (1592-1655)
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3b Dispersive Media
• Sound in some media (e.g. metal plates)
is very dispersive.
• Speed of sound usually increases with
frequency (in metal)
• This creates “inharmonics” in
instruments (bells, xylophones, strings)
References & Notes
• Ruben’s Tube Demo:
– http://www.youtube.com/watch?v=kQ6jYR0-svE
– http://www.youtube.com/watch?v=JD90K6KfQes
– http://www.youtube.com/watch?v=EhnbhOoPIBc
– Mythbusters: http://www.youtube.com/watch?v=ynqzeIYA7Iw
• Dispersion of sound waves
– Part 1: http://www.youtube.com/watch?v=HtUGYKjG09g
– Part 2: http://www.youtube.com/watch?v=rm8cMV95gWY
• Echo Tube (shows dispersion of sound)
– http://www.youtube.com/v/H9LocUJCR5c
– http://www.youtube.com/watch?v=H9LocUJCR5c
– http://www.youtube.com/watch?v=IYRoWutoyxM
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Things to do
• x