Transcript Welcome to PHY 1371

Chapter 1 (Hall)
The Nature of Sound
Dr. Jie Zou
PHY 1071
1
Outline

Introduce some of the basic concepts of
sound.





Acoustics and music
Organizing our study of sound
The Physical nature of sound
The speed of sound
Pressure and sound amplitude
Dr. Jie Zou
PHY 1071
2
Acoustic and music


Acoustics: The study of the
physical nature of sound.
Audible sound:



Dr. Jie Zou
Music: Includes those intentional
combinations of sounds that we
choose to hear for esthetic enjoyment
and usually depends on an orderly
pattern of sounds for its pleasing
effect.
Speech: Much in common with music
but differs in the purpose. Speech
sounds communicate the entire range
of human ideas through word
symbols rather than by conveying
emotions directly.
Noise: Unorganized, unpleasant, or
unwanted sounds.
PHY 1071
3
Organizing our study of sound

Study sound in three main areas:
(1) Production: How is it created?

Sound originates from vibration.
(2) Propagation: How does it travel from one place
to another?

Sound waves need a medium to propagate.
(3) Perception: How does it affect the senses and
emotions of a listener?



Dr. Jie Zou
Pitch: The sensation of how “high” or “low” a sound is.
Treble: High-pitched sounds. Bass: Low-pitched
sounds.
Loudness: The sensation of strength or weakness in a
sound.
PHY 1071
4
The physical nature of sound

What is sound in physics?





Vibration: A rapid back-and-forth
movement of a single object or of a single
small piece of some large object.
Wave: A disturbance traveling outward
from a vibrating source.
Connection between vibrations and
waves:


Dr. Jie Zou
Sound entails a disturbance (vibration) of
the air through which it moves.
Sound in air consists of longitudinal waves
carrying energy outward from their source.
The passage of a wave through any region
causes each little piece of material in that
region, in turn, to vibrate.
For a sound wave, it is not the air that
travels across the room; it is the signal
(the disturbance) in the air.
PHY 1071
5
Representation of a sound
wave
Ray

Crest
A simplified, stylized
representation of sound waves





Dr. Jie Zou
Ray: The rays indicate the direction
in which the wave is moving.
Crests: Points of greatest
compression.
Wavelength : The distance from
one crest to the next along the
direction of travel. For audible sound
  2 cm to 20 m.
Corresponding more literal
representation (see lower left
picture)
PHY 1071
6
The speed of sound



The speed of sound in dry afternoon temperature (T
= 20°C) is v = 344 m/s or 1130 ft/s, or 770 mi/hr.
All sounds, whether high or low in pitch (frequency),
travel through the air at the same speed.
If the air temperature changes, so does the speed of
sound.

If the temperature is higher, the random molecular motions
are faster, neighboring molecules collide more often, and
they can pass the sound disturbance faster from one region
to another. The sound speed increases about 0.6 m/s for
each degree of temperature rise on the Celsius scale.
Dr. Jie Zou
PHY 1071
7
Example#1

How far does sound travel during 0.002
second if the temperature is 20°C?
Dr. Jie Zou
PHY 1071
8
Pressure and sound amplitude

Displacement amplitude: The distance each bit
of air moves to either side of its normal position
during its vibration.


Pressure amplitude: The maximum increase of
air pressure (above normal atmospheric pressure)
in a sound wave compression.



Dr. Jie Zou
For ordinary sound waves, the displacement
amplitude is on the order of millionths of a meter or
less – displacement is very difficult to measure.
Pressure = Force/Area, 1 atm = 1.01 x 105 N/m2.
Pressure amplitudes are also typically very small, but
they are easily measured because microphone
diaphragms respond directly to the pressure
fluctuations.
The pressure amplitudes of sound waves at
comfortable listening levels range from about 0.01 to
1 N/m2, or 10-7 to 10-5 atm.
PHY 1071
9
Example #2

Suppose a particular sound wave
momentarily creates an extra pressure
of 10-4 atm upon a microphone
diaphragm that has an area of 1 cm2.
What total force in newtons (N) does
this make on the diaphragm?
Dr. Jie Zou
PHY 1071
10
Homework

(Hall) Ch. 1, P. 15, Exercises: #3, 11,
12.
Dr. Jie Zou
PHY 1071
11