Transcript II. Nature of Sound

~ Nature of Sound ~
1. What is sound?
2. Human Hearing
3. Properties of Sound
4. Interactions of Sound
5. Sound Quality
I. WHAT IS
SOUND?
I. WHAT IS SOUND?



A Sound wave is a LONGITUDINAL WAVE
caused by vibrations of particles back and forth
through a medium.
Sound waves are mechanical waves and NEED
A MEDIUM TO TRAVEL
Sound waves travel in all directions from the
source
II. Human Hearing
II.Human Hearing
sound wave
vibrates ear drum
amplified by bones
converted to nerve
impulses in cochlea

http://www.hearingusa.com/ears01.mov
III. Properties of
Sound
A. SPEED OF SOUND
 345
m/s in air at 20°C
 Depends on:

Type of medium
travels better through liquids and solids
 can’t travel through a vacuum


Temperature of medium

travels faster at higher temps

http://www.stmary.ws/highschool
/physics/home/notes/waves/C
haracteristicsPeriodicWaves.htm
B. PITCH

Pitch



highness or
lowness of a
sound
depends on
frequency of sound
wave
human range: 20 20,000 Hz
ultrasonic waves
subsonic waves
C. LOUDNESS

Loudness



volume of sound
depends on energy (amplitude) of sound wave
measured in decibels (dB)
Loudness measured in Decibels
DECIBEL SCALE
100
70
40
0
10
18
80
110
120
D. DOPPLER EFFECT

Have you ever noticed how the sound
of a police car siren changes as it
moves past you? The changes in pitch
you hear are due to a phenomenon
known as the Doppler effect. Look at
the illustration of the police car to
answer these questions.

The police car has passed the woman
standing on the sidewalk, but has not
yet passed the man. Which observer
hears the siren at a higher pitch—the
woman or the man?
D.DOPPLER EFFECT
Doppler Effect




change in wavelength due to the motion
of the source. As the source moves, the
waves become bunched up and the
pitch (frequency) of the sound changes.
moving toward you - pitch sounds
higher
moving away from you - pitch
sounds lower
Doppler Effect

http://www.fearofphysics.com/Sound/doppler.htm
l
Doppler Effect

http://www.colorado.edu/physics/2000/applets/do
ppler.html

http://www.pbs.org/wgbh/nova/universe/moving.h
tml
IV. INTERACTIONS OF
SOUND
Reflection
Interference
Resonance
A. REFLECTIONS OF SOUND
WAVES



Reflections of sound
waves are called echos!
Sound waves reflect best
off smooth, hard surfaces
ECHOLOCATION

USE OF REFLECTED
SOUND WAVES TO FIND
OBJECTS
Ultrasonic waves are used to see with
sound using reflection!

Ultrasonic waves - above 20,000 Hz
SONAR
Medical Imaging
“Sound Navigation Ranging”
B. Interference

Interference

When 2 or more waves overlap
Constructive - louder
Destructive - softer
SONIC BOOM

When a jet plane reaches the speed of sound,
the sound waves in front of the jet combine by
constructive interference. The result is a high
density compression (high amplitude)
compression called the sound barrier.
WATCH THESE VIDEOS!

http://www.youtube.com/watch?v=d9A2oq1N38&feature=related

http://www.youtube.com/watch?v=QjpikTn2qAk&f
eature=related
C. RESONANCE

Forced Vibration



when one vibrating object forces
another object to vibrate at the
same frequency
results in a louder sound
because a greater surface area
is vibrating
used in guitars, pianos, etc.
= Resonance =

Resonance



special case of forced
vibration
object is induced to
vibrate at its natural
frequency
http://www.youtube.com/
watch?v=17tqXgvCN0E
= Resonance =
“Galloping Gertie”
The Tacoma Narrows Bridge
Disaster
http://www.youtube.com/watch?v=j-zczJXSxnw&feature=related
Wind through a narrow waterway caused the bridge to vibrate until it reached its natural frequency.
= Resonance =

Beats


variations in sound
intensity produced by
2 slightly different
frequencies
both constructive
and destructive
interference occur
V. Sound Quality
A. Music vs. Noise

Music



specific pitches and sound quality
regular pattern
Noise


no definite pitch
no set pattern
B. ACOUSTIC

Acoustics


the study of sound
Reverberation

echo effect produced
by the reflection of
sound
Anechoic chamber - designed
to eliminate reverberation.