~ Nature of Sound ~

1. What is sound?2. Human Hearing3. Properties of Sound4. Interactions of Sound5. 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 Hearingsound 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/CharacteristicsPeriodicWaves.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

7080

100110

120

40

1810

0

DECIBEL SCALE

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.html

Doppler Effect http://www.colorado.edu/physics/2000/applets/

doppler.html

http://www.pbs.org/wgbh/nova/universe/moving.html

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

Medical Imaging

SONAR

“Sound Navigation Ranging”

Constructive - louder

B. Interference Interference

When 2 or more waves overlap

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&feature=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

Wind through a narrow waterway caused the bridge to vibrate until it reached its natural frequency.

http://www.youtube.com/watch?v=j-zczJXSxnw&feature=related

= 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.