~ Nature of Sound ~
1. What is sound?2. Human Hearing3. Properties of Sound4. Interactions of Sound5. Sound Quality
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 Hearingsound wave
vibrates ear drum
amplified by bones
converted to nerve impulses in cochlea
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
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)
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.colorado.edu/physics/2000/applets/
doppler.html
http://www.pbs.org/wgbh/nova/universe/moving.html
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
A. Music vs. Noise
Music specific pitches and sound quality regular pattern
Noise no definite pitch no set pattern