Chapter 21: Sound

Pages 532 - 557

• Sound is a form of energy produced by the vibration of matter.

• Sound is a compressional or longitudinal wave

• Sound is transmitted through solids, liquids, and gases.

• Sound is transmitted better through solids and liquids. Why?–More dense

• Gases transmits sound a lot farther than a solid and liquid. Why?–Less dense; not as many

particles to interfere.

Sound• So what causes sounds to travel better

through some substances and not others?–The greater the elasticity, the

greater the speed.–The greater the density, the slower

the speed.–The best conductors of sound are

elastic substances.

• Sound can not be transmitted through a vacuum.–Sound needs a medium in order

for it to be transmitted.• Radio waves can travel through a

vacuum; no medium is needed.–This is why astronauts can use

radio signals to talk in space.

Speed of Sound

• 344 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

The denser the medium, the faster sound will travel.

The higher the temperature, the faster the particles of the medium will move and the faster the particles will carry the sound.

Sound travels through different media.

We hear sound which usually travels through air. Sound travels through other media as well, such as water and various solids.

Sound travels different speeds in different media. Sound typically travels faster in a solid that a liquid and faster in a liquid than a gas.

Parts of the ear

• Outer ear: pinna• Ear canal• Ear drum: tympanum

–Vibrates the ear drum

• Middle ear: three small bones–Anvil–Hammer–Stirrup

• Inner ear–Cochlea: filled with fluid

•Hair-like nerve endings•Auditory nerves

Brain

• Damage done to the hairs causes permanent hearing loss. The hairs never grow back.

B. Human Hearingsound wave

vibrates ear drum

amplified by bones

converted to nerve impulses in cochlea

Human Hearing

• Pitch– highness or

lowness of a sound

– depends on frequency of sound wave

– human range: 20 - 20,000 Hz

ultrasonic waves

subsonic waves

FREQUENCIES YOU CANNOT HEAR

•Ultrasonic: higher than 20,000 HzUses: clean jewelry, medical

applications•Infrasonic: lower than 20 Hz

Found: in the atmosphere and in the crust when plates move; also an indication motion sickness

Human Hearing

• Intensity– volume of sound– depends on energy (amplitude) of sound wave– measured in decibels (dB)

Human Hearing

7080

100110

120

40

1810

0

DECIBEL SCALE

Above 120 dB can cause hearing loss

Doppler Effect

• Doppler Effect– change in wave frequency

caused by a moving wave source

moving toward you - pitch sounds higher

moving away from you - pitch sounds lower

What is the Doppler Effect?

The Doppler Effect is the apparent change in frequency detected when the sound is moving relative to the hearer.

Reflection of Sound Waves

• Echoes• Echolocation: process using reflected sound

waves to find objects– Bats– Whales– SONAR– Ultrasonography

Seeing with Sound

• Ultrasonic waves - above 20,000 Hz

Medical Imaging SONAR“Sound Navigation and Ranging”

What is sonar?

Sonar is a system that uses the reflection of underwater sound waves to detect objects. This has been used to find sunken ships and schools of fish.

Diffraction

• Bends of waves around or through a barrierExamples:

1. Thunder2.Someone in the hallway

on the other side and wecan hear them.

Constructive - louder

D. Interference

• Interference– the ability of 2 or more waves to combine to form a

new wave

Destructive - softer

D. Interference

• Beats– variations in sound

intensity produced by 2 slightly different frequencies

– both constructive and destructive interference occur

Interference

–The effects caused by 2 or more waves.

•Ex. Several instruments produce interference in a band.

Cool Interference Examples:• The Sound Barrier: the point at which the

source of a sound accelerates to the speed of sound

• Sonic Booms: the explosive sound heard when a shock wave reaches your ears

• 1st time sound barrier broken: Oct. 14, 1947 by Chuck Yeager (speed of sound is called Mach 1); so Mach 6 is going 6 times the speed of sound

Sound Barrier and Sonic Booms

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

Harmonics

• Fundamental– the lowest natural frequency of an object

• Overtones– multiples of the fundamental frequency

• Examples

–Fundamental – 100 Hz–1st Overtone – 200 Hz–2nd Overtone – 300 Hz

Music vs. Noise

• Music– specific pitches and sound quality– regular pattern

• Noise– no definite pitch– no set pattern

Acoustics

• Acoustics– the study of sound

• Reverberation – echo effect produced by

the reflection of sound

Anechoic chamber - designed to eliminate reverberation.

Acoustics