Chapter 26: Sound

The Origin of Sound

All sounds are produced by the vibrations of material objects

Pitch – our subjective impression of sound A young person can normally hear pitches with

frequencies from 20 to 20,000 Hz; as we grow older, this range shrinks

Infrasonic – sound waves with frequencies below 20 Hz

Ultrasonic – sound waves with frequencies above 20,000 Hz

Sound Waves and Frequency

Sound in Air

Compression – a pulse of compressed air

Rarefaction – pulses of lower pressure Most sound waves are combinations of

compressions and rarefactions which create a longitudinal wave

Sound in Air

Media That Transmits Sound

Most sounds you hear are transmitted through the air

Sound also travels through solids and liquids

Solids and liquids are generally better conductors of sound than air, sound waves travel faster in solids and liquids

Sound cannot travel through a vacuum (there is nothing to compress!)

Bell in a Vacuum

Speed of Sound

You hear thunder after you see lightning (evidence that sound is much slower than light)

The speed of sound in dry air at 0ºC is ~330 m/s (1/100000000 the speed of light)

For each degree increase in temperature, the speed of sound increases by 0.60 m/s (at normal room temperature of 20ºC, sound travels at 340 m/s)

The speed of sound in a material depends not on the density, but on its elasticity (ability to change shape in response to an applied force)

Loudness The intensity of sound is proportional to the

square of the amplitude of a sound wave Sound intensity is objective and can be measured

by an oscilloscope Loudness is physiological sensation sensed in

the brain The unit of intensity for sound is the decibel (dB),

after Alexander Graham Bell Starting with zero at the threshold of hearing for a

normal hear, an increase of each 10 dB means that sound intensity increases by a factor of 10

Human hearing is approximately logarithmic

Source IntensityIntensity

Level# of Times

Greater Than TOH

Threshold of Hearing (TOH)

1*10-12 W/m2 0 dB 100

Rustling Leaves 1*10-11 W/m2 10 dB 101

Whisper 1*10-10 W/m2 20 dB 102

Normal Conversation

1*10-6 W/m2 60 dB 106

Busy Street Traffic 1*10-5 W/m2 70 dB 107

Vacuum Cleaner 1*10-4 W/m2 80 dB 108

Large Orchestra 6.3*10-3 W/m2 98 dB 109.8

Walkman at Maximum Level

1*10-2 W/m2 100 dB 1010

Front Rows of Rock Concert

1*10-1 W/m2 110 dB 1011

Threshold of Pain 1*101 W/m2 130 dB 1013

Military Jet Takeoff 1*102 W/m2 140 dB 1014

Instant Perforation of Eardrum

1*104 W/m2 160 dB 1016

Forced Vibration

Forced Vibration – the vibration of an object which is made to vibrate by another vibrating object that is nearby

The mechanism in a music box is mounted on a sounding board (the object being forced to vibrate); without the sounding board, the sound would be almost inaudible

The vibration of guitar strings in an acoustic guitar follow the same principle

Forced Vibration

Natural Frequency

Natural Frequency – an objects own special set of frequencies, which together form its special sound

The natural frequency depend on factors such as the elasticity and shape of the object

Bells and tuning forks vibrate at their own characteristic frequencies

Even planets and atoms vibrate at one or more natural frequencies

Natural Frequency

Resonance

Resonance – occurs when the frequency of a forced vibration on an object matches the object’s natural frequency, a dramatic increase in amplitude occurs

In order for something to resonate, it needs force to pull it back to its starting position and enough energy to keep it vibrating

Interference

Sound waves, like any waves, can be made to interfere

Interference affects the loudness of sounds Destructive interference of sound waves is usually

not a problem, because there is enough reflection of sound to fill in canceled spots; although, “dead spots” are often evident in poorly designed arenas

Destructive sound interference is used in anti-noise technology

Interference Patterns

Beats

Beats – the periodic variation in the loudness of sound

Beats can be heard when two slightly mismatched tuning forks are sounded together; when the forks are in step, the sound is at a maximum, when the forks are out of step, the sound is at a minimum

Beats

Light

Chapter 27

Early Concepts of Light

Light has been studied for thousands of years Up until the time of Newton, most scientists

thought that light consisted of particles The Dutch scientists, Christian Huygens,

argued that light was also a wave In Einstein’s photoelectric effect, light consists

of particles—massless bundles of concentrated electromagnetic energy—called photons

Now scientists agree to a dual nature for light, part particle and part wave

Photoelectric Effect

The Speed of Light The first demonstration that light travels at a

finite speed was supplied by the Danish astronomer Olaus Roemer ~1675, using the relationship between Io and Jupiter

The most famous experiment for the speed of light was done by the American physicist Albert Michelson in 1880, using reflected light from a mirror in the distance and an eyepiece (won the Nobel Prize)

We now know that the speed of light in a vacuum is a universal constant (c = 3.0x108 m/s)

Michelson-Morley Experiment

Electromagnetic Waves Light is energy that is emitted by accelerating

electrons in atoms, it travels in a wave that is partly electric and partly magnetic – electromagnetic wave

Light is a small portion of the large family of electromagnetic waves (radio waves, microwaves, X-rays, etc.)

Electromagnetic Spectrum – the range of electromagnetic waves

Infrared – electromagnetic waves of frequencies lower than the red of visible light

Ultraviolet – electromagnetic waves of frequencies higher than those of violet

Electromagnetic Spectrum

Light and Transparent Materials When light is incident upon matter, electrons in the

matter are forced to vibrate How a receiving material responds when light is

incident depends on the frequency of the light and the natural frequency of electrons in the material

Transparent – materials that allow light to pass through

When light passes through a transparent material, there will be a slight time delay as it must force the electrons in the material to vibrate, but the light will have the same frequency as before when it reemerges

Transparent Objects

Opaque Materials Opaque – materials which absorb light without

reemission and thus allow no light through them

Any coordinated vibration given by light to the materials atoms is transformed into random kinetic energy (become warmer)

Metals appear shiny because of a release of free surface electrons, by light vibrations, into the visible spectrum

Our atmosphere is transparent to visible light and infrared, but almost opaque to high-frequency ultraviolet waves

Opaque Materials

Shadows Ray – a thin beam of light When light shines on an object, some of the

rays may be stopped while others pass on Shadow – formed where light rays cannot

reach Sharp shadows are produced by a small

light source close to the object There is usually a dark part on the inside

and a lighter part around the edges Umbra – total shadow Penumbra – partial shadow

Shadows

Eclipses

Polarization Light travels in waves The waves are transverse, not longitudinal,

demonstrated by polarization When the vibrations of the light wave are

back and forth, the wave is polarized A wave can be polarized in either the

horizontal or the vertical A pair of polarizing sunglasses cuts out the

waves in one direction, allowing the other waves to go through

Polarization

Java Applet

Assignment

Read Chapter 26 (pg. 390-400) Do Ch. 26 Assessment # 21-39 (pg. 402-

403) Read Chapter 27 (pg. 404-418) Do Ch. 27 Assessment #24-36 (420);

Appendix F #1-15 (pg. 684-685)