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/ 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

SourceIntensity Level # of Times Greater Than TOH Threshold of Hearing (TOH) 1* W/m 2 0 dB10 0 Rustling Leaves1* W/m 2 10 dB10 1 Whisper1* W/m 2 20 dB10 2 Normal Conversation 1*10 -6 W/m 2 60 dB10 6 Busy Street Traffic1*10 -5 W/m 2 70 dB10 7 Vacuum Cleaner1*10 -4 W/m 2 80 dB10 8 Large Orchestra6.3*10 -3 W/m 2 98 dB Walkman at Maximum Level 1*10 -2 W/m dB10 Front Rows of Rock Concert 1*10 -1 W/m dB10 11 Threshold of Pain1*10 1 W/m dB10 13 Military Jet Takeoff1*10 2 W/m dB10 14 Instant Perforation of Eardrum 1*10 4 W/m dB10 16

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 = 3.0x10 8 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 )  Do Ch. 26 Assessment # (pg )  Read Chapter 27 (pg )  Do Ch. 27 Assessment #24-36 (420); Appendix F #1-15 (pg )