Download presentation
Presentation is loading. Please wait.
Published byMalcolm Sparks Modified over 9 years ago
1
Chapter 26: Sound
2
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
3
Sound Waves and Frequency
4
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
5
Sound in Air
6
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!)
7
Bell in a Vacuum
8
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)
9
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
10
SourceIntensity Level # of Times Greater Than TOH Threshold of Hearing (TOH) 1*10 -12 W/m 2 0 dB10 0 Rustling Leaves1*10 -11 W/m 2 10 dB10 1 Whisper1*10 -10 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 dB10 9.8 Walkman at Maximum Level 1*10 -2 W/m 2 100 dB10 Front Rows of Rock Concert 1*10 -1 W/m 2 110 dB10 11 Threshold of Pain1*10 1 W/m 2 130 dB10 13 Military Jet Takeoff1*10 2 W/m 2 140 dB10 14 Instant Perforation of Eardrum 1*10 4 W/m 2 160 dB10 16
11
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
12
Forced Vibration
13
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
14
Natural Frequency
15
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
16
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
17
Interference Patterns
18
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
19
Beats
20
Light Chapter 27
21
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
22
Photoelectric Effect
23
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)
24
Michelson-Morley Experiment
25
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
26
Electromagnetic Spectrum
27
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
28
Transparent Objects
29
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
30
Opaque Materials
31
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
32
Shadows
33
Eclipses
34
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
35
Polarization Java Applet
36
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)
Similar presentations
© 2025 SlidePlayer.com. Inc.
All rights reserved.