Download Chapter 4: Sound

Wave Motion, Sound and Light
Wave Motion and Sound - Introduction
We will investigate the function of stereo speakers, microphones, VCR's,
DVD's, CD's and AM and FM radio stations. In addition we will discuss how
you hear sound and the waves that transmit sound and electrical signals.
The goal of this chapter will be an understanding of how you hear sound and
the physics of sound.
Objectives
Upon
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completion of this chapter you should be able to:
Explain how a speaker work
Explain how a microphone work
Explain the difference between longitudinal and transverse waves
Explain how information is stored on a VCR, floppy disc, CD and DVD
Explain how you hear sound
Click here to watch the video for “Wave Motion, Sound and Light –
Introduction”.
Longitudinal Waves
Sound waves are made of compressions and rarefactions of air molecules.
The compressions cause an increase in air pressure above normal
atmospheric pressure and the rarefactions cause a reduction in air pressure.
The louder the sound the greater the pressure differences compared to
atmospheric pressure. The pitch of the sound is controlled by the frequency.
The frequency is the number of compressions and rarefactions that occur per
second and are measured in cycles per seconds known as Hertz. This type of
wave is known as a longitudinal wave. Humans have a hearing range
between 20 and 20,000 Hertz. The following two links provide a good
visualization of a longitudinal wave. Note the locations of the compressions
and rarefactions in the wave in the two demonstrations. Demonstration 1:
http://www.gmi.edu/%7Edrussell/Demos/waves/wavemotion.html
Demonstration 2:
http://surendranath.tripod.com/Applets/Waves/LWave01/LW01.html.
Click here to watch the video for “Wave Motion, Sound and Light –
Longitudinal Waves”.
Human Ear
We hear a sound
because the ear drum
senses either an
increase or reduction
of air pressure. This
compression or
rarefaction causes the
ear drum to vibrate
back and forth. The
internal working of the
ear then changes this
vibration to an impulse
that can be
transmitted to the
audio nerve and
received by the brain. For additional information about the middle ear click
on the hyper link below:
http://www.iurc.montp.inserm.fr/cric/audition/english/ear/inear/inear.htm#
animation.
Click here to watch the video for “Wave Motion, Sound and Light – Human
Ear”.
Transverse Waves
A transverse wave has
an amplitude
perpendicular to the
direction of
propagation. The
vibration is up and
down while the wave
moves to the side,
much like a waves in
the ocean. The greater
the amplitude of the
wave the greater the
energy contained
within the wave.
The frequency is determined by the number of crests or peaks that pass a
given point in a second. The trough is the lowest part of the wave, and the
wavelength is the distance from one point on a wave to the corresponding
point on the next wave.
Click here to watch the video for “Wave Motion, Sound and Light –
Transverse Waves”.
Electromagnetism
In this section we will determine how a speaker produces sound. When a
current flows in a wire it will create a magnetic field. The direction of the
magnetic field in a single wire is coaxial to the wire. The farther from the
wire the lower the magnetic field intensity. Wire wrapped around a ferrous
material will intensify the magnetic field and induce a magnetic field in the
iron. More wraps makes a stronger field. If the iron is a soft type it can
become temporarily magnetized only while the current is flowing in the wire.
If it is a harder core it will be a permanent magnet.
If a magnetic core is used and allowed to move in a coil of wire a device has
been created that can be used in an electric circuit to open and close items
or to turn on and off items. Remember from your previous knowledge that
like poles of a magnet will repel and unlike poles will attract.
Speakers & Microphone
The cone of a speaker is connected
to a magnet with a coil of wire
wrapped around it. The stereo
generates an electrical signal that is
sent through the coil of wire. The
higher the sound level the greater
the current. The greater the current
in the coil the larger the magnetic
field produced and the greater either
the attraction or repulsion of the
magnet. If the current is positive the
magnetic field will be in one
direction. If it is negative it will be in
the opposite direction and thus affect
whether the magnetic core is
attracted or repelled.
Figure 4.11 Speaker
To fully understand this interaction
we must first understand sound and waves.
Click here to watch the video for “Wave Motion, Sound and Light – Speaker
and Microphones”.
Radio Waves
Radio and TV signals are transmitted through the atmosphere in the form of
a electromagnetic wave. There are two different ways the signal of the radio
or TV is encoded onto the transverse wave. One way is amplitude
modulation and the other is frequency modulation.
Human voice and music has a frequency range of 20 to 20,000 hertz and
this is known as the signal.
Amplitude Modulation is done in AM radio stations. The frequency of the
wave is in the range of 550,000 to 1,500,000 Hertz. So when you set your
AM radio to 880 on the dial this is 880 kilohertz or 880,000 Hertz. This is the
frequency of the carrier wave and upon this constant frequency carrier is
placed the signal of the music or voice this is done by varying the amplitude
of the signal. This variation of amplitude is interpolated by the electronics of
the radio into an electrical signal that is sent to the amplifier and then to the
speakers where it is converted from transverse waves into the longitudinal
waves that can be sensed by our ears.
Frequency Modulation is the method used on FM radio stations. The
frequency range of FM radio stations is in the 100 megahertz range or
100,000,000 hertz. This is known as the carrier wave. The carrier frequency
(radio station assigned frequency) is changed slightly by frequency of the
signal. For example: if the carrier frequency is 98,000,000 hertz and signal
is 10,000 hertz, the frequency the station transmits will vary by this 10,000
hertz. The radio station will broadcast at 97,990,000 and an instant later at
98,010,000 hertz on the radio wave. It is carried both positive (+10,000
hertz) and negative (-10,000 hertz) since the speaker will vibrate forward
and backwards. When the radio receives the radio wave the carrier
frequency is removed (leaving the original signal) and the radio will amplify
the signal and send it to the speakers. Our ears cannot detect the radio
signal but can detect the sound coming from the speakers. Most FM radio
stations are stereo so therefore they broadcast two slightly different
wavelengths which your stereo radio decodes for the left and right speaker.
Click here to watch the video for “Wave Motion, Sound and Light – Radio
Waves”.
Digital Encoding
We need to understand how a computer and other digital devices work.
Computers work on what is known as the binary system, a system of 1's and
0's. All the numbers, letters and other information are converted into series
of 1's and 0's. For examples see the table on the next page.
Click here to watch the video for “Wave Motion, Sound and Light – Digital
Encoding”.
VCR, Camcorders & Disc Drivers
These devices include tape
recorders, computer floppy discs
and hard disk drives which use a
magnetic medium to record
information. This information is
recorded by magnetic particles on
the medium that create areas of
high magnetic concentration and
areas of lower magnetic
concentration. This information is
recorded with a write head which is
a device that converts an electrical
signal into a magnetic image that is
recorded on the magnetic medium.
This information can be retrieved with a read head. This read head senses
the magnetic field that is stored on the magnetic medium and converts the
magnetic image into an electrical current. This current is converted by the
computer, tape recorder or the VCR into information that can be used by the
device for its particular purpose. A disk drive spins the medium and the read
head moves in and out. A tape record, VCR, and Camcorder all have a tape
that moves from one spool to another past the read head. The information
can be stored either in analog format or digital. For additional information
about VCRs and how they work go to
http://electronics.howstuffworks.com/vcr2.htm and read the article entitled
"How VCRs Work". Make sure you watch the video entitled "Introduction to
How VCRs Work" that is inside this article. A second video about VCRs can
be seen by going to http://www.youtube.com/watch?v=HTZsBVFDdxQ.
CD, DVD, & CD ROMs
CD's, DVD's (http://www.howstuffworks.com/dvd2.htm) and CD ROMS
(http://micro.magnet.fsu.edu/electromag/java/cd) all work off the same
general principles. A CD uses a laser which sends a beam that strikes the CD
and is either reflected off the surface of the laser disc or is scattered by pits.
The reflected signal is detected with a sensor that can read the reflected
laser beam. The reflections and pits represent 0's and 1's and thus the
information that is stored on the disc can be loaded into the computer (a
DVD player or CD player are forms of a computer). The computer then takes
this information and reforms it into computer information, a video or music
or a combination of more than one of these. If it is music it is converted into
an electrical signal that is sent to the speakers. If it is video it is converted
to a signal that can be seen on a TV or monitor.
A CD-ROM burner is one of the ways to create a CD for later retrieval of the
information. The CD burner, using a laser beam, creates the pits and
reflective areas to represent the data being stored. This type of device is
common in today's personal computers. Mass production of CD's (like a
program or game) is done by a pressing method instead of the burner. A
DVD device works similar to CD except that it uses a shorter wavelength
laser and can store data in a smaller area.
Click here to watch the video for “Wave Motion, Sound and Light – CD, DVD
& CD ROMs”.
Click here to watch the video for “Wave Motion, Sound and Light –
Conclusion”.
Light - Introduction
The purpose of this section will be to discuss the physical principle of how
fiber optics works and is used in telecommunications. Fiber optics is
extremely small diameter glass that carries light which contains voice and
data. Each fiber optical cable is coated with a plastic type coating and
multiple strands of fiber are placed together to form a cable. Other
materials are added to the cable to give it strength. Each strand of fiber can
carry a much larger amount of data then copper wire and thus we say it has
a much greater bandwidth (more data per period of time). Each strand of
fiber is connected to a solid state laser that takes the electrical signal and
converts it to an optical signal and at the other end of the cable it is
connected to a receiving device which converts the light into an electrical
signal. If the fiber is long it may have an amplification station which
strengthens the signal. This may be repeated numerous times depending on
length of the fiber. The data is coded in binary (see discussion at the end of
Electricity section).
Objectives:
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Explain the basic properties of light
Demonstrate an understanding of the principles of reflection
Demonstrate an understanding of the principles of refraction
Explain how total internal reflection occurs
Explain how total internal reflection is used in fiber optics
Explain the basic principles of a LASER
Click here to watch the video for “Wave Motion, Sound and Light – Light
Introduction”.
Properties of Light
The basic rule of optics states that light travels in a straight line unless it
strikes an object which causes it to be bent or absorbed, if it is bent it will
then continue in a new straight path, the exception to this is when diffraction
occurs (light passes through a narrow opening). The two basic ways in which
bending occurs in optics is by reflection and refraction. Before we can fully
understand the bending in fiber optics we must understand the basic
principles of reflection and refraction. Light travels at a constant speed and
this is the fastest speed which can be obtained. When light travels into
another optical medium it will slow. The speed of light in a vacuum is
186,000 miles per second or
meters per second. At this speed light
takes about one and a half seconds to go from the earth to the moon. Light
can be thought of as traveling in waves or it can also be thought of as a
particle. Some principles of optics only will work for a particle model of light
and other areas of optics will only work for an electromagnetic wave model.
Many of the basic principles work for either model. The particles of light are
called photons. Different photons have different amounts of energy.
Different colors of light can be thought of being composed of different
colored photons. A photon is a massless particle composed of pure energy
and thus the amount of energy is directly related to the color of light. From
the wave model discussion the different wavelengths of light (the distance
from one part of a wave to the next corresponding part of the wave) will
make up the different colors of light. This duality of light is a model that
makes the understanding of the principles of light easier and for our
purposes will not make a difference in our discussions.
Click here to watch the video for “Wave Motion, Sound and Light –
Properties of Light”.
Electromagnetic Spectrum
The electromagnetic spectrum is a family of light like radiation that has both
a particle and wave nature. This family includes: gamma rays, X-rays,
ultraviolet, light, infrared, microwaves and radio waves. The list was from
most energetic with the shortest wavelength first to the least energetic with
the longest wavelengths. The properties of light previously discussed are
also true for members of this family; the speed is still 3 x 108 m/s, reflection
and refraction still occurs. Light will reflect off a glass mirror and a gamma
ray will pass through the mirror, therefore the actions are the same as
previously discussed but the substances will change that will allow for the
interaction. All these forms of radiation are invisible to the human eye.
Gamma Rays are a form of nuclear radiation and normally discussed in
nuclear physics. The gamma ray is created in the nucleus of an atom. Some
substances in nature are naturally radioactive and as they go through
radioactive decay they will release gamma rays to lower the total energy of
the atomic nucleus. Gamma rays can penetrate many difference substances;
lead is normally used to stop the gamma rays because of its high density.
Gamma rays are used in some cancer treatments.
Click here to watch the video for “Wave Motion, Sound and Light –
Electromagnetic Spectrum - Gamma Rays”.
X-rays are a form of atomic radiation which is emitted by the electrons that
orbit the nucleus of an atom. X-rays can penetrate many substances, but
can be blocked by thin sheet of lead. X-rays are used in the medical field to
look at hard substances (bones) in the human body; they are also used in
some types of radiation therapy for cancer patients.
Click here to watch the video for “Wave Motion, Sound and Light –
Electromagnetic Spectrum - X Rays”.
Ultraviolet Radiation is used in the sorting of mail to insure that postage
has been attached to the envelope. It is used to make certain kinds of paint
glow, what is sometimes referred to as black light. If you observe a black
light and you see a purple color that is visible radiation not the ultraviolet
radiation. It is used to sterilize medical equipment to eliminate bacteria. It
can cause damage to the eye if the retina of the eye is exposed for length
periods of time. Certain photographic films will detect ultraviolet waves and
they will be converted into a violet or blue color on the film, which causes
the sky to appear bluer than it really is.
Click here to watch the video for “Wave Motion, Sound and Light –
Electromagnetic Spectrum - Ultraviolet Radiation”.
Light has been discussed previously and will not be repeated at this
location.
Click here to watch the video for “Wave Motion, Sound and Light –
Electromagnetic Spectrum - Light”.
Infrared Radiation is normally discussed as two different types, one being
near infrared and the other thermal infrared. The near infrared is that part of
the electromagnetic spectrum with wavelengths just longer then visible red
light. Near Infrared is detected by some satellites and aircraft to look for
diseases in plants such as corn blight. The thermal infrared is the area of the
spectrum that is known as heat. You experience thermal infrared when you
walk near a heat source and you feel the heat on your body; your body is
detecting the thermal infrared. Thermal infrared is used detect heat source
and is used to detect cancer cells; especially breast cancer. It is also used to
detect when parts of a machine is wearing out and has too much friction;
such as the bearings in a railroad car.
Click here to watch the video for “Wave Motion, Sound and Light –
Electromagnetic Spectrum - Infrared Radiation”.
Microwaves are used in many ways in our everyday lives from
telecommunication to cooking. If you have a satellite dish on your home
then it is receiving microwaves, long distance telephone communication uses
microwaves and of course cooking with a microwave oven. A microwave
oven causes the molecules in the substance to vibrate thus creating friction
which causes the substance to heat up. The more dense the substance
usually the faster it will heat.
Click here to watch the video for “Wave Motion, Sound and Light –
Electromagnetic Spectrum - Microwaves”.
Radio Waves are used for many different types of communications and the
name sometimes causes people to believe it is only referring to radio. These
wavelengths are used in cell phones, cordless phones, wireless networking,
radar, television, CB’s and of course AM and FM radio. Radio waves are the
longest least energetic of all the members of the electromagnetic spectrum.
Click here to watch the video for “Wave Motion, Sound and Light –
Electromagnetic Spectrum - Radio Waves”.
Reflection
Figure 1.1 Reflection
When light reflects by striking a
mirror or other reflective surface the
principle is very simple. The angle of
the incoming light ray is equal to the
angle of the light after reflection. A
normal is a line that is perpendicular
to the surface of the object (mirror).
Therefore the gray line represents a
mirror the normal (green line) is
perpendicular to the surface of the
mirror. The Law of Reflection
states that angle of incident
(incoming ray) is equal to the angle
of reflection (the outgoing ray). The
angle is measured between the
incident ray and the normal and the
reflected ray and the normal.
(0.1)
When light strikes a curved mirror the rules are the same. You can think of a
mirror as being composed of multiple straight sections which are extremely
small. The reflection occurs in the same way. Each little straight section has
the incident ray equaling the reflected ray. The normal for each of these
sections will be the radius of the curve for spherical shaped mirror, such as
those used for security in department stores. This area of physics is called
geometric optics and how the images are formed in a security mirror or a
make mirror is interesting to understand but do not lead to the direct
principle of understanding fiber optical cable.
Click here to watch the video for “Wave Motion, Sound and Light –
Reflection”.
Refraction
Refraction
occurs
when
light
travels from one substance into
another substance. If the light
enters
the
other
substance
perpendicular to the surface then
only the speed is changed. If you
are going from an optically less
dense medium to an optical denser
medium then the light will slow. So
if light is going from air into glass
the light travels faster in the air
then it does in the glass. The
opposite is true if you go from
glass into air that light increases in
speed. Light has a maximum
speed, i.e. the speed of light, and
the medium with the least optical
dense medium is a vacuum.
Figure 1.2 Refraction
If the light does not strike the surface perpendicular to the surface then the
angle of the light ray also changes. If we are going from less dense to more
dense then the light ray will bend toward the normal and if we are going
from denser to less dense it will bend away from the normal. The law which
applies to refraction is known as Snell’s Law and it is as follows.
(0.2)
is the index of refraction for the first optical medium,
is the index of
refraction of the second optical medium. The index of refraction of air is
approximately 1.
is the angle of incident which the angle between the
is the angle of refraction which is measured
normal and the incident ray.
between the normal and the refracted ray. We will not mathematically solve
this relationship. The sine function is a trigonometry function, the sine of 90
degrees is 1 and the sine of 0
degrees is 0.
The angle for this type of problem
cannot exceed 90 degrees.
If we have a ray of light trying to exit
the denser medium into the less
dense medium then as previously
noted it will bend away from the
normal toward the surface of the
medium.
Figure 1.3 Refraction
Click here to watch the video for “Wave Motion, Sound and Light –
Refraction”.
Refraction Simulation
An online experiment will be used to assist your understanding this concept.
You will move the light source around observing both the incident and
refracted or reflected rays. Note the angles of these rays will be stated. To
move the light source simply click and drag the source and the incident ray
will move.
Try making the substance water above the horizontal line and make the
substance below water (you can play with other combinations). Do several
examples of going from air to water and record you results (measurements
given). Move the light source below the water and repeat.
Every student will need to make sure that they are able to complete the
table below by answering the following questions:
1. At what angle did reflection occur?
2. What was the critical angle in the online experiment when refraction
no longer occurred and reflection began? Click here to reach the online
experiment.
Contact your instructor if you have any questions about the Refraction
Simulation.
Click here to watch the video for “Wave Motion, Sound and Light –
Refraction”.
Refraction on a Curved Surface
Figure 1.4
Refraction on a Curved Surface
The surface of the fiber optical
cable must also be polished to a
smooth curved surface so that
light properly enters the cable
the shape of the front surface
must be convex. A convex
surface is higher in the center
and shallower at the edges. The
light that enters the convex
surface is focused toward the
center of the fiber, thus
eliminating much of initial
losses. The curved surface can
be thought of as a number of
straight surfaces just as we
discussed in the mirrors
previously.
Total Internal Reflection
If we assume the less dense medium is air we can develop the critical case
when the equation is no longer possible to be solved. Therefore using
equation Error! Reference source not found. and if
(air) and the
largest angle that
can be is 90o (this is when the light refracts along the
surface of the interface) and the sin of 90o is 1. Therefore if we substitute
into equation Error! Reference source not found. we get the following:
(0.3)
Therefore if angle of incidence exceeds 41.8 degrees for these mediums then
the equations of Snell’s no longer can be solved. At this point the interface of
the medium becomes a reflective surface and no longer a refractive surface.
This is known as total internal reflection and is the principle in which fiber
optics works on. You will not be required to do any calculations using the
sine function, it is shown to you to confirm the observations you have made
visually in the online laboratory. Repeat the online lab if you need to explore
this principle again.
For additional information about "Total Internal Reflection" visited the links
provided below:
http://laser.physics.sunysb.edu/~wise/wise187/2001/reports/andrea/report.
html
http://theory.uwinnipeg.ca/mod_tech/node114.html
Fiber Optics
The optical fiber has a higher dense then the air surrounding it. Therefore if
the angle of incident is greater than the critical angle total internal reflection
will occur. Therefore, as the light strikes the walls of the surface they are
reflected instead of being refracted and the light continues down the fiber.
Note the higher the ratio of optical density
of the mediums the small the angle
becomes to have total internal reflection.
Thus light still has the same physical
properties and continues to travel in a
straight line, but is bent continuously as it
reflects of the outer wall of the fiber.
Therefore, the fiber can be bent at different
angles as is required for proper installation.
For additional information about "Fiber
Optics" visited the links provided below:
Figure 1.5 Fiber Optics Cable
http://www.timbercon.com/Total-Internal-Reflection.html
http://electronics.howstuffworks.com/fiber-optic6.htm.
The surface of the fiber optical cable must also be polished to a smooth
curved surface so that light properly enters the cable the shape of the front
surface must be convex. A convex surface is higher in the center and
shallower at the edges. The light that enters the convex surface is focused
toward the center of the fiber, thus eliminating much of initial losses. The
curved surface can be thought of as a number of straight surfaces just as we
discussed in the mirrors previously.
Click here to watch the video for “Wave Motion, Sound and Light – Fiber
Optics”.
Lasers
The optical information that is placed in the fiber cable is done by a solid
state laser diode in general. We will not go into the depth of understanding
how laser diodes function, but will discuss the principles of a laser and how it
is different from regular light.
There are many types of lasers and many different uses for lasers. In this
chapter we will discuss what a laser produces and how we can transmit
information by using a laser. LASER stands for light amplification by
stimulated emission of radiation. A laser produces a single wavelength of
coherent light. If we think of a light bulb it produces most all the colors in
the rainbow, using a prism we can observe the colors of light in the light
bulb. A laser in general will produce only a single wavelength of light (a
single color), but just by having a single color of light does not create a
laser. If you take a light bulb and pass the light through a color filter you
will get a single color of light, this is not a laser. The laser light must be
coherent. One of the ways to think of light is that it is composed of
transverse waves (transverse waves will be discussed in Electricity section; a
transverse wave is like a water wave). If we have a single color of light then
we will have all the waves having the same length (wavelength). Yet to be
coherent light all the waves must start at the same time (all the peaks of the
waves must be aligned and all the troughs must be aligned). A laser creates
coherent light rays of a single wavelength.
How the laser produces it light from an equation understanding is above the
level of this class mathematically and since we are not discussing physical
optics and the atomic spectrum in this class we will not discuss the actual
principles of lasing. If you wish to explore more detail on this subject go to
the following website:
http://www.middlebury.edu/~PHManual/Photos/heliumneon/fig2.html.
There are several different types of lasers and we use some of these lasers
in our everyday life. What are some examples of the use of a laser that you
can name? Some lasers use gases, other use liquids and some use solids.
You probably have seen a laser pointer; this type of laser is a solid state
device and is similar to the laser used in telecommunications.
If the laser is turned on and off rapidly then a code of information can be
transmitted by the device much as Morse code was used with the telegraph
a century ago. This code can be composed of on and off bits of information;
all letters and numbers can be transmitted using this coding. All images can
be made to be represented by numbers and the numbers converted into this
system. This system is known as binary and is discussed in the Electricity
section when we discuss how a CD player works.
The laser is connected to the end of the fiber optical cable and transmits the
information to a receiving station at the other end of the fiber. The electronic
equipment then decodes the signal and sends the information on to the
appropriate computer or other telecommunication device. If the distance is
great a repeater may be employed. A repeater is a receiver and another
sending device.
Click here to watch the video for “Wave Motion, Sound and Light – Lasers”.