Chapter 4: Sound - Introduction
In this chapter 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 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
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 at:
http://www.gmi.edu/%7Edrussell/Demos/waves/wavemotion.html , and Demonstration 2
at:
http://surendranath.tripod.com/Applets/Waves/TwaveRefTran/TwaveRefTranApplet.html.
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 to go:
http://www.iurc.montp.inserm.fr/cric/audition/english/ear/inear/inear.htm#animation .
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 then next
wave.
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.
To fully understand this interaction
we must first understand sound and
waves.
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.
Digital Encoding
We need to understand how a computers 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 below:
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 VCR
(http://www.udayton.edu/%7Ecps/cps460/notes/displays/tvset/vcrs.html ),
computer or tape recorder 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.
CD, DVD, & CD ROMs
CD's, DVD's (http://www.howstuffworks.com/dvd.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.