Vern J. Ostdiek
Donald J. Bord
Chapter 8
Electromagnetism and EM Waves
(Section 4)
8.4 Applications to Sound Reproduction
• A hundred years or so ago, the only people who
listened to music performed by world class
musicians were those few who could attend live
performances.
• Today, people in the most remote corners of the
world can hear concert-quality sound from large
home entertainment systems, pocket-sized or
smaller MP3 players, and many devices in
between.
8.4 Applications to Sound Reproduction
• The first Edison phonographs were strictly
mechanical and did a fair job of reproducing
sound.
•
It was the invention of electronic recording and
playback machines that brought true high fidelity to
sound reproduction.
• The sequence that begins with sound in a
recording studio and ends with the reproduced
sound coming from a speaker in your home,
headphones or earbuds, or car includes
components that use electromagnetism.
8.4 Applications to Sound Reproduction
• The key to electronic sound recording and
playback is first to translate the sound into an
alternating current and then later retranslate the
AC back into sound.
•
The first step requires a microphone, and the
second step requires a speaker.
• Although there are several different types of
microphones, we will take a look at what is called
a dynamic microphone.
8.4 Applications to Sound Reproduction
• Although there are
several different types of
microphones, we will
take a look at what is
called a dynamic
microphone.
•
It consists of a magnet
surrounded by a coil of
wire attached to a
diaphragm.
8.4 Applications to Sound Reproduction
• The coil and diaphragm are free to oscillate
relative to the stationary magnet.
• When sound waves reach the microphone, the
pressure variations in the wave push the
diaphragm back and forth, making it and the coil
oscillate.
•
Because the coil is moving relative to the magnet,
an oscillating current is induced in it.
8.4 Applications to Sound Reproduction
• The frequency of the AC in the coil is the same as
the frequency of the diaphragm’s oscillation, which
is the same as the frequency of the original sound.
•
That is all it takes.
• This type of dynamic microphone is referred to as
a moving coil microphone.
•
The alternative is to attach a small magnet to the
diaphragm and keep the coil stationary—a moving
magnet microphone.
8.4 Applications to Sound Reproduction
• Let’s skip ahead now to when the sound is played
back.
• The output of the CD player, radio, or other audio
component is an alternating current that has to be
converted back into sound by a speaker.
•
The basic speaker is quite similar to a dynamic
microphone.
8.4 Applications to Sound Reproduction
• In this case, the coil (called the voice coil) is
connected to a stiff paper cone instead of to a
diaphragm.
8.4 Applications to Sound Reproduction
• Recall that an alternating current in the voice coil
in the presence of the magnet will cause the coil to
experience an alternating force.
• The voice coil and the speaker cone oscillate with
the same frequency as the AC input.
•
•
The oscillating paper cone produces a longitudinal
wave in the air—sound.
The tiny speakers built into earbud earphones now
commonly used with iPods and other portable
music devices operate by these same principles.
8.4 Applications to Sound Reproduction
• They are made possible by the use of small but
powerful permanent magnets made of an alloy of
the elements neodymium, iron, and boron (NIB).
•
•
The extreme strength of NIB magnets (which in
some cases can approach that of large medical
MRIs) makes them capable of reproducing a very
broad range of frequencies with exceptional fidelity.
Coupled with their small size, this has made them
indispensable in the design of compact earphones.
8.4 Applications to Sound Reproduction
• Microphones and speakers are classified as
transducers:
•
They convert mechanical oscillation from sound into
AC (microphone), or they convert AC into
mechanical oscillation and sound (speaker)
• They are almost identical.
•
•
In fact, a microphone can be used as a speaker,
and a speaker can be used as a microphone.
But, as with motors and generators, each is best at
doing what it is designed to do.
8.4 Applications to Sound Reproduction
• Most sound recording, from simple cassette
recorders to sophisticated studio tape machines, is
done on magnetic tape.
•
•
The tape is a plastic film coated with a thin layer of
fine ferromagnetic particles that retain magnetism.
Sound is recorded on the tape using a recording
head, a ring-shaped electromagnet with a very
narrow gap.
8.4 Applications to Sound Reproduction
• During recording, an AC signal (from a
microphone, for example) produces an alternating
magnetic field in the gap of the recording head.
•
•
As the tape is pulled past the gap, the particles in
each part of the tape are magnetized according to
the polarity of the head’s magnetic field at the
instant they are in the gap.
The polarity of the particles changes from north–
south to south–north, and so on, along the length of
the tape.
8.4 Applications to Sound Reproduction
• To play back the recording, the tape is pulled past
a playback head, often the same head used for
recording.
•
The magnetic field of the particles in the tape
oscillates back and forth and induces an oscillating
magnetic field in the tape head.
• This oscillating magnetic field induces an
oscillating current (AC) in the coil—
•
electromagnetic induction again
8.4 Applications to Sound Reproduction
• Magnetic recording is not limited to sound
reproduction.
•
•
Television videocassette recorders (VCRs) record
both sound and visual images on magnetic tape.
Computers store information magnetically on tapes,
floppy discs, and hard discs.
8.4 Applications to Sound Reproduction
Digital Sound
• A revolution in sound reproduction occurred in the
1980s with the advent of digital sound
reproduction, the method used in compact discs
(CDs) and various computer sound file formats,
including MP3.
• In a process known as analog-to-digital
conversion, the sound wave to be recorded is
measured and stored as numbers.
8.4 Applications to Sound Reproduction
Digital Sound
• For CDs, the actual voltage of the AC signal from
a microphone is measured 44,100 times each
second.
8.4 Applications to Sound Reproduction
Digital Sound
• Note that this frequency is more than twice the
highest frequency that people can hear.
• The waveform of the sound is “chopped up” into
tiny segments and then recorded as numerical
values.
•
These numbers are stored as binary numbers using
0s and 1s, just as information is stored in
computers.
8.4 Applications to Sound Reproduction
Digital Sound
• To play back the sound, a digital-to-analog
conversion process reconstructs the sound wave
by generating an AC signal whose voltage at each
instant in time equals the numerical value
originally recorded.
•
After being “smoothed” with an electronic filter, the
waveform is an almost perfect copy of the original.
8.4 Applications to Sound Reproduction
Digital Sound
• A huge amount of data is associated with digital
sound reproduction—millions of numbers for each
minute of music. CDs (and DVDs) store these data
in the form of microscopic pits in a spiral line
several miles long.
8.4 Applications to Sound Reproduction
Digital Sound
• A tiny laser focused on the pits reads them as 0s
and 1s.
•
•
The amount of information stored on a 70-minute
CD is equivalent to more than a dozen full-length
encyclopedias.
A standard DVD can store about seven times as
much data, and new Blu-ray discs (which employ
special blue lasers to scan the pits) can handle as
much as 40 times more.
• Little wonder that CDs and DVDs have also been
embraced by the personal-computer industry as a
way to store huge amounts of information in
durable, portable form.
8.4 Applications to Sound Reproduction
Digital Sound
• The superior quality of digital sound comes about
because the playback device looks only for
numbers.
•
•
It can ignore such things as imperfections in the
disc or tape, the weak random magnetization in a
tape that becomes tape hiss on cassettes, and the
mechanical vibration of motors that we hear as a
rumble on phonographs.
A sophisticated error-correction system can even
compensate for missing or garbled numbers.
8.4 Applications to Sound Reproduction
Digital Sound
• Because the pickup device in a CD player does
not touch the disc, each CD can be played over
and over without the slow deterioration in quality
that results from a needle moving in a phonograph
groove or from the constant unwinding and
rewinding of a cassette tape over the recorder
heads.
• This combination of high fidelity and disc durability
made the CD system an immediate hit with
consumers.
8.4 Applications to Sound Reproduction
Digital Sound
• This is just a glimpse of some of the factors in
state-of-the-art high-fidelity sound reproduction.
• Perhaps we are all so accustomed to it that we
cannot appreciate how much of a technological
miracle it really is.
•
The next time you listen to high-quality recorded
music, remember that it is all possible because of
the basic interactions between electricity and
magnetism.