Chapter 8 Electromagnetism and EM Waves (Section 4)

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Chapter 8Electromagnetism and EM Waves(Section
4)
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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.

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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.

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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.

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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.

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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.

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8.4 Applications to Sound Reproduction

• The frequency of the AC in the coil is the same
  as the frequency of the diaphragms 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 stationarya
  moving magnet microphone.

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8.4 Applications to Sound Reproduction

• Lets 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.

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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.

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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 airsound.
• The tiny speakers built into earbud earphones now
  commonly used with iPods and other portable music
  devices operate by these same principles.

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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.

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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.

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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.

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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 heads magnetic field at
  the instant they are in the gap.
• The polarity of the particles changes from
  northsouth to southnorth, and so on, along the
  length of the tape.

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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

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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.

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8.4 Applications to Sound ReproductionDigital
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.

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8.4 Applications to Sound ReproductionDigital
Sound

• For CDs, the actual voltage of the AC signal from
  a microphone is measured 44,100 times each second.

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8.4 Applications to Sound ReproductionDigital
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.

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8.4 Applications to Sound ReproductionDigital
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.

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8.4 Applications to Sound ReproductionDigital
Sound

• A huge amount of data is associated with digital
  sound reproductionmillions 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.

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8.4 Applications to Sound ReproductionDigital
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.

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8.4 Applications to Sound ReproductionDigital
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.

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8.4 Applications to Sound ReproductionDigital
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.

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8.4 Applications to Sound ReproductionDigital
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.