Chapter 8 Electromagnetism and EM Waves ( Section 4)

Vern J. OstdiekDonald J. Bord

Chapter 8Electromagnetism 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.


• 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.


• 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.


• 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.


• 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.


• 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.


• 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.


• In this case, the coil (called the voice coil) is connected to a stiff paper cone instead of to a diaphragm.


• 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.


• 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.


• 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.


• 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.


• 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.


• 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


• 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 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.


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


• 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.


• 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.


• 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.


• 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.


• 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.


• 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.


• 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.

Documents

Chapter 8 Electromagnetism and EM Waves ( Section 4)