FUN WITH MAGNETS: MICROPHONES, SPEAKERS, TAPE ...

400007_CH08_p065-076.pdf 5/26/05 8:03 AM Page 65CHAPTER EIGHTFUN WITH MAGNETS:MICROPHONES, SPEAKERS,TAPE RECORDERS, RECORDPLAYERS, GUITAR PICK-UPSYou probably know that your stereo speakers have magnets in them; that’swhy they are so heavy. It turns out that magnets are used in many commonhousehold items, many of them having to do with audio equipment, such asmicrophones, tape recorders, record players, and electric guitar and bass pick-ups.All of these devices can be easily understood once you understand one physical concept:electromagnetic induction. Don’t worry, it’s not as bad as it sounds! Electromagneticinduction simply means that a changing magnetic field induces, or creates,an electric current in a wire which is within the influence of the magnetic field. Theconverse is also true: a changing current produces a changing magnetic field aroundthe current-carrying wire. This is almost everything you need to know to understandall the devices listed above.MICROPHONESThere are basically two different classes of microphones: dynamic mics, which useelectromagnetic induction, and condenser mics, which use capacitors (electrostatics).I will explain both, but the dynamic mic is more useful to our general discussionbecause the way it works also applies to the other devices we will discuss. A

400007_CH08_p065-076.pdf 5/26/05 8:03 AM Page 6666IS THERE A LASER IN THE HOUSE?condenser microphone contains a capacitor, which is an electronic component whichstores electric charge. A battery is nothing more than a capacitor which stores chargefor a very long time. A capacitor can be made in several different ways, but the easiestone to understand is the one contained in the condenser mic. It consists of twometal plates separated by a certain distance. If there is electrical charge stored on theplates of the capacitor, then there will be a voltage between the two plates, and thisvoltage will depend on the separation between the plates; the larger the separation,the larger the voltage. In a condenser mic, one of the capacitor plates is fixed so thatit cannot move, and the other is attached to the movable diaphragm at the front ofthe mic. The diaphragm is a thin membrane that vibrates when sound waves hit it.The vibrations cause the attached capacitor plate to move, which changes the spacingbetween it and the other capacitor plate. This creates a changing voltage acrossthe capacitor, which can be transformed into a changing current, which is then sentto an amplifier. In this way, sound waves are transformed into electrical current (seeFigure 8.1). Condenser mics usually need “phantom power,” which is a voltage whichmust be applied to the microphone in order to keep a charge on the capacitor plates.This voltage is typically 48 volts and can usually be supplied by the mixing console.A dynamic mic performs the same function, but uses a different physical mechanismto transform the sound waves into electric current. The diaphragm in this caseis attached to an ordinary bar magnet, with a north and south pole.This bar magnet is surrounded by, but not touching, a coil of electrical wire. Theprinciple of electromagnetic induction says that a changing magnetic field will inducean electric current in a wire that is within the magnetic field. So, when this bar magnetis sitting still, no current flows in the wire. If sound waves cause the diaphragmattached to the magnet to vibrate, the magnet will now move in and out of the coil ofwire, in response to the vibrations. As the magnet moves, the magnetic field experiencedby the coil of wire changes, and hence a current starts to flow in the wire. As themagnet reverses direction, so does the current. In this way, an AC electric current isgenerated in response to sound waves hitting the diaphragm of the dynamic microphone(see Figure 8.2). A stronger sound wave will produce a stronger change in themagnetic field, and a larger amount of electric current will result. The amount of currentis therefore directly proportional to the amount of sound hitting the microphone.It is also possible to design the microphone so that the diaphragm is attached to the

400007_CH08_p065-076.pdf 5/26/05 8:03 AM Page 67FUN WITH MAGNETS 67diaphragmcapacitor withone moving plate,one fixed platevariable voltage signalto amplifiersound wavesFigure 8.1 Illustration of how a condenser microphone works. Sound waves hitthe diaphragm which is located just behind the protective covering (not shown) ofthe mic. The diaphragm causes the spacing between the capacitor plates to change,which creates a changing voltage, which can create a changing current through aresistor (not shown). The current gets sent to the amplifier.diaphragmbar magnetsound wavescoil ofwireinduced current fromcoil sent to amplifierFigure 8.2 Illustration of how a dynamic microphone works. Sound waves hittingthe diaphragm cause relative motion between a coil of wire and a bar magnet,which induces a current to flow in the coil. This current is then amplified.coil instead of the bar magnet, which is usually the way it is done because the coil islighter than the magnet and the mic can therefore be more sensitive. It doesn’t matter,scientifically, which way it is done, as long as there is relative motion between the coiland the bar magnet when sound strikes the diaphragm. Practically though, it is betterto have the lighter element attached to the diaphragm so that it can have good transientresponse, meaning the ability to respond to rapidly changing sounds.

400007_CH08_p065-076.pdf 5/26/05 8:03 AM Page 6868IS THERE A LASER IN THE HOUSE?In case you’re wondering what the difference between condenser mics anddynamics mics is with regard to performance, condenser mics tend to have slightlybetter frequency and transient response (they are better at reproducing higher frequenciesand faster-changing sounds) whereas dynamic mics tend to be morerugged. This is because the capacitor plate in the condenser mic is easier to movethan is the coil or magnet in a dynamic mic, so the diaphragm in the condenser miccan be lighter and better able to respond to higher frequencies or fast changes. Thisalso makes it more fragile. In fact, if dropped even once, a condenser mic can break.It is not universally true that every condenser mic has better frequency response thanevery dynamic mic; this is a generalization. There are $3,000 dynamic mics whichcan blow $100 condenser mics out of the water!LOUDSPEAKERSStereo speakers work almost like dynamic mics in reverse. Their job is to transform anelectrical signal into sound waves. They do it via the interaction of electric current,magnetic field, and the resulting force which is generated. We need to add the conceptof force to our arsenal in order to understand the loudspeaker, and you need to thinkthree-dimensionally to get this. Picture a coil of electric wire wrapped around one endof a bar magnet, say, the north-pole end, and picture two bar magnets with their southpoleends close by, one above and the other below the coil, as shown in Figure 8.3.Magnetic field lines point from the north pole to the south pole of a magnet. Thecurrent which will flow through the coil will keep changing directions because thesignal which goes to the loudspeaker is an AC signal, but, as you can see fromFigure 8.3, it will always be perpendicular to the magnetic field lines. There is a lawin physics which says that if you have current flowing in a direction perpendicularto a magnetic field, there will be a force generated which is perpendicular to boththe current and the magnetic field. In Figure 8.3, using your right hand, if you putyour thumb in the direction of the magnetic field at the top of the coil (up), yourindex finger in the direction of the current in the coil (let’s say into the page as thecoil is drawn), then your middle finger, held perpendicular to the other two fingers,will point in the direction of the force; in this case, the force will point to the right.This is called the right-hand rule. The force will act on the paper cone which is wrapped

400007_CH08_p065-076.pdf 5/26/05 8:03 AM Page 69FUN WITH MAGNETS 69speakerconemagneticfieldSNcoilSFigure 8.3 Illustration of a loudspeaker. A coil of wire is wrapped around a barmagnet so that when a current flows through the coil, a force is generated whichpushes and pulls on the speaker cone, generating sound waves.around the magnet. As the current in the coil changes directions, so will the force.The result will be that the paper cone will be pushed and pulled at the frequency ofthe audio signal coming through the wire, which will create compressions (regionsof greater air density) and rarefactions (regions of lesser air density) in the air nearthe cone; this is how sound waves are produced. Sound waves are just variations inair pressure where the frequency of the variation falls somewhere between 20 Hzand 20,000 Hz, the normal range of human hearing. In fact, if you take the dustcover off of your speaker, you can see the cone vibrating, but don’t touch it becauseit is very fragile. Sound waves are called longitudinal waves because the waves travelin the same direction as the oscillation, as shown in Figure 8.4, unlike electromagneticwaves, where the direction of travel was perpendicular to the direction of oscillation(see Chapter 5, description of transverse waves).Sound waves with frequencies above 20,000 Hz are called “ultrasound” and cannotbe heard by humans, but many animals use ultrasound for navigation and forlocating food. Dolphins and bats do this (sonar). Sound with frequencies below 20 Hzis called “infrasound” and animals can hear this too; it is generated by thunder,Earthquakes, and nuclear explosions. This is one reason why a dog can becomefrightened of a thunder storm long before you hear it. Infrasound waves are very

400007_CH08_p065-076.pdf 5/26/05 8:03 AM Page 7070IS THERE A LASER IN THE HOUSE?oscillation direction andtravel direction of soundwavecompressionrarefactionFigure 8.4 Illustration of a longitudinal (sound) wave. Compressions are regionsof greater material density and rarefactions are regions of lesser material density.long and diffract around most objects, and therefore can carry great distances. Thisis also why you can hear the bass of someone’s music from far away when you can’thear the higher frequencies; the bass diffracts more and can travel farther. Subwoofersinstalled in floors of dance clubs also emit infrasound to make you “feel”the music. The human chest cavity has a resonance (natural frequency of maximumvibration) at around 8 Hz, so this frequency will make your chest feel the “rumble.”TAPE RECORDERS AND RECORD PLAYERSTape recorders also use the principle of electromagnetic induction. Audio tape iscoated with iron oxide, which is easily magnetized. A coil of wire, which willcarry the audio signal to be recorded, is placed very close to the tape. When theaudio signal passes through the coil, a magnetic field is created surrounding thecoil. The tape passes through this magnetic field in the recording process and themagnetic field causes the little magnetic particles on the surface of the tape toalign themselves according to the strength and direction of the magnetic fieldcoming from the coil. As the audio signal changes in strength and polarity (direction),so does the alignment of the magnetic particles (see Figure 8.5). Betterqualityrecordings are obtained when there are more magnetic particles, so thismeans using wide tape and fast speeds, to expose the maximum amount of thetape to the signal, produces the best quality recording. During the erase process,a uniform magnetic field is used to align all the particles in one direction, thusdestroying whatever information was encoded in them (but not completely; some

400007_CH08_p065-076.pdf 5/26/05 8:03 AM Page 71FUN WITH MAGNETS 71AC audiosignalfromsoundsourcechanging current in coilfrom AC source createschanging magnetic fieldwhich gets stored ontape as it passesaudio tape passes coil andgets magnetizedalignedmagneticparticlesFigure 8.5 Illustration of how an electrical signal is stored as a magnetic signal ona piece of audio tape.Side“A”tracksSide“B”tracksFigure 8.6 Illustration of tracks on an audio tape; shown here are four tracks, typicalof home-use audio cassette tapes. Two tracks, one for left channel and one forright channel, are recorded in each direction, corresponding to “Side A” and “Side B”of the tape.residual remains). Figure 8.5 is to illustrate the scientific concept only; real tapemachines use a magnetic “gap” and a different physical configuration, but theidea is the same.To play the tape back, the process is reversed. The magnetized tape is passedclose to the coil of wire, and the changing magnetic field presented by the tapecauses a current to flow in the coil, again depending on the strength and directionalityof the signal that was encoded on the tape. High-quality tape recorders haveseparate record, erase, and playback heads because each of these functions requiresdifferent working distances and magnetic field strengths in order to function at theirbest. Cheap tape recorders do it all with a single head.

400007_CH08_p065-076.pdf 5/26/05 8:03 AM Page 7272IS THERE A LASER IN THE HOUSE?There is only one working “side” to a recorded tape. The tape gets divided intoseparate “tracks” along its width, so when you flip a tape over to “play the otherside” you are really just repositioning the tape so that the heads are reading the othertrack, or the other set of tracks. A typical cassette tape for home use has four trackson it, two in each direction, to obtain stereo sound, as shown in Figure 8.6.RECORD PLAYERSRecord players also use electromagnetic induction. The needle slides along thegroove of the record, which is encoded with bumps to represent the audio signal.The needle rides up and down on these bumps; inside the stylus, there is a bar magnetattached to the moving part of the needle. This bar magnet moves in and out ofa fixed coil, which then has a current induced in it by the motion of the magnet, andthis current then gets sent to an amplifier and becomes the music signal. It works inmuch the same way as the dynamic microphone, except in this case the motionbetween the magnet and the coil is produced by bumps on a record rather than bysound waves hitting a diaphragm. Also, it doesn’t matter if the coil is fixed and themagnet moves, or if the magnet is fixed and the coil moves; the key is that there mustbe relative motion between the coil and the magnet in order to induce a current toflow in the coil of wire. Records are, of course, subject to scratching, and thescratches get transmitted to the magnet just as the “real” signals do, which is whywe hear scratches on records. Record players require much compensation for thedifferent bump sizes needed to get equal volumes at different sound frequencies andmuch mechanical stabilization as well, but that is beyond the scope of this book.GUITAR AND BASS PICK-UPSElectric guitar and bass pick-ups also use electromagnetic induction to transform amechanical vibration into an electrical signal. There is a magnet and a coil containedwithin the pick-up, or sometimes 2 coils to reduce hum, and the metal guitar stringsits directly over the pick-up. This means that the string sits within the magneticfield created by the pick-up. When the string is plucked, it moves through the magneticfield, which disturbs it; this creates a change in the magnetic field strength,

400007_CH08_p065-076.pdf 5/26/05 8:03 AM Page 73FUN WITH MAGNETS 73magnetic fieldemanating from magnetmetalstringcoilmagnetcurrent toamppick-upFigure 8.7 Simplified illustration of how an electric guitar or bass pick-up couldwork. Plucking the metal string disturbs the magnetic field, which then induces acurrent to flow in the coil. This current is then sent to the amplifier.which can then induce a current in the coil within the pick-up. This current is proportionalto the amount of string movement; the harder you pluck the string, thelouder the sound. The electrical signal generated within the pick-up is amplified andsent to the loudspeaker. Figure 8.7 shows a simple concept for how an electric guitaror bass pick-up could work; these are, of course, more complex in reality but thebasic idea is given here. The placement of the pick-up with respect to the wavesvibrating on the string creates massively different tonal effects in the electric guitarand bass output. That is why you often see sets of pick-ups distributed at differentlocations on the body of the instrument; some closer to, some farther from, thebridge. Artists have lots of fun playing with the tonal nuances created by the physicsof where the pick-up magnet is in relation to how the string is vibrating.CHAPTER 8 SUMMARY• Electromagnetic induction is the basis of operation for dynamic microphones,loudspeakers, tape recorders, record players, and electric guitar and bass pick-ups.• Condenser microphones use a variable capacitor and generally have better frequencyresponse than dynamic mics.

400007_CH08_p065-076.pdf 5/26/05 8:03 AM Page 7474IS THERE A LASER IN THE HOUSE?• Electromagnetic induction means that a changing magnetic field will induce acurrent to flow in an electrical wire that is within the magnetic field.• Loudspeakers work because an electrical current flowing through a coil in thepresence of a magnetic field generates a force which moves the speaker coneand creates sound waves.• Dynamic mics convert sound waves to electrical AC current by having a movablediaphragm which responds to sound waves connected to a coil of wirewrapped around a magnet.• Tape recorders record sound on a magnetic tape by creating a changing magneticfield in the vicinity of the tape, which records this magnetic field pattern.During playback, the changing magnetic field on the tape induces a current toflow in the coil of wire which is in close proximity to the tape.• Record players work by having the needle attached to a coil of wire which surroundsa magnet; the needle moves up and down on the bumps in the groovesof the record, creating motion between the coil and the magnet.• Electric guitar and bass pick-ups work by having a metal string vibrating in thepresence of a magnetic field (which hence changes) near a wire, which sends acurrent to an amplifier.IN CASE YOU DIDN’T KNOW . . .A coil of wire can function in the same way as a bar magnet. If a currentflows through the coil of wire, it creates a magnetic field that exists inthe center of the coil and also surrounding it, and it has the same fieldpattern as a bar magnet. This is called an “electromagnet” and it is beneficialbecause it can be turned on and off, unlike a bar magnet.Electromagnets are used in security doors. The door is locked orunlocked by throwing a switch.CHAPTER 8 PROBLEMS1. Why does a condenser microphone have better frequency response than adynamic microphone?2. Could a microphone function as a loudspeaker?

400007_CH08_p065-076.pdf 5/26/05 8:03 AM Page 75FUN WITH MAGNETS 753. Why do better tape recorders have separate erase, record, and play heads?4. Does a current get induced in a piece of wire sitting near a bar magnet whenneither the wire nor the magnet are moving?5. How does a record player pick up a stereo signal from the record? Stereorequires two separate channels of information (left and right channels).6. Why can’t you use plastic strings on an electric guitar?

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