Transducer Designs
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Part 4 in our beginner’s series M i c r o p h o n e s 1 — Transducer Designs electing the right microphone is a Le ver aging this principle, ca l l e d rec o r ding music on magnetic tape or Sconstant part of the job. Ba n d : el e c t r omagnetic induction, you can pl a ying music back through a loud- The Has Beens. Song #5: The Hair I ge n e r ate your own electricity if you sp e a ke r . Mo r e on all that in future Used to Have. Ove r dub #16: ukulele. wan t . Just persuade someone to hop episodes of Nuts & Bolts; for now Whi c h microphone should be used? onto a bicycle modified so that the le t ’ s apply electromagnetic induction Depending on the studio, the engi- rear tire is a coil of wire. Set it up so to microp h o n e s . neer has to choose among maybe a that the wire rotates through the gap Mi c r ophones that rel y on electro- do z en or maybe even a hundred of a magnet when he or she pedals. magnetic properties to convert an mi c ro p h o n e s . The y come from count- If he or she pedals hard enough and acoustic event into an electrical sig- less manufa c t u re rs , offering severa l if the coil and magnet are big nal are called el e c t r o d ynamic (m o r e model num b e rs . en o u g h , you could power all you r co m m o n l y ‘d ynamic’) mics. The r e are What will a given microp h o n e favorite equipment free (assuming tw o types of dynamic microp h o n e s sound like on a particular instru- you don’t pay this perso n ) . We don’t used in the studio: moving coil and ment in a specific style of music in kn o w people willing to do that, so ri bb o n . And they both are appealing- this unique rec o r ding space? Aa a rg h ! instead we have power companies. ly strai g h t fo r wa r d devi c e s . The r e is no end to the possibilities. The moving coil dynamic micro- One develops insight and intuition phone converts sound into electricity about which mic to try for a given with essentially three components: a situation through exp e r i e n c e . But we di a p h r agm that moves with the air, a can help our experience along by We can gain insight coil that is moved by the diaphrag m , learning how they work . and a magnet that induces electrical It ’ s helpful to break down the vas t about which mic to cu r r ent onto the coil when it moves . range of microphone possibilities This type of mic takes advan t a g e into some subgrou p s . In the rec o r d- tr y for a given of the motion of air particles during ing studio, the rec o r ding engineer an acoustic sound to move a coil of ty p i c a l l y chooses among three types si t u ation by learning wi r e through the magnetic field of a of microphone designs: moving coil, permanent magnet. The coil move- ri bb o n , or condenser. how they work. ment creates an electrical signal whose voltage changes as a direc t How do they work? result of the acoustic even t . It ’ s a sat- Un usual in our wor ld of complicat- is f y i n g l y simple proc e s s . ed gear (ever open up a digital 8- The ribbon microphone takes tra ck ? ) , the microphone is an ele- Power companies use giant steam- ad vantage of the same electrody n a m - ga n t l y simple, co m p l e t e l y knowabl e powered turbines to spin generators ic principle we’ v e discussed. As a te ch n o l og y . And knowing how the that rely on this same fundamental ma c hine that converts acoustic ener- thing wor ks gives us some insight physical property. And not only does gy into electrical energy , it is even into how to use it. a magnetic field induce a current on simpler than the moving coil system. A fascinating parallel betwee n a wire that moves through it, but The ribbon microphone clever ly com- electricity and magnetism exists and also a changing current on a wire bines the diaphragm and the coil seems tailor-made for audio. creates a magnetic field around it. abo ve into a single device: a ribbo n . Whe n e ver an electrical conductor— That is, electromagnetic induction That is, the thing that moves in the li k e a wire— m o ves through a mag- also works in reverse. air is also the conductor of electricity. netic field, an electrical current is Using electricity to create a mag- The ribbon is a piece of metal sus- induced onto it. netic field is a basic necessity when pended between the poles of magnet. RECORDING OCTOBER 1999 When a musical instrument plays , ai r Once upon a time this type of elec- molecules move. The air molecules trical component was called a co n - near the ribbon for ce it to move; the de n s e r . While the component is today motion of the ribbon through the ge n e ra l l y called a capacitor instead, magnetic field induces electrical cur- the microphone built around this rent onto the ribbon itself. Vol t a g e te ch n o l o gy hangs on to the name changes that are a perfect analogy to co n d e n s e r . the acoustic event are crea t e d . A condenser microphone is noth- A third microphone tran s d u c e r ing more than a var i a ble capacitor te ch n o l o gy employed in the studio dr i ven by acoustic sound waves . On e do e s n ’ t rel y on electrom a g n e t i c plate of the capacitor is the induction at all. The condenser micro- di a p h r agm whose motion is a res u l t phone relies on the electrical prop e r - of the changing sound pres s u r e ty of capacitance instead. We know ar ound it. As the diaphragm moves , that if we hook up a voltage source the capacitance cha n g e s . The electri- (e . g . a battery) across a wire, el e c t r i - cal output of the microphone is a cal current will flow. If we cut that pattern of voltage changes derived wi re , the current stops. fr om this change in capacitance. It turns out that there is something Mission accomplished: acoustic in between a closed circuit (the wire) music in, electrical signal out. and an open circuit (the severe d wi re ) . Imagine that after cutting the Which one do I use? wi r e we bring the two ends of the Knowing the type of transducer wi r e rea l l y close to each other with- technology a microphone employs out touchi n g . It ’ s easy to imagine gives the engineer some insight into th a t , without current actually flow- how it might sound and what appli- ing across the gap we’ v e made in the cations it is best suited for. But let wi re , the two ends would influence me preface this discussion with ea c h other electrically. some very important, really good A capacitor is a component that news: we’re lucky to be in the audio does this on purpo s e . Whe r e the wire biz in 1999. was broke n , plates of metal are The quality of the design, ma t e r i - at t a ch e d . And these two plates are al s , and manuf acturing techn i q u e s br ought up ver y close to each other, used today is enabling all microp h o n e again without touchi n g . The result is te ch n o l o gies to conver ge towar d a that an electrical cha r ge builds up co n s i s t e n t ,h i g h - q u a l i t y , hi g h - d u r abi l i - on the plates, pulled by the influ- ty pro d u c t .B e l ow I discuss some gen- ence of the voltage source across the er al properties of microphones based gap in between the plates. on the type of transducer used. This is The ability to store a cha rg e , or a good starting point for deciding capacitance(hence the name capaci- wh i c h mic to use in a given situation. to r ) , is a function of the vol t a g e And it’s certainly helpful when using ac r oss the plates, the size of the the ever -popular older microp h o n e s .