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 . pl a t e s , and the distance between the Tak e note, ho wever , that some new tw o plates. As the plates separat e , mi c r ophones have addr essed many of th e y become more like a fully bro- the historic design weaknesses clev- ken circuit and the cha r ge dissipates. er ly, cr eating mics that are often As the plates converg e , th e y have a ap p r opriate in a broad range of st r onger and stronger influence on rec o r ding situations. ea c h other and try to approa c h the So with the caveat that these gen- be h a vior of a completed circu i t — t h e er alities don’t apply to all mics, co n - cha r ge on the plates then increa s e s . sider the fol l ow i n g .

RECORDING OCTOBER 1999 Durability Mo ving coil dynamic microp h o n e s Mo ving coil microphones are often ar e the largest mechanism used for co n s i d e r ed to be the heartiest of the co n verting acoustic waves into elec- bu n ch . As a result they are often the trical ones. Not surpr i s i n g l y, th e n , tr ansducer of choice for live sound th e y general l y have a natural high ap p l i c a t i o n s , wh i c h are ver y tough fre q u e n c y roll-off as the ability of on delicate equipment. the device to transduce diminishes At the other end of the durabi l i t y at higher freq u e n c i e s . chain is the ribbon mic. The ribbo n Consider the fol l o wing hyp o t h e t i - itself is pretty frag i l e — e s p e c i a l l y on cal exp e d i t i o n . Be fo r e the session the vintage (i.e. exp e n s i ve) ribbo n be g i n s , you go food shopping for the mi c r ophones avai l a ble at some studios. band you are wor king with, and the Re m e m b e r , the job of the ribbon is drummer helps. The shopping list to react instantly to any change in consists solely of potato chips and the air pres s u r e around it. And if be e r , but in enough quantity to get th e r e is, sa y, a 10 kHz component to the band through a two- w eek session. the music you are rec o rd i n g , th e n You and the drummer go to the the ribbon has to be able to move neighborhood Chomp ‘n Gulp, get two ba c k and forth ten thousand times a shopping carts, and you fill one cart se c o n d . Ph ysics asks it, th e re fo re , to with chips while the drummer fills ha ve as little weight as possible. the other with beer. ‘T his has nothing Un fo r t u n a t e l y, as the ribbon loses to do with microp h o n e s , ’ you—and my mass it necessarily loses stren g t h . editor—think to you rs e l ve s . Some ribbon microphones are still But consider these questions: ma nu fa c t u r ed today, and the ribbo n Whi c h cart is easier to drive? Whi c h within those mics is certainly cart is easier to stop and start? The tougher than the ribbons in Gran ny ’ s chip cart and the beer cart can go mi c ro p h o n e s . pr etty muc h the same speed, but the But nobody dares stick a ribbo n beer cart needs a stronger shove to mi c r ophone in the high amplitude get going. Cl e ver drummers start wor ld of a roc k and roll kick drum. emptying (i.e. drinking) some cans Some new ribbons are designed to be for this ver y rea s o n . tough enough for screaming voc a l s For our microp h o n e s , the movi n g and thundrous electric guitar. Bu t coil is more like the beer cart. Qu i t e th e y all want a chance at the horns si m p l y, the diaphragm/coil assembly (not too close, thank you ) , the piano, is too big to react quickl y, as and the acoustic guitar, among others. req u i r ed by ver y high freq u e n c i e s .

Un u s u al in our world of complicated gear (ever open up a digital 8-trac k ? ) , the microphone is an elegan t l y simple, co m p l e t e l y knowable technology.

In the durability category, co n - The ribbon microphone is more de n s e r s general l y fall somewh e r e in li k e the chip cart. Consisting of a sin- be t w een moving coil and ribbo n gle moving part (the ribbon) it is a de s i g n s . As a res u l t , you’ll certainly lighter mecha n i s m . As a result the find some of them performing on ri b bon transducer is typically more stage or placed near the ver y loud agile than a moving coil, ach i ev i n g instruments such as kick drum, tr u m - mo r e sensitivity at the high freq u e n - pe t , tro m b o n e , and so for t h . cies as a res u l t . The condenser microphone is gen- Sound quality era l l y lightest of all, be h a ving more Though microphones of all types li k e an empty shopping cart in the seem to be improving in capabi l i t y , it an a l o gy abo ve. The only moving part, is worth making some general i z a t i o n s the diaphrag m , can be an ext re m e l y about how a microphone of a given thin plastic membrane with just the tr ansducer techn o l o gy might sound. lightest coating of a metal to make it

RECORDING OCTOBER 1999 conduct electricity. As a result the pelling after the subtle reshaping of condenser offers the best opportuni- the transient that a moving coil ty to capture the detail of a tran - mi c r ophone introd u c e s . si e n t , or the ver y high freq u e n c y por- As soon as a session permits, tr y tion of a high hat. using a moving coil and a condenser

The apparent transient response weakness in the moving coil design can in fact be quite a handy engineering tool.

The apparent transient response on the same instrument, placed as weakness in the moving coil design near to the same location as possi- can in fact be quite a handy engi- bl e . Then listen critically to the dif- neering tool. By reacting slowly to a fe r ent coloration of each mic. Mo s t sudden increase in amplitude, it acts ap p a r ent will be the freq u e n c y mechanically as a compressor might response differen c e s , with the con- act electrically. It reduces the ampli- denser sounding a little brighter at tude of the peaks of a transient the high end while the moving coil sound. of f e r s perhaps a presence peak in This is helpful for two major rea - the upper mid-ran g e . so n s . Firs t , this reduction of peaks But listen beyond this, to the cha r - can help prevent the sort of distor- acter of the attack of the instrument. tion that comes from overl o a d i n g Depending on the application, you your electron i c s . The true spike of will often find that the moving coil amplitude that comes off a conga dynamic microphone squashes the might easily distort the microp h o n e tr ansients into a more exc i t i n g , mo r e pr eamplifier or over load the tape you intense sound. ar e rec o r ding onto. The use of a By the time the trac k you are dynamic mic might be just the right rec o r ding gets combined with all the solution to capture the sound without other trac ks in the mul t i t ra c k proj e c t , di s t o r t i o n . and after the signal makes its way

Be yond this issue of audio fidelity fr om mic to tape, th r ough the console and the prevention of distortion, and various effects proc e s s o r s to the dynamic microphones with their nat- lo u d s p e a ke rs , the rec o r ded sound ur al lethargy are often used for cre- fr om a moving coil microphone often at i ve rea s o n s . The sound of a clave, just seems to wor k better. sn a re , ki ck , du m b e k , and many other Meantime the ribbon microp h o n e , instruments is often muc h more com- offering more high freq u e n c y content

RECORDING OCTOBER 1999 than the typical moving coil micro- of applying different microphones in phone but less high freq u e n c y rea c h di f f e r ent musical situations. than most condensers, still finds its Let the other engineers wor k their place in the rec o r ding studio. way through the microphone closet,

Many instruments have a rather pai n f u l amount of high end. Close-miking them, as we so often must do in the studio, on l y makes this worse.

Ma n y instruments have a rat h e r ran d o m l y trying different micro- painful amount of high end. Cl o s e - phones in different applications. We miking them, as we so often must do can orga n i z e our experiences based in the studio, on l y makes this wors e . on what we know about how the The natural high freq u e n c y attenua - mi c r ophone work s .

tion of a ribbon is often just the Try to rat i o n a l i z e what you actual- right touch to make a trumpet, a ly hear with how you think it should cym b a l , a tambourine, a triangle, so u n d , and you’ll bring some order to and others become beautiful, ai r y, an otherwise chaotic part of the and sparkl i n g , without being shrill, rec o r ding gig. pi e rc i n g , th i n , or edgy. In our next Nuts & Bolts episode Alex Case (case@rec o r d i n g m a g . c o m ) we’ll look at other microphone prop e r - is the director of eFr m a ta whe r e he ties like directionality and proxi m i t y records and produces music he vloe s . Excerpted from the October edition of RECORDING magazine. effect so that we can make more sense He hopes you have a similar job. ©1999 Music Maker Publications, Inc. Reprinted with permission. out of the vast range of options micro- 5412 Idylwild Trail, Suite 100, Boulder, CO80301 Tel: (303) 516-9118 Fax: (303) 516-9119 phones offer.We can look for wa r d to a For Subscription Information, call: 1-800-582-8326 ca re e r -long exp l o r ation of the beauty

RECORDING OCTOBER 1999