PRELAB 9: TRANSDUCERS

Transcription

1 I. INTRODUCTION PRELAB 9: TRANSDUCERS Transducton s a process that changes energy (or normaton) rom one orm to another. Mcrophones transduce acoustcal energy nto electrcal energy (voltage); loudspeakers transduce voltage back nto sound. Tape deck record and playback heads transduce electrcal sgnals nto magnetzed regons on magnetc tape, and vce versa. CD players transduce the normaton recorded on a CD back nto electrcal energy. Thus, transducers are a crucal part o the study o acoustcs. A mcrophone s a transducer that converts acoustc energy to electrcal energy. There are ve key types o mcrophone you may use Movng col (dynamc mcrophone) Rbbon Condenser Electret Crystal All employ derent mechansms to convert sound energy to electrcal energy. Hence all have derent advantages and dsadvantages. Parameters o mcrophone nclude senstvty, requency response and drectonal response. Snce the mcrophone operates nto HIGH ELECTRICAL IMPEDANCE such that there s (approxmately) zero current and the VOLTAGE s the output varable, t s not expected to delver electrcal power. Consequently, t s conventonal to talk o the "Open Crcut" response. The mcrophone may be senstve to any combnaton o acoustc varables, but the smplest case s a mcrophone that s responsve to pressure nput. II Mcrophone senstvty A mcrophone s senstvty (pressure senstvty) s dened as the voltage generated n response choosng a mcrophone to a certan pressure nput. The common unt s: M 0 (Volts/Pascal). For example: A mcrophone s exposed to 94dB SPL generates 50mV output, ts senstvty s 50mV/Pa because a SPL 94dB corresponds to an RMS acoustc pressure o 1Pa. An alternatve unt or a mcrophone s senstvty s expressed n logarthmc orm, relatve to a reerence,.e. db re 1 V/Pa. To convertng to ths scale, we have 0log(M 0 ) db re 1 V/Pa. The above mcrophone senstvty s 0log(0.05)= 6 db re V/Pa or 6 dbv/pa. Queston: A mcrophone s rated 5 db re 1 V/Pa. I an acoustc pressure o 5 10 Pa s ncdent on the mcrophone, what s the open-crcut output voltage o the mcrophone? PRELAB 09: Transducer; Page 1

2 III FARADAY S LAW OF ELECTROMAGNETIC INDUCTION The magnetc eld H, s measured n amperes per meter. However, the commonly called Magnetc eld B s known as magnetc lux densty that has the SI unts TESLA. One Tesla s equal to or 10 4 Gauss. The magnetc eld lnes pont away rom N-pole (or North Pole) and pont toward S-pole (or the South Pole). Typcally, the Earth magnetc eld s about 0.5 Gauss pontng toward Earth North Pole. Thus the Earth North Pole s a magnetc South Pole. Typcal magnetc eld strength o magnets o speakers and mcrophones s 0.1 T to 1 T. Pace makers should avod magnetc eld strength larger than 5 Gauss, and typcal rergerator magnets are about 50 G. The operaton o most transducers s based on Faraday s Law, whch says that you move a wre through a magnetc eld (or vary the magnetc eld strength through a col o wre), a voltage wll be produced n the wre. The nduced electromotve orce (EMF) s equal to the rate change o the magnetc lux,.e. E= Φ/Δt, where the negatve sgn arses rom the Lenz s law, E s the nduced EMF, Φ s the magnetc lux n [Tesla-m ], gven by (magnetc eld B) (area o an electrc crcut loop), and t s the tme n [seconds]. The unt o EMF s [V]=[T m ]/[s]. Example: A bar magnet s moved rapdly toward a 40-turn, crcular col o wre. As the magnet moves, the average value o magnetc lux densty B over the area o the col ncreases rom T to T n 0.5 s. I the radus o the col s 3.05 cm, the magntude o the nduced em can be ound as ollows: Answer: The ntal and nal magnetc luxes through the col are Φ Φ = B A = (0.015T) π (0.0305m) = B A = (0.45T) π (0.0305m) The change o magnetc lux s ΔΦ=Φ Φ. The nduced voltage over the 40-turn col s Φ ε = N t = 40 ( In a dynamc mcrophone, the nduced voltage s small, and the nduced voltage s o the order o mv (10 3 V) that needs amplcaton or an audble sound. Rbbon mcrophone uses Faraday s law or transducton. IV Condenser mcrophone Φ t Two conductor sheets can hold charges between two plates. The amount o charge depends on the dstance between these two sheets. When the sound wave exerts pressure on the condenser sheet, the dstance change and the charge on each plate vares. Ths sets up electrc current. Ths process converts sound energy to electrc energy. What s the derence between a speaker and a mcrophone? 3 = = Tm Tm Tm ) / 0.5s = 0.05V Acoustc Lab 09: Transducer; Page

3 The mcrophone s a devce that converts sound nto electrcal energy. They are used n rado broadcastng, recordng, and sound amplyng systems. Its basc component s a daphragm that responds to the pressure or velocty o the sound waves. Varous orms o mcrophone were developed ndependently n 1877 by Emle Berlner, Davd E. Hughes, Thomas A. Edson, and others. It was rst used as a telephone transmtter. Varety o mcrophones ncludes nto Condenser mcrophone (ncludng Electret condenser mcrophone), Dynamc mcrophone, Rbbon Mcrophone, Carbon mcrophone, Pezoelectrc mcrophone, Fber optc mcrophone, Laser mcrophone, Lqud mcrophone, MEMS mcrophone, and Speakers as mcrophones. In a condenser mcrophone, also called a capactor mcrophone or electrostatc mcrophone, the daphragm acts as one plate o a capactor, and the vbratons produce changes n the dstance between the plates. There are two methods o extractng an audo output rom the transducer thus ormed: DC-based and rado requency (RF) or hgh requency (HF) condenser mcrophones. Wth a DC-based mcrophone, the plates are based wth a xed charge (Q). The voltage mantaned across the capactor plates changes wth the vbratons n the ar, accordng to the capactance equaton (C = Q / V), where Q = charge n coulombs, C = capactance n arads and V = potental derence n volts. The capactance o the plates s nversely proportonal to the dstance between them or a parallel-plate capactor. RF condenser mcrophones use a comparatvely low RF voltage, generated by a lownose oscllator. The oscllator may ether be ampltude modulated by the capactance changes produced by the sound waves movng the capsule daphragm, or the capsule may be part o a resonant crcut that modulates the requency o the oscllator sgnal. Demodulaton yelds a low-nose audo requency sgnal wth very low source mpedance. The absence o a hgh bas voltage permts the use o a daphragm wth looser tenson, whch may be used to acheve wder requency response due to hgher complance. The RF basng process results n a lower electrcal mpedance capsule, a useul byproduct o whch s that RF condenser mcrophones can be operated n damp weather condtons that could create problems n DC-based mcrophones wth contamnated nsulatng suraces. An electret mcrophone s a relatvely new type o capactor mcrophone nvented at Bell laboratores n 196 by Gerhard Sessler and Jm West. The externally appled charge descrbed above under condenser mcrophones s replaced by a permanent charge n an electret materal. An electret s a erroelectrc materal that has been permanently electrcally charged or polarzed. The name comes rom electrostatc and magnet; a statc charge s embedded n an electret by algnment o the statc charges n the materal, much the way a magnet s made by algnng the magnetc domans n a pece o ron. Due to ther good perormance and ease o manuacture, hence low cost, the vast majorty o mcrophones made today are electret mcrophones; a semconductor manuacturer estmates annual producton at over one bllon unts. Nearly all cell-phone, computer, PDA and headset mcrophones are electret types. They are used n many applcatons, rom hgh-qualty recordng to bult-n mcrophones n small sound recordng devces and telephones. Though electret mcrophones were once consdered low qualty, the best ones PRELAB 09: Transducer; Page 3

4 can now rval tradtonal condenser mcrophones n every respect and can even oer the long-term stablty and ultra-lat response needed or a measurement mcrophone. Unlke other capactor mcrophones, they requre no polarzng voltage, but oten contan an ntegrated preampler that does requre power (oten ncorrectly called polarzng power or bas). Ths preampler s requently phantom powered n sound renorcement and studo applcatons. Dynamc mcrophones use electromagnetc nducton. They are robust, relatvely nexpensve and resstant to mosture. Coupled wth ther potentally hgh gan beore eedback, they are deal or on-stage use. A small movable nducton col, postoned n the magnetc eld o a permanent magnet, s attached to the daphragm. When the sound wave moves the daphragm, the col moves n the magnetc eld and produces a varyng current n the col through electromagnetc nducton. A sngle dynamc membrane does not respond lnearly to all audo requences. Some mcrophones or ths reason utlze multple membranes or the derent parts o the audo spectrum and then combne the resultng sgnals. Combnng the multple sgnals correctly s dcult and desgns that do ths are rare and tend to be expensve. Rbbon mcrophones are smlar to movng col mcrophones n the sense that both produce sound by means o magnetc nducton. Basc rbbon mcrophones detect sound n a b-drectonal (also called gure-eght) pattern because the rbbon, whch s open to sound both ront and back, responds to the pressure gradent rather than the sound pressure. Though the symmetrcal ront and rear pckup can be a nusance n normal stereo recordng, the hgh sde rejecton can be used to advantage by postonng a rbbon mcrophone horzontally, or example above cymbals, so that the rear lobe pcks up only sound rom the cymbals. Crossed gure 8, or Blumlen par, stereo recordng s ganng n popularty, and the gure 8 response o a rbbon mcrophone s deal or that applcaton. A carbon mcrophone, also known as a carbon button mcrophone (or sometmes just a button mcrophone), use a capsule or button contanng carbon granules pressed between two metal plates lke the Berlner and Edson mcrophones. A voltage s appled across the metal plates, causng a small current to low through the carbon. One o the plates, the daphragm, vbrates n sympathy wth ncdent sound waves, applyng a varyng pressure to the carbon. The changng pressure deorms the granules, causng the contact area between each par o adjacent granules to change, and ths causes the electrcal resstance o the mass o granules to change. The changes n resstance cause a correspondng change n the current lowng through the mcrophone, producng the electrcal sgnal. Carbon mcrophones were once commonly used n telephones; they have extremely lowqualty sound reproducton and a very lmted requency response range, but are very robust devces. A crystal mcrophone or pezo mcrophone uses the phenomenon o pezoelectrcty the ablty o some materals to produce a voltage when subjected to pressure to convert vbratons nto an electrcal sgnal. An example o ths s Rochelle salt (potassum sodum tartrate KNaC 4 H 4 O 6 4H O), whch s a pezoelectrc crystal that works as a transducer, both as a mcrophone and as a slmlne loudspeaker component. Crystal mcrophones were once commonly suppled wth vacuum tube (valve) equpment, such as domestc tape recorders. Ther hgh output mpedance matched the hgh nput Acoustc Lab 09: Transducer; Page 4

5 mpedance (typcally about 10 MΩ) o the vacuum tube nput stage well. They were dcult to match to early transstor equpment, and were quckly supplanted by dynamc mcrophones or a tme, and later small electret condenser devces. The hgh mpedance o the crystal mcrophone made t very susceptble to handlng nose, both rom the mcrophone tsel and rom the connectng cable. A ber optc mcrophone converts acoustc waves nto electrcal sgnals by sensng changes n lght ntensty, nstead o sensng changes n capactance or magnetc elds as wth conventonal mcrophones. Laser mcrophones are oten portrayed n moves as spy gadgets. A laser beam s amed at the surace o a wndow or other plane surace that s aected by sound. The slght vbratons o ths surace dsplace the returned beam, causng t to trace the sound wave. The vbratng laser spot s then converted back to sound. In a more robust and expensve mplementaton, the returned lght s splt and ed to an ntererometer, whch detects movement o the surace. The ormer mplementaton s a tabletop experment; the latter requres an extremely stable laser and precse optcs. Early mcrophones dd not produce ntellgble speech, untl Alexander Graham Bell made mprovements ncludng a varable resstance mcrophone/transmtter. Bell's lqud transmtter conssted o a metal cup lled wth water wth a small amount o sulurc acd added. A sound wave caused the daphragm to move, orcng a needle to move up and down n the water. The electrcal resstance between the wre and the cup was then nversely proportonal to the sze o the water menscus around the submerged needle. Elsha Gray led a caveat or a verson usng a brass rod nstead o the needle. Other mnor varatons and mprovements were made to the lqud mcrophone by Majoranna, Chambers, Vann, Sykes, and Elsha Gray, and one verson was patented by Regnald Fessenden n These were the rst workng mcrophones, but they were not practcal or commercal applcaton. The amous rst phone conversaton between Bell and Watson took place usng a lqud mcrophone. The MEMS (McroElectrcal-Mechancal System) mcrophone s also called a mcrophone chp or slcon mcrophone. The pressure-senstve daphragm s etched drectly nto a slcon chp by MEMS technques, and s usually accompaned wth ntegrated preampler. Most MEMS mcrophones are varants o the condenser mcrophone desgn. Oten MEMS mcrophones have bult n analog-to-dgtal converter (ADC) crcuts on the same CMOS chp makng the chp a dgtal mcrophone and so more readly ntegrated wth modern dgtal products. A loudspeaker, a transducer that turns an electrcal sgnal nto sound waves, s the unctonal opposte o a mcrophone. A conventonal speaker s constructed much lke a dynamc mcrophone (wth daphragm, col and magnet), t can actually work n reverse" as mcrophones. The result, though, s a mcrophone wth poor qualty, lmted requency response (partcularly at the hgh end), and poor senstvty. In practcal use, speakers are sometmes used as mcrophones n applcatons where hgh qualty and senstvty are not needed such as ntercoms, walke-talkes or Vdeo game voce chat perpherals, or when conventonal mcrophones are n short supply. PRELAB 09: Transducer; Page 5

6 Acoustc Lab 09: Transducer; Page 6