{{B}}tExt 4{{/B}}

moDEm tEChnology mAy not hAvE improvED thE worlD All thAt muCh, But. it CErtAinly hAs mADE liFE noisiEr. un-muFFlED motorCyClEs, BlAring CAr AlArms, AnD roving Boom BoxEs ComE First, sEConD, AnD thirD on my list oF most oBnoxious noisE oFFEnDErs, But EvEryonE CoulD ComE up with his own vErsion oF AurAl hEll--iF hE CoulD just FinD A quiEt spot to ponDEr thE mAttEr. yEt whAt tEChnology hAs DonE, othEr tEChnology is now stArting to unDo, using ComputEr powEr to zAp thosE EAr-splitting noisEs into silEnCE. prEviously silEnCE sEEkErs hAD littlE rECoursE ExCEpt to stAy insiDE, ClosE thE winDows, AnD plug thEir EArs. rEmEDiEs likE thEsE ArE quAintly tErmED "pAssivE" systEms, BECAusE thEy plACE physiCAl BArriErs AgAinst thE unwAntED sounD、now ComputEr tEChnology is proDuCing A FAr morE EFFECtivE "ACtivE" systEm, whiCh DoEsn’t just ContAin, DEFlECt, or mAsk thE noisE But AnnihilAtEs it ElECtroniCAlly. thE systEm works By CountEring thE oFFEnDing noisE with "Anti-noisE", A somE whAt sinistEr sounDing tErm thAt CAlls to minD AntimAttEr, BlACk holEs, AnD othEr populAr sCiEnCE minDBEnDErs But thAt ACtuAlly rEFErs to somEthing quitE simplE. just As A wAvE on A ponD is FlAttEnED whEn it mErgEs with A trough thAt is its ExACt oppositE (or mirror imAgE), so CAn A sounD wAvE By mEEting its oppositE. this gEnErAl thEory oF sounD CAnCEllAtion hAs BEEn ArounD sinCE thE 1930s. in thE FiFtiEs AnD sixtiEs it mADE or A kinD oF mAgiC triCk Among lABorAtory ACoustiCiAns plAying ArounD with thE First Clunky mAinFrAmE ComputErs. thE ADvEnt oF low-Cost, high-powEr miCroproCEssors hAs mADE ACtivE noisE-CAnCEllAtion systEms A CommErCiAl possiBility, AnD A hAnDFul oF smAll ElECtroniCs Firms in thE unitED stAtEs AnD ABroAD ArE Bringing thE First onEs onto thE silEnCE mArkEt. silEnCE BuFFs might BE hoping thAt thE noisE-CAnCEling AppArAtus will tAkE thE shApE oF thE 44 mAgnum wiElDED ByDirty hArry, But in FACt ACtivE sounD Control is not quitE thAt ACtivE. thE systEm might morE propErly hE DEsCriBED As rEACtivE in thAt it rEsponDs to sounD wAvEs AlrEADy hEADED towArD humAn EArs. in thE ConFigurAtion thAt is usuAl For suCh systEms miCrophonEs DEtECt thE noisE signAl AnD sEnD it to thE systEm’s miCroproCEssor, whiCh Almost instAntly moDEls it AnD CrEAtEs its invErsE For louDspEAkErs to FirE At thE originAl.BECAusE thE two sounDs oCCupy’ thE sAmE rAngE oF FrEquEnCiEs AnD tonEs, thE invErsE sounDs ExACtly tikE thE noisE it is mEAnt to EliminAtE: thE Anti-noisE CAnCElingBEEthovEn’s FiFth symphony is hEArD AsBEEthovEn’s FiFth. thE only DiFFErEnCE is thAt EvEry positivE prEssurE proDuCED on thE Air By thE orChEstrA is mAtChED By A nEgAtivE prEssurE proDuCED By thE ComputEr, AnD thErEBy silEnCing thE sounD、thE systEm is most EFFECtivE As A kinD oF muFFlEr, in whiCh miCrophonEs, miCroproCEssor, AnD louDspEAkEr ArE All in A unit EnCAsing thE DEviCE, thAt proDuCEs thE sounD, stiFling it At its sourCE.But it CAn work As A hEADsEt, too, nEgAting thE sounD At thE lAst momEnt BEForE it DisturBs onE’s pEACE oF minD、

whiCh oF thE Following stAtEmEnts is not truE ACCorDing to thE pAssAgEA．in thE pAst, pEoplE somEtimEs pluggED thEir EArs to Fight AgAinst thE oFFEnDing noisE.
B．An ACtivE noisE-CAnCEllAtion systEm Follows thE prinCiplEs oF A wAvE BEing FlAttErED By mEEting its ExACt oppositEs.
C．thE First ACtivE noisE-CAnCEllAtion systEm wAs mADE in thE 1930s.
D．ACtivE noisE-CAnCEllAtion systEms ArE now AvAilABlE on thE mArkEt.