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Vin o lng vit namVietnam metrology institute*

Cc khi nim c bn v m thanh

Vin o lng vit namVietnam metrology institute

Tm tt

Bi ging ny gii thiu mt s khi nim v m thanh v o lng m bng cch m t p sut m thanh, mc m thanh v cc i lng o lng m thanh. Trc khi thc hin cc php o m thanh, iu quan trng l phi lm quen vi cc thut ng m thanh, cc quy tc c bn trong truyn m, v tnh nng ca cc thit b o m thanh. Vin o lng vit namVietnam metrology institute*


m thanh l mt phn ca cuc sng hng ngy. C th l nhng m thanh du dng th v nh ting nhc hay ting chim ht. N cho php chng ta giao tip vi nhau hoc l dng cnh cnh bo, bo ng - v d nh ting chung in thoi, hoc ting ci. m thanh cng cho php chng ta thc hin nh gi cht lng v chn on t ting m m van ca mt chic xe, hoc ting p ca nhp timVin o lng vit namVietnam metrology institute*


m thanh v ting nHu ht m thanh trong cuc sng hin i u khin chng ta bc dc. Nhng m thanh khng mong mun hoc lm phin n chng ta th gi l ting n. Tuy nhin, mc phin toi khng ch ph thuc vo cht lng ca m thanh, m thi ca chng ta i vi chng. V d c nhng loi hnh m nhc rt c yu thch bi mt s ngi nhng c th c coi nh l ting n i vi nhng ngi khc, c bit l nu n qu to. V i khi m thanh cng khng cn phi qu ln gy nn phin h. Mt ting sn p p, mt ting co ln h s, hoc nhng m thanh lin tc ca mt vi nc chy nh git c th gy kh chu nh mt ting sm to. V to ca m thanh cng c nh gi ty vo thi gian din ra trong ngy. V du: mt ting ng to c th d c chp nhn vo ban ngy hn ban m. m thanh cng c th gy tn hi v ph hy. Mt ting n ln c th lm v ca s v lm bong thch cao ra khi bc tng. Nhng trng hp ng tic nht l khi m thanh lm thit hi c ch tinh t nht c thit k nhn c n l tai ngi.Vin o lng vit namVietnam metrology institute*


Cc thut ng v m thanhm thanh c nh ngha l bt k s thay i p sut no m tai ngi c th pht hin ra v c phm vi rt rng t m thanh yu nht n mc c th gy hi cho thnh gic.Cc nghin cu v m thanh c gi l m thanh hc ACOUSTICS bao gm tt c cc lnh vc ca m thanh nh: cc sm phm m thanh, qu trnh truyn m, thu m, cho d to ra v thu nhn bi con ngi hoc my mc v dng c o lng.Ting n l mt phn tt yu ca cuc sng hng ngy v s pht trin cng ngh dn n s gia tng mc ting n t my mc, nh xng, giao thng vv.. V s dn ti vic gim ting n, do ting n khng phi l mt ci g m chng ta phi chp nhn. Trong cuc chin chng li ting n, vic u tin l chng ta phi c kh nng nh gi ting n mt cch ng n v d nh l thc hin cc php o lng m thanh ng tin cy. Tuy nhin, trc khi cp n vic o ting n, iu quan trng l phi lm quen vi cc thut ng v cc nguyn tc c bn ca php o lng m thanh.

Vin o lng vit namVietnam metrology institute*dBRMSpeakFastSlowImpulseTrng sLeq L10

L90

Thang logarit1/1 v 1/3 c tap sut m thanhPascal


p sut v cng sut mTrc khi m t cc tnh cht vt l ca m thanh, chng ta hy lm mt php so snh gia m thanh,v mt i lng vt l c th c bit n nhiu hn - nhit. Mt ci l si in c th to ra ra mt lng nng lng nht nh trong mt n v thi gian [J/s] tc l n c mt cng sut nht nh tnh bng W [Watt = Joule / giy]. y l cch n gin nh gi lng nhit n c th sinh ra v c lp vi iu kin mi trng xung quanh. Nhit lng t l silm tng nhit cc phn khc nhau ca cn phng v nhit nyc th co bngmt ci nhit kv tnh bng Choc F.. Tuy nhin, nhit ti mt im c th s khng ch ph thuc vo cng sut ca my si v khong cch t l si, m cn ph thuc vo lng nhit hp th bi cc bc tng, v lng nhit truyn qua cc bc tng v ca s.Mt ngun m to ra mt nng lng nht nh trong mt n v thi gian [J/s] v do c mt cng sut nht nh tnh bng W [Watt = J/s]. y cng l mt phng php c bn nh gi bao nhiu nng lng m thanh n c th to ra, v c lp vi mi trng xung quanh ca n. Nng lng m thanh pht i t ngun m hnh thnh nn mt p sut m thanh nht nh trong phng. Khi o p sut m thanh ny s khng ch ph thuc vo cng sut ca ngun m v khong cch gia ngun v cc im o, m cn ph thuc vo lng nng lng m thanh hp th bi cc bc tng v lng nng lng m thanh c truyn qua cc bc tng v ca s



Cc i lng m thanh c bnKhi m thanh to ra bi mt ngun m c cng sut, P, mt nng lng c truyn t ngun n cc phn t khng kh lin k v truyn n cc phn t xa hn v do , nng lng c lan rng ra xa t ngun ging nh gn sng trn h. Nng lng chy theo mt hng nht nh qua mt n v din tch nht nh c gi l cng m. Nng lng i qua mt im c th trong khu vc xung quanh ngun m s lm tng p lc m thanh, p, ti im . l mt khng kh, c l vn tc m.Cng m l mt i lng vc t bao gm thng tin v bin v hng.p sut v cng m c th o trc tip bng cc thit b ph hp. Cng sut m c th c tnh ton t t cc gi tr o p sut hoc cng v iu kin mi trng tin hnh o. Cng dng chnh ca vic xc nh cng sut m l xc nh ln n ca my mc v cng m s dng xc nh v tr v ln ca ngun n. Khi cp n vic nh gi tc hi v s phin toi ca cc ngun ting n th p sut m thanh l mt tham s quan trng.

Vin o lng vit namVietnam metrology institute*Vc t cng m thanh miu t lng v hng ca dng nng lng m ti mt im cho trcCng sut: P (W)Cng :I (J/s/m2) = W/m2p sut:p (Pa=N/m2)


Phng truyn sngKhi mt l xo b nn li, ch b nn s chy dc theo l xo. iu tng t cng xy ra khi cc phn t khng kh b nn v dn ra; cc ch 'nn' v dn' hoc thay i p sut c di chuyn v ta ra trong khng kh.



p sut mKhi mt ngun m thanh nh mt ci tuning fork rung n s gy ra s bin i p sut trong khng kh xung quanh. S lan truyn ca s thay i p sut ny c th c so snh vi nhng gn sng trong mt ci ao khi nm mt hn xung nc. Cc gn sng ny lan ra t im hn chm mt nc. Tuy nhin nc th khng t n di chuyn ra xa m ch di chuyn ln xung to ra cc gn sng trn b mt. . m thanh cng ging nh vy, ci ao l khng kh, v gn sng l kt qu ca sng m



p sut mp sut m chng thm vo p sut tnh ca mi trngVin o lng vit namVietnam metrology institute*


Phm vi ca p sut m thanhNu so snh vi p sut tnh th dao ng ca p sut m thanh nghe c rt nh t 20Pa n 100 Pa. 20Pa l m thanh nh nht m mt ngi bnh thng c th nghe thy v do n c gi l ngng nghe. p sut m thanh ln ti xp x 100 Pa c th gy au v do c gi l ngng au. T s gia hai thi cc ny ln n hn1 triu ln, do o vic s dng thang o tuyn tnh trong o lng m thanh s dn n nhng con s khng l v kh s dng. Thm na tai ngi c phn ng khng tuyn tnh m theo hm m i vi i lng ny. V cc nguyn nhn ny ngi ta tm cch th hin thc t hn cho thng s m thanh bng mt t l logarit ca gi tr o c mt gi tr tham chiu c gi l decibel hoc dB.Vin o lng vit namVietnam metrology institute*


Phm vi ca p sut m thanhu im ca vic s dng ca dB l thy r trn hnh. Thang tuyn tnh vi s cng knh ca cc con c chuyn i thnh mt thang d kim sot hn t 0 dB ti ngng nghe (20 Pa) n 130 dB ti ngng au



dB xi benMc p sut m thanh Lp (dB) c nh ngha bng 20 log p/p0 , trong p l p sut o c bng Pa, v p0 l p sut m tham chiu c tiu chun ha bng 20Pa . Lu y t mc c thm vo p sut m thanh ch ra rng i lng ny c mt mc nht nh trn so vi mt gi tr tham chiu v k hiu cho mc p sut m thanh l LP . Trn hnh minh ho l hai v d v cch s dng cc cng thc tnh mc dB v hai l hai mc thng c s dng trong thit b hiu chun my o n.



dB xi benMt s tng 3 dB (tng ng vi 1,4 ln) v mc p sut m thanh ch mi khin ngi ta hi cm nhn c. Khi tng 10 dB (tng ng vi 3,16) ;ln cho thy to tng gp 2 ln. Hon ton khng c quan h tuyn tnh v to theo dB v cm nhn ca tai ngi.


S thay i mc mCm nhn s thay i v to3Ch hi cm nhn c5Thy c khc bit ng lu 10To gp 2 ln15Thay i ln20To gp 4 ln


S dng bng i sang dBThay v s dng cng thc i t n v p sut sang mc (hoc ngc li) ngi ta c th s dng mt biu n gin nh hnh trn iVin o lng vit namVietnam metrology institute*


S dng biu i sang dBThay v s dng cng thc i t n v p sut sang mc (hoc ngc li) ngi ta c th s dng mt biu n gin nh hnh trn i.



Quy tc chuyn i n ginQuy tc ny kh hu ch khi ang thc hin o lng m thanh v cn chuyn i gia thang tuyn tnh sang thang lo ga rt. Trn hnh l mt s gi tr thng dng.Vin o lng vit namVietnam metrology institute*


S truyn m trong khng khMt chic xe ti 10 tn pht ra ting n l bao nhiu? iu ph thuc rt nhiu vo vic bn ang ng cch bao xa v c ro cn ng trc hay ng sau n khng. Ngoi ra c rt nhiu yu t khc nh hng n mc ting n, v kt qu o c th lch nhau hng chc decibel cho cng mt ngun ting n. Nguyn nhn ca s chch lch c th do cc yu t sau:

Vin o lng vit namVietnam metrology institute*Loi ngun m (im hay ng)Khong cch t ngunS hp th ca kh quynGiNhit , mVt cnS hp th ca mt tS phn xLng ma


Mt s dng ngun mNgun im Ngun m m c kch thc nh hn so vi khong cch n ngi nghe th gi l ngun m im. Nng lng m c lan ra theo hnh cu v mc p sut m ti tt c cc im cch u ngun m l nh nhau v theo quy tc khi khong cch tng gp i th p sut m gim mt na v tng ng l mc m gim i 6 dB. Mt ngun m im c cng sut m l Lw t nm trn mt t th mc p sut m ti bt k khong cch r (m) t ngun m c th ct nh t cng thc:


Lp =Lw -20lg(r)-8 dB


Mt s dng ngun mNgun ng Nu ngun m hp tri theo mt hng v di so vi khong cch ngi nghe th gi l ngun m dng ng. V d nh mt ci ng cha y dung dch chy bn trong hoc mt con ng c lu lng giao thng cao. Nng lng m c tri di theo hnh tr, v do mc p sut m ti tt c cc im cch u ng l bng nhau v gim 3dB khi khong cch tng gp i. Mt ngun ng c cng sut m trn mt (Lw/m) nm trn mt t th, mc p sut m LP ti mt im cch r (m) bt k tnh theo cng thc:Vin o lng vit namVietnam metrology institute*

Lp =Lw -10lg(r)-5 dB


Mt s dng ngun mNgun phngMt loi ngun m him gp trong thc t l dng ngun phng. Theo nguyn l th mt ngun m phng bao gm mt piston t nng lng c pht x vo trong mt ng hnh thnh nn sng phng trong ng . Gi s khng c tn tht nng lng qua thnh ng, cng , hay l nng lng m thanh chy qua ng, l c lp khong cch t ngun. V cng l nh nhau khp mi ni trong ng, nn p sut m thanh s khng gim khi tng khong cch t piston



Ro chn n gim do ro chn ph thuc hai yu t: lch gia ng truyn thng v ng truyn vt ln trn ro chnCc thnh phn tn s ca ting nVin o lng vit namVietnam metrology institute* gim m ph thuc khong cch v tn s m

** m thanh tn s thp rt kh b cn li bi ro chn gim m ph thuc chiu cao ca ro chn


S hp th m ca kh quynS gim m trong khng kh ph thuc vo cc yu t nh:Khong cch t ngun mTn s mNhit mi trng mp sut kh quynHai yu t u tin c xem l nh hng ln nht v c bin th trong s di y. Tn s thp cng t b hp bi kh quyn.Vin o lng vit namVietnam metrology institute*


GiTc gi tng theo cao, lm b cong ng i ca m thanh hng gi v lm hnh thnh mt vng ti v pha ngc chiu giTi sao li o theo hng gi: khong cch gn, ln n 50m gi c nh hng rt nh n mc p sut m. khong cch ln hn, gi s nh hng ng k hn. Theo chiu gi, mc m c th tng ln bi mt vi dB, ty thuc vo tc gi, nhng o ngc chiu gi, mc m c th gim hn 20 dB, ty thuc vo tc gi v khong cch. y l l do ti sao o theo hng gi o c la chn, kt qu o c lch nh hn v kt qu cng m bo hn.Vin o lng vit namVietnam metrology institute*


Nhit Nhit cng to ra hiu ng tng t nh gi, ngoi tr vic ng nht theo tt c cc hng. Vo ngy tri nng, khng gi, nhit gim theo cao, to ra mt hiu ng vng ti ca m thanh. Trong mt n tri quang, nhit c th tng ln theo cao (nhit o ngc), m thanh tp trung trn mt tVin o lng vit namVietnam metrology institute*


Mt tm thanh phn x bi mt t lm nh hng n nhng ng truyn m trc tip. nh hng ca mt t khc nhau ty thuc vo cng m thanh ca b mt v d b mt cng nh b tng hoc nc, mm nh c, cy xanh hay thm thc vt v b mt hn hp. S suy gim do mt t thng c tnh ton theo tng bng tn. Lng ma c th nh hng n s hp th trn mt t. Tuyt c th lm tng s hp th m ca mt t. V thng thng chng ta s khng tin hnh o trong nhng iu kin hi tit nh vy.Vin o lng vit namVietnam metrology institute*nh hng ca mt t khong cch 100m gia ngun v ngi nhn, ngun v ngi nhn cch mt t khong 2m


Trng m thanhTrong thc t, phn ln cc php o m thanh c thc hin trong phng khng hn l phng vang hay phng cm - nhng u gia hai loi ny. iu ny dn n kh khn trong vic tm ra v tr o ng.Thng thng trong thc hnh khng gian xung quanh ngun n, v d nh my mc c chia thnh bn trng: trng gn, trng xa, trng t do v trng vang.Trng gn l khng gian cn k ngun n, ti mc p sut m c th thay i ng k khi thay i v tr. Khu vc ny ko di n khong cch nh hn bc sng ca tn s thp nht pht ra bi ngun n hoc t nht bng hai ln kch thc ln nht ca ngun n. Nn trnh php o mc p sut m trong vng ny. Trng xa c chia thnh trng t do v trng vang. Trong trng t do sng m din ra nh trong khng gian m khng c vt cn can thip vo ng truyn ca n. iu c ngha l trong vng ny p sut m gim 6 dB khi khong cch tng gp i.Trong trng vang, sng phn x t cc bc tng v cc vt khc c th cng ln nh sng trc tip pht ra t ngun.



p sut m tng ti b mt cc bc tngMc p sut m Lp gn vi b mt phn x s b lp li v c xem nh l c 2 mc p sut m cng pha v bin . Do p sut m gn b mt s c nhn iL= Lp + 6 dBVin o lng vit namVietnam metrology institute*


Phng cm v phng vangNng lng m thanh khng phi lc no cng c pht x mt cch t do t ngun m. Khi sng m p vo cc b mt nh tng, trn v snmt phn nng lng s phn x, v mt phn b hp th v truyn qua cc b mt . Trong mt phng c cc b mt cng, tt c nng lng s b phn x v hnh thnh nn mt trng hn lon vi nng lng m thanh c phn b ng u trong c phng. Mt phng nh vy c gi l phng vang.Trong mt cn phng m c cc b mt c hp th cao, ton b nng lng s b hp th vo cc b mt ny v nng lng m c lan xa t ngun nh l ngun m trong mt trng t do. Mt cn phng nh vy c gi l phng cm.



Trng p sutTrong mt trng p sut, bc sng m di hn so vi khch thc ca khng gian kn, v p sut m l ng u trong khng gian y. Trng ny thng c s dng trong cc thit b hiu chun my o n.Vin o lng vit namVietnam metrology institute*


p sut m t hai ngun mNu hai ngun m cng pht x nng lng th chng u gp phn vo mc p sut m ti mt im. Nu hai ngun ny pht x nng lng bng nhau th ti mt im cch u hai ngun ny cng m tng gp hai ln nu ch mt ngun pht x, trong kh p sut m s tng ln ln tng ng vi 3 dB.Lu rng khi cng mc p sut m ti 1 im t hai ngun m th khng phi l php cng tng ca hai gi tr dB



Php cng hai gi tr dBNu hai ngun m c nng lng pht khc nhau, th tng mc p sut m s c tnh bng cch i cc gi tr dB v gi tr tuyn tnh, cng li v i tr li thnh dB. Trn hnh l mt cch n gin hn cng hai mc dB.Tnh hiu L ca hai mc p sut mS dng ng cong trong hnh tm gi tr L+Cng thm gi tr L+ vo mc p sut m ln hnCh nu L =0, nh trong slice trc th gi tr cng thm vo l 3 dBNu lch gia hai gi tr ln hn 10 dB, th c th b qua phn ng gp ca ngun tnh lng hn.



Php tr gi tr dBTrong mt vi trng hp, v d ta ting hnh o n ca mt my mc c th trong mi trng c mt n nn nht nh, khi ta s cn bit ting n o c l t my mc, tp m nn, hay l c hai. bit c u ta s lm nh sau:1 o n kt hp t my mc v tp m nn: LS+N2 Tt my i v o tp m nn LN 3 Tnh lch L = LS+N LN v s dng biu sau tm n gy ra bi my mc



Php tr gi tr dBNu L nh hn 3 dB, khi tp m nn qu cao c th o c chnh xc n do my pht raNu L ln hn 10 dB th c th b qua ga tr tp m nn.Nu L nm gia 3 v 10 dB, th ta c th dng biu trn tm L- . Khi n do my pht ra s l LS+N L-



Cng nhiu gi tr dBVin o lng vit namVietnam metrology institute* cng nhiu gi tr dB ta c th s dng cng thc


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