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082-Engineering-Mathematics-Anthony-Croft-Robert-Davison-Martin-Hargreaves-James-Flint-Edisi-5-2017

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90 Chapter 2 Engineering functions

Using the dB measurement for voltage gain (see Engineering application 2.10)

this corresponds to voltage gain (dB) =20log V o

V i

which equals 20log10. So,

ln10 (Np) = 20log10 (dB). Thus,

1Np= 20log10 dB ≈ 8.685 89 dB

ln 10

Using the value quoted earlier of an attenuation factor of 0.3 dB km −1 , this corresponds

to

0.3 ÷8.685 89 Np km −1 = 0.034539 Np km −1

It is more usual to quote values of Np m −1 and so this value becomes 3.4539 ×

10 −5 Np m −1 .

Engineeringapplication2.13

Referencelevels

We saw in Engineering application 2.11 that the suffix ‘m’ is used in dBm to indicatetheprovisionofaspecificreferencelevel.Alternativesuffixesareusedtodenote

other reference levels and quantities, which do not necessarily have to be related to

electricalpower.Forexample,whenmeasuringsoundpressure,P,inairtheconventionalreferencelevelforsoundpressureis20

µPar.m.s.Thisischosentocorrespond

totheapproximatethresholdofhumanhearingfora1kHzsinusoidalsignal.Theunit

forsoundpressureisthereforequotedwithreferencetoaninputpressureof20 µPa.

ThisiscommonlywrittenasdBre20 µPar.m.s.orusingtheshorthanddBSPL(dB,

soundpressure level). Inotherwords wehave

( )

P

soundpressure level (dBSPL) = 20log

20 ×10 −6

Asaconsequenceofthechoiceofthehumanhearingthresholdasthereferencelevel,

anegativevalueofdBSPLcorrespondstoasoundthatistooquiettobeheardbythe

averageperson;0dBSPLisasoundthatcanjustbeheardandanythingabovethisis

fullyaudible.Anofficemighthaveanambient(background)levelof30dBSPLand

a person talking to you at the next desk might produce 60 dB SPL, both quantities

being measured atyour hearing position.

Logarithmfunctions

Thelogarithm functionsaredefined by

f(x)=log a

x x>0

whereaisapositive constant called the base.

In particular the logarithm functions f (x) = logx and f (x) = lnx are shown in Figure2.32andsomevaluesaregiveninTable2.5.Thedomainofbothofthesefunctionsis

(0,∞)andtheirrangesare (−∞,∞).Weobservefromthegraphsthatthesefunctions

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