Lanjutan … materi 3

Lanjutan…materi 3

Noise Measurement

Audible Range

• Audible range : 20 Hz – 20 KHz. • Telinga manusia sangat sensitif pada frekuensi

3000 Hz – 4000 Hz dan kurang sensitif pada bunyi-bunyi frekuensi rendah.

• Pada frekuensi sensitif, dalam keadaan tertentu manusia bahkan mampu mendengar bunyi pada kekuatan -5 dB.

• Respons telinga berbeda untuk setiap frekuensi yang muncul mekanisme dengar

Phon Scale

• Mekanisme dengar grafik yang tidak linear • Test-test yang dilakukan menghasilkan

serangkaian kurva : equal loudness level contours atau disebut juga Fletcher-Munson equal loudness contours.

• Kurva ini telah distandarkan secara internasional sebagai kurva referensi untuk menunjukkan respons telinga normal pada tingkat kekuatan bunyi tertentu pada frekuensi tertentu.

Phon Scale: Phon scale dapat diartikan sebagai tingkat kekerasan bunyi pada frekuensi tertentu yang sama dengan tingkat dB pada frekuensi 1000 Hz pada kontur atau kurva tersebut.

• Bunyi dengan tingkat 60 dB dirasa lebih keras, yaitu dalam tingkat 60 phon, bila berada pada frekuensi 1000 Hz, namun dirasa lebih lemah, yaitu hanya 30 phon ketika frekuensinya 50 Hz.

5Dept. of Mech. EngineeringUniversity of Kentucky

Sound Weighting Sound level meters incorporate frequency weighting to simulate this effect.• Respons telinga yang

berbeda-beda terhadap bunyi pada frekuensi tertentu akhirnya mengelompokkan bunyi-bunyi dalam bobot tertentu, sesuai kesan atau sensasi yang diterima oleh telinga sound weighting


The A-weighted Sound Level

The A-weighting is a filter that approximates the ear’s response at low levels. It filters and integrates the actual spectrum to yield a single value, e.g., 95 dB(A) overall sound level.

A-weightingresponse

100 Hz 1000 Hz

(Ear’s response at approximately 40 dB)

19 dB


Example of A-Weighting

Sound Pressure Level

50

60

70

80

90

100

110

0 500 1000 1500 2000

Frequency (Hz)

SP

L (d

B)

Unweighted (114.4 dB)

A-weighted (109.6 dBA)


The A-weighting Curve Approximation

A-weightingresponse

100 Hz 1000 Hz

19 dB

0 dB

An approximate function that can be used to calculate the response of the A-weighting curve is:

Hz)in (log516806.12251.212.49867.121290

10

2345

ffx.xxxxx.

This function is accurate to within 0.6 dB maximum error at any f.


Historical Notes

• Octave and one-third octave filters predate by many years the use of the FFT (introduced in the early 1970’s) to obtain the frequency spectrum of complex sounds.

• Many standards were written around octave and one-third octave filter frequency analysis and have yet to be fully updated.

• The A-weighting system is, strictly speaking, only valid at low sound levels (around 40 dB), but it is commonly used to rate sounds having levels over 100 dB.

• The A-weighting curve is often mistakenly used to rate noise for “annoyance” or other subjective (psychological) measures of sound quality. It is, in fact, only a measure of the physical response of the ear mechanism.


Working with Decibels

• In as much as decibels are logarithmic quantities, we cannot manipulate them arithmetically.

• Instead, we must convert decibels to their “base” values, perform the arithmetic operation, and then convert back to decibels

• Addition of sound levels. Sound levels are added by adding their mean-square sound (i.e., rms2) pressures:

1022

223

22

21

2

10 iLrefi

NTotal

pp

ppppp

2

2

1010 log10log20ref

rms

ref

rmsP p

pppL


Examples

1. Two sources each have a sound pressure level of 95 dB at a distance of 5 m. What is the combined level?

dB0.98

1032.6log10log10

1032.6

1016.310

dB95

9102

2

10

292

21

29109522

1

21

Tot

ref

TotTot

refTot

refref

L

ppL

pp

pppp

LL


Examples (2)

2. A machine has a sound pressure level of 95 dB in the presence of background noise. A background noise measurement made with the machine off yields a level of 92 dB. What is the true level of the machine?

dB0.92

1058.1log10log10

1058.11058.116.3

1058.110

1016.310

9102

2

10

29292

29109222

29109522

Machine

ref

MachineMachine

refrefMachine

refrefBackground

refrefTot

L

ppL

ppp

ppp

ppp


3. A machine has a sound pressure level of 86 dB at a distance of 3 m. If 6 identical machines are arranged in a circle about the measurement point, what is the total level?

Examples (3)

For each machine: 28108622 1098.310 refrefi ppp

dB8.938.786

6log101098.3

log10

1098.36

102

28

10

282

Tot

ref

refTot

refTot

L

pp

L

pp

Truth behind noise

THANK YOU

HAVE A NICE DAY

Lanjutan … materi 3

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