BA7667-11, 1

Measuring Sound

Contents: The Microphone The Sound Level Meter Leq

Statistics and Noise Dose

BA7667-11, 2

Measuring Microphones

860550/1

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The Polarised Condenser MicrophonePrinciple of Operation

ZV

in

CA

d

VQ

C

Q

Ad V

Q

Ad

Q = CV

860552/1

BA7667-11, 4

How much does the diaphragm move ?

VV

dd

=

For typical measurement microphone:

– diameter 12.5 mm – thickness of diaphragm 5 µm – distance between diaphragm and backplate 20 µm – polarisation voltage 200 V – sensitivity 50 mV/Pa

For 94 dB = 1 Pa the diaphragm moves

V dV

50 mV 20 µm200 V

==d

= 5 nm

Diameter ofdiaphragm

Pressure(level re 20µPa

Diaphragm’smovement

12.5mm 1Pa (94dB) 5nm (5 x 10–9 m)

12.5mm 0.02Pa (60dB) 1 Å (10–10 m)

12500km(thickness of

diaphragm 5km)

0.02Pa (60dB) 0.1m (10–1 m)

0.0002Pa (20dB) 0.001m (10–3 m)

BA7667-11, 5

Directional Characteristics

90° 90°

90° 90°

0° 180° 0°

2 kHz

8 kHz

12 kHz

°°

180°

860558/1

BA7667-11, 6

Types of Microphones

Random incidence

Pressure

Free field

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Free Field Correction

p0

pm

10

20 logdB

p0

pm

f

27k

1/8 1/4 1/2 1 2 4

2k 5k 10k 20k 50k 100k

D

(D=1/2”)Hz

D

BA7667-11, 8

Free Field Correction

LdB

10

50°

30°

60°

R

90°

120°

Frequency

0°

90°

BA7667-11, 9

Measuring in Accordance with Standards:

IEC ANSI

70° – 80°

BA7667-11, 10

Use of Free Field Microphones

BA7667-11, 11

Use of Random Incidence Microphones

8605566/1

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Measuring Sound

Contents: The Microphone The Sound Level Meter Leq

Statistics and Noise Dose

BA7667-11, 13

HoldHold

The Sound Level Meter

RMSPeak

FastSlow

Impulse

87.2

Time

Time

Lp

p

BA7667-11, 14

Sound Level Parameters

pPa

Peak Peak – Peak

RMS =

Average =

pPa

1 2

0Tx t dt

T

( )

1

0Tx d t

T

Time

Time

Average

Crest factor =PeakRMS

RMS

BA7667-11, 15

Basic Sound Level Parameters

pPa

Peak RMS

pPa

Crest factor =PeakRMS

Time

Time

BA7667-11, 16

Time Weighting

p

Lp

Time

Time

Slow (1 s) Fast (125 ms)Impulse (35 ms)

Impulse (1.5 ) Slow (1 s) Fast (125 ms)

Lp

BA7667-11, 17

Peak Hold

p

Lp

Time

Time

Lp

BA7667-11, 18

Time Weighting

p

Lp

Time

Time

ImpulseFastSlow

Lp

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Time Weighting

Fast

Lp

Time

Slow

Time

Lp

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The Digital Display

Lp

Time1 2 3 4 5 6 7

79.082.4 79.3 76.8 76.0 80.1 80.1

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Measuring Sound

Contents: The Microphone The Sound Level Meter Leq

Statistics and Noise Dose

BA7667-11, 22

Equivalent Level, Leq

Leq

T

Lp

Time

L

T

pt

pdteq

T

10

110

00

2

log

BA7667-11, 23

Lp

Time

Leq

Measuring Leq

BA7667-11, 24

Leq for Transient Noise

Lp

Time

Leq

Transient

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Sound Exposure Level and its Origin

Lp

Time

SEL 1 SEL 2

BA7667-11, 26

Time

Sound Exposure Level, SEL

1 s

Leq

LdB(A)

SEL

SEL L eq

10 log

t

1s

t

BA7667-11, 27

Measuring Sound

Contents: The Microphone The Sound Level Meter Leq

Statistics and Noise Dose

BA7667-11, 28

Level Sampling

Time

dB(A)

40

50

60

70

40

50

60

70

Time

dB(A)

Sample

BA7667-11, 29

Noise Level Distribution

Distributionof levels

40

50

60

70

Time

dB(A)

40

50

60

70

dB(A)

Time

Time40

50

60

70

Noise Level Distribution

dB(A)

Sample

BA7667-11, 30

Cumulative Distribution

Cumulative Distribution

L10

L50

L90

40

50

60

70

40

50

60

70

10%50%90%

Time

dB(A)

dB(A)

Distributionof levels

40

50

60

70

Time

dB(A) Noise Level Distribution

BA7667-11, 31

Percentile Levels

Cumulative Distribution

L10

L50

L90

40

50

60

70

40

50

60

70

10%50%90%

Time

dB(A)

dB(A)

Noise Level Distribution

Time

L10

L50

L90

dB(A)

BA7667-11, 32

Statistical Analysis of Noise Levels

Cumulative Distribution

L10

L50

L90

40

50

60

70

40

50

60

70

10%50%90%

Time

dB(A)

dB(A)

Distributionof levels

40

50

60

70

Time

dB(A) Noise Level Distribution

Time

dB(A)

40

50

60

70

Sample

BA7667-11, 33

Percentile Levels, LN

Time

L10

L90

dB(A)

100% Time

a b

10% time

a + b = 10% time

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Noise Dose Measurement in Practice

BA7667-11, 35

Definition of Noise Dose

8hours

in

= 100 % Noise DoseThe actual dB level depends on National

Legislations. In some countries the level is 80 dB

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Noise Dose Examples

8 hours4 hours

85

100%

50%

100%

200%

dB(A)

8 hours4 hours

85

dB(A) 8 hours4 hours

85

dB(A)

8 hours4 hours

85

dB(A)

BA7667-11, 37

Daily Personal Noise Exposure, LEP,d

L L 10logT

TEP,d Aeq,Te 10e

0

Time

dB(A)

70

80

90

100

T0 = 8 hoursWork periodTe

LAeq,Te

Example:

LAeq,Te = 89.2 dB and Te = 4 hours

L89.210log4

889.2386.2dBEP,d 10

Morning Lunch Go Home

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Standards ISO 9612

Guidelines for the measurement and assessment of exposure to noise in a working environment

ISO 1999Determination of occupational noise exposure and estimation of noise-induced hearing impairment

IEEC Directive EEC/86/188The protection of workers from the risk related to the exposure to noise at work

OSHAOccupational Safety and Health Act

International

International

The European

Union

USA andothers

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Exchange Rate

Time

LdB(A)

Leq = (85 + Exchange Rate) dB

Leq = 85 dB

8 hours4 hours

Exchange Rate, q

8hours

4h.

BA7667-11, 40

ISO vs. OSHA

Leq dB(A)

100

95

90

85

1 hour 2 hours 4 hours 8 hours Time

3 dB 5 dBISO (q = 3)

OSHA (q = 5)

Logarithmic time axis

BA7667-11, 41

Noise Dose and Peak

Time70

80

90

100

LEP, d

LC, pkNo peaks above this level allowed

110

120

130

Lp

dB

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Conclusion

The sensitivity of a microphone depends on the direction of the noise - that is why there are free-field and random-incidence microphones

Sound level meters are integrated measurement equipment with standardized time weightings and measurement parameters

The Leq is the energetic average of the noise over a period of time and is one of the most important noise measurement parameters

The Noise Dose and Daily Personal Noise Exposure (LEP,d) are ways of showing how much noise exposure a person has received in relation to legal limits

BA7667-11, 43

Literature for Further Reading

Acoustic Noise Measurements

Brüel & Kjær (BT 0010-12)

Noise Control - Principles and Practice

Brüel & Kjær (188-81)