Measuring Sound Contents l The Microphone l The

Measuring Sound Contents: l The Microphone l The Sound Level Meter l Leq l Statistics and Noise Dose BA 7667 -11, 1

Measuring Microphones BA 7667 -11, 2 860550/1

The Polarised Condenser Microphone Principle of Operation Z in V Q = CV BA 7667 -11, 3 860552/1

How much does the diaphragm move ? V d = V d 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 m. V/Pa BA 7667 -11, 4 For 94 d. B = 1 Pa the diaphragm moves d = V d 50 m. V 20 µm = V 200 V = 5 nm Diameter of diaphragm Pressure (level re 20µPa Diaphragm’s movement 12. 5 mm 12500 km (thickness of diaphragm 5 km) 1 Pa (94 d. B) 0. 02 Pa (60 d. B) 0. 0002 Pa (20 d. B) 5 nm (5 x 10– 9 m) 1Å (10– 10 m) 0. 1 m (10– 1 m) 0. 001 m (10– 3 m)

Directional Characteristics 90° 2 k. Hz 8 k. Hz 0° 180° 12 k. Hz 90° ° BA 7667 -11, 5 90° ° 860558/1

Types of Microphones Random incidence Free field Pressure BA 7667 -11, 6

Free Field Correction p 0 20 log d. B p 0 pm 10 pm D D 1/8 1/4 1/2 2 k BA 7667 -11, 7 1 2 4 f 5 k 10 k 20 k 50 k 100 k Hz (D=1/2”) 27 k

Free Field Correction L d. B 10 0° 5 0° 30° Frequency 60° 90° R 90° 120° BA 7667 -11, 8

Measuring in Accordance with Standards: IEC ANSI 70° – 80° BA 7667 -11, 9

Use of Free Field Microphones BA 7667 -11, 10

Use of Random Incidence Microphones BA 7667 -11, 11 8605566/1

Measuring Sound Contents: l The Microphone l The Sound Level Meter l Leq l Statistics and Noise Dose BA 7667 -11, 12

The Sound Level Meter p Time RMS Peak Hold Fast Slow Impulse 87. 2 BA 7667 -11, 13 Lp Time

Sound Level Parameters p Pa RMS = Time Peak – Peak Average = RMS p Pa Crest factor = Time BA 7667 -11, 14 Peak RMS

Basic Sound Level Parameters p Pa Time Peak RMS Crest factor = p Pa Time BA 7667 -11, 15 Peak RMS

Time Weighting p Time Lp Lp Impulse (1. 5 ) Slow (1 s) Fast (125 ms) Impulse (35 ms) BA 7667 -11, 16 Time

Peak Hold p Time Lp Lp Time BA 7667 -11, 17

Time Weighting p Time Lp Lp Impulse Fast Slow Time BA 7667 -11, 18

Time Weighting Lp Fast Time Lp Slow Time BA 7667 -11, 19

The Digital Display 82. 4 79. 3 76. 8 76. 0 80. 1 79. 0 Lp 1 BA 7667 -11, 20 2 3 4 5 6 7 Time

Measuring Sound Contents: l The Microphone l The Sound Level Meter l Leq l Statistics and Noise Dose BA 7667 -11, 21

Equivalent Level, Leq Lp Leq Time T BA 7667 -11, 22

Measuring Leq Lp Leq Time BA 7667 -11, 23

Leq for Transient Noise Lp Transient Leq Time BA 7667 -11, 24

Sound Exposure Level and its Origin Lp SEL 1 SEL 2 Time BA 7667 -11, 25

Sound Exposure Level, SEL L d. B(A) SEL Leq t BA 7667 -11, 26 1 s Time

Measuring Sound Contents: l The Microphone l The Sound Level Meter l Leq l Statistics and Noise Dose BA 7667 -11, 27

Level Sampling d. B(A) 70 60 50 40 Time Sample d. B(A) 70 60 50 40 BA 7667 -11, 28 Time

Noise Level Distribution d. B(A) 70 60 50 40 Time Sample Noise Level Distribution d. B(A) 70 70 60 BA 7667 -11, 29 60 Distribution of levels 50 40 d. B(A) Time 50 Time 40

Cumulative Distribution d. B(A) L 10 L 50 70 L 90 50 60 40 90% 50% 10% Cumulative Distribution Noise Level Distribution d. B(A) 70 70 60 BA 7667 -11, 30 60 Distribution of levels 50 40 d. B(A) Time 50 Time 40

Percentile Levels d. B(A) L 10 L 50 70 L 90 50 Time 60 40 90% 50% 10% Cumulative Distribution Noise Level Distribution d. B(A) 70 60 50 Time BA 7667 -11, 31 40

Statistical Analysis of Noise Levels d. B(A) 70 60 L 10 L 50 50 L 90 50 70 40 60 40 Time 90% Noise Level Distribution d. B(A) 70 60 60 Distribution of levels 50 BA 7667 -11, 32 10% Cumulative Distribution Sample 40 50% Time 50 Time 40

Percentile Levels, LN 10% time d. B(A) L 10 L 90 a b a + b = 10% time 100% Time BA 7667 -11, 33 Time

Noise Dose Measurement in Practice BA 7667 -11, 34

Definition of Noise Dose in 8 hours = 100 % Noise Dose The actual d. B level depends on National Legislations. In some countries the level is 80 d. B BA 7667 -11, 35

Noise Dose Examples d. B(A) 85 85 100% 4 hours d. B(A) 85 85 4 hours 8 hours 200% 50% 4 hours BA 7667 -11, 36 8 hours 4 hours 8 hours

Daily Personal Noise Exposure, LEP, d d. B(A) 100 LAeq, Te 90 80 70 Time Work period Te Morning T 0 = 8 hours Lunch Go Home Example: LAeq, Te = 89. 2 d. B and Te = 4 hours BA 7667 -11, 37

Standards l ISO 9612 Guidelines for the measurement and assessment of exposure to noise in a working environment l ISO 1999 l IEEC Directive EEC/86/188 l OSHA na tio na l Int Determination of occupational noise er exposure and estimation of noise-induced nati on hearing impairment al Th e. E u The protection of workers from the risk Un rop ion ea related to the exposure to noise at work n Occupational Safety and Health Act BA 7667 -11, 38 Int er US ot A an he d rs

Exchange Rate L Exchange Rate, q d. B(A) Leq = (85 + Exchange Rate) d. B Leq = 85 d. B Time 8 hours 4 h. BA 7667 -11, 39 8 hours

ISO vs. OSHA Leq d. B(A) 100 OSHA (q = 5) 95 90 ISO (q = 3) 5 d. B 3 d. B 85 1 hour 2 hours 4 hours Logarithmic time axis BA 7667 -11, 40 8 hours Time

Noise Dose and Peak Lp d. B LC, pk 130 120 No peaks above this level allowed 110 100 LEP, d 90 80 70 BA 7667 -11, 41 Time

Conclusion BA 7667 -11, 42 l The sensitivity of a microphone depends on the direction of the noise - that is why there are free-field and randomincidence microphones l Sound level meters are integrated measurement equipment with standardized time weightings and measurement parameters l The Leq is the energetic average of the noise over a period of time and is one of the most important noise measurement parameters l 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

Literature for Further Reading BA 7667 -11, 43 l Acoustic Noise Measurements Brüel & Kjær (BT 0010 -12) l Noise Control - Principles and Practice Brüel & Kjær (188 -81)