Post on 27-Mar-2015
transcript
BA7667-11, 1
Measuring Sound
Contents: The Microphone The Sound Level Meter Leq
Statistics and Noise Dose
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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
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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)
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Directional Characteristics
90° 90°
90° 90°
0° 180° 0°
2 kHz
8 kHz
12 kHz
°°
180°
860558/1
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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
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Free Field Correction
LdB
10
50°
30°
60°
R
90°
120°
Frequency
0°
90°
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Measuring in Accordance with Standards:
IEC ANSI
70° – 80°
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Use of Free Field Microphones
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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
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HoldHold
The Sound Level Meter
RMSPeak
FastSlow
Impulse
87.2
Time
Time
Lp
p
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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
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Basic Sound Level Parameters
pPa
Peak RMS
pPa
Crest factor =PeakRMS
Time
Time
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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
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Peak Hold
p
Lp
Time
Time
Lp
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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
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Equivalent Level, Leq
Leq
T
Lp
Time
L
T
pt
pdteq
T
10
110
00
2
log
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Lp
Time
Leq
Measuring Leq
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Leq for Transient Noise
Lp
Time
Leq
Transient
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Sound Exposure Level and its Origin
Lp
Time
SEL 1 SEL 2
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Time
Sound Exposure Level, SEL
1 s
Leq
LdB(A)
SEL
SEL L eq
10 log
t
1s
t
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Measuring Sound
Contents: The Microphone The Sound Level Meter Leq
Statistics and Noise Dose
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Level Sampling
Time
dB(A)
40
50
60
70
40
50
60
70
Time
dB(A)
Sample
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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
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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
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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)
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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
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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
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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)
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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.
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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
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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
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Literature for Further Reading
Acoustic Noise Measurements
Brüel & Kjær (BT 0010-12)
Noise Control - Principles and Practice
Brüel & Kjær (188-81)