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ANNEX 5 : CURVE CHOICE FOR SPECTRAL ANALYSIS
THOUGHTS ABOUT WEIGHTING CURVE SELECTION FROM INTERMEDIATE OR LINEAR CURVE, USED FOR
SPECTRAL ANALYSIS BY THE MEASUREMENT CHAIN.
SNA 50-2 measurement chain is in 1.1 diagram.
98-114 decree advise to measure under A weighting.
Fig 1.2 shows A weighting template. It is equivalent to human ear for low acoustic pressure. At this level human ear is more sensitive
between 400 HZ to 4KHz, comparatively to higher or lower frequencies.
At level around 100dB, this is not the same : for instance at low pressure the difference is 35dB between medium and 40Hz, but at
100dB it drops to 14dB.
M filter shows the human ear sensitivity around 100dB, and takes more in account the low frequencies for analysis and regulation
purpose.
On the other side, if A curve is used for analysis, the low frequencies wont be much processed, and the equipment will not perform in
case of noise harm to external environment. Indeed this is most of the time, the low frequency that makes more problems outside,
because the acoustic insulation is less effective at low frequencies. So M filter could more suitable.
According to the desired result, you can choose A, M filter or linear. In every situation the law will be fulfilled because wit M filter or
linear , a wider frequency range is taken in account than with A filter.
Sensor
Sound level measurement channel diagram
fig 1.1 -
RANGE
DISPLAY SCALE
(dB)
10
25
50
LIN
V
V
V
V
0
0
0
1
LIN
-2
-5
-10
0.75
-4
-10
-20
0.5
M
-6
-15
-30
0.25
-8
-20
-40
0
-50
20
AMIX
A weighted filter
A
Digital Pot
M
Filters shapes
-40
-20
cal.
dBm
0
+20
uncal.
dBV
50
100
200
A
50
100
200
fig 1.2 - Filter curves -
line
A
Regulator
filter
M
500
1k
2k
500
1k
2k
47
140
dB
120
100
80
60
40
0
20
0
10
20
40
100
200
400
1
2
Hz
frequency f
Human ear bandwidth
fig 1.3 -
5k
10k
20k
LIN
M
5k
10k
20k
2
10
W/m²
0
10
-2
10
-4
10
-6
10
-8
10
-10
10
-12
10
4
10 20
KHz
-
A
40k
SNA 50-2
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