CESVA SC310, Руководство пользователя

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74
6.2 Definition of functions
6.2.1 Sound pressure level with fast and slow time weightings
L
F
(Fast) RMS value with 125 ms fast exponential averaging, in decibels.
L
S
(Slow) RMS with 1 s slow exponential averaging, in decibels.
p(t): instantaneous sound pressure
To obtain a stable reading, the sound level meters feature two kinds of
responses: ‘F’ and ‘S’. The ‘F’ response has an exponential averaging circuit
time constant of
τ
= 125 ms, and the ‘S’ response of
τ
= 1 s.
‘F’ response is recommended for measuring sound levels that fluctuate
relatively little, such as vehicle noise, while ‘S’ response is recommended for
noises that very more.
⎟⎟
⎟
⎟
⎟
⎟
⎟
⎠
⎞
⎜⎜
⎜
⎜
⎜
⎜
⎜
⎝
⎛
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
⋅
⋅ =
∫
∞
o
2
1
T
-
)/ - (t - 2
F S,
p
d e ) ( p
1
log 0 2 L
ζ ζ
τ
τ ζ
p(t): instantaneous sound pressure
p
0
: reference sound pressure (20
µ
Pa)
6.2.2 Sound pressure level with ‘I’ time weighting
L
I
(Impulse) Maximum short-term RMS value with exponential averaging of 35 ms, in
decibels.
p(t): instantaneous sound pressure
The ‘I’ characteristic is designed to detect impulse noise, like shots or blows.
The ‘I’ function has a very fast exponential averaging circuit time constant:
τ
=
35 ms, and a peak detector that retains the measured value long enough for it
to be displayed.
( )
2
EXPONENTIAL
AVERAGING
L
I
...
τ
= 35 ms
( )
2
0
p
log 10
⋅
p
0
= 20
µ
Pa
L
I
p(t)
PEAK
DETECTOR
τ
= 1,5 s
( )
2
EXPONENTIAL
AVERAGING
L
F
...
τ
= 125ms
L
S
...
τ
= 1s
( )
2
0
p
log 10
⋅
p
0
= 20
µ
Pa
L
F
L
S
p(t)