CESVA SC-30, Руководство пользователя, Страница: 51 / 76

SC-30
User’s Manual
49
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
F
...
τ
= 125ms
L
S
...
τ
= 1s
( )
2
0
p
log 10
⋅
p
0
= 20
µ
Pa
L
F
L
S
p(t)
( )
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