Instruction Manual
Sound Level Meter Types 2245 –
Instruction Manual
Page 28 of 110
Table 2.2 Exponential time constants and corresponding averaging times
Time Weighting
Time Constant
(seconds)
Averaging
Time
(seconds)
Fast
0.125
0.25
Slow
1
2
Time-weighted Sound Level, I Time-weighted
The I (Impulse) time-weighted sound level,
L
xI
(t)
, is defined as ten times the logarithm
to base ten of the ratio of a given mean-square sound pressure to the square of the
reference sound pressure,
p
0
, followed by a peak detector with a decay time constant
of 1500 ms. The mean-square sound pressure being obtained with a frequency
weighting,
x
, and time weighting with a 35 ms time constant, where:
x
is A for A-weighted, B for B-weighted, C for C-weighted or Z for Z-weighted
p
0
is the reference sound pressure, equal to 20 μPa
The I time-weighted sound level is a continuous function of time and is expressed in
decibels (dB).
L
xI
(t)
is not displayed but is the base for
L
xI
(T
n
)
,
L
xI
(SPL)(Tn)
,
L
xImax
(T)
,
L
xImin
(T)
and
L
xIm
(T)
.
Instantaneous Time-weighted Sound Level
The instantaneous time-weighted sound level,
L
xy
(T
n
)
, is defined as the time-weighted
sound level,
L
xy
(t),
sampled at
t
=
T
n
where:
x
is A for A-weighted, B for B-weighted, C for C-weighted or Z for Z-weighted
y
is F for Fast-weighted, S for Slow-weighted or I for Impulse-weighted
T
n
=
t
0
+n·Δ
t
t
0
is some starting time
n
is an incrementing integer
Δ
t
is the display update interval
The instantaneous time-weighted sound level is, in other words, updated every Δ
t
second and is expressed in decibels (dB).
The symbols used by the sound level meter for instantaneous time-weighted sound
levels are (for A, B, C and Z frequency weighting and F, S and I time weighting):
L
AF
,
L
AS
,
L
AI
,
L
BF
,
L
BS
,
L
BI
,
L
CF
,
L
CS
,
L
CI
,
L
ZF
,
L
ZS
,
L
ZI