Instruction Manual
Sound Level Meter Types 2245 –
Instruction Manual
Page 27 of 110
2.14 Measured Quantities
This section gives a precise mathematical definition of the measured quantities and
defines the abbreviations used on the display.
2.14.1 Instantaneous Broadband Measurements
These measurements are done continuously, independent of measurement Start,
Pause and Stop. They cannot be saved and are only displayed.
Overload
For Instantaneous Measurements, the
Overload
indication is displayed as long as the
overload condition exists, or for 1 s, whichever is the greater.
Overload is indicated as a flashing (colour: red) on the screen and by a flashing red
“traffic light” indicator. Overload is common to all results of Instantaneous
Measurements.
Underrange
The
Underrange
indication is displayed as long as the underrange condition exists,
or for 1 s, whichever is the greater.
The Underrange condition is present if any measurement of time-weighted sound level,
time average sound level, or sound exposure level is less than the specified lower limit
of a linear operating range.
Time-weighted Sound Level, F and S Time-weighted
The time-weighted sound level,
L
xy
(t)
, is defined as twenty times the logarithm to the
base ten of the ratio of a given root-mean-square sound pressure to the reference
sound pressure, root-mean-square sound pressure being obtained with a frequency
weighting,
x
, and standard time weighting,
y
, 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 or S for Slow-weighted
The time-weighted sound level is a continuous function of time and is expressed in
decibels (dB).
L
xy
(t)
is not displayed, but is the base for
L
xy
(Tn)
,
L
xy
(SPL)(T
n
)
,
L
xy
max(T)
and
L
xy
min(T)
.
In symbols, frequency-weighted and time-weighted sound level,
L
xy
(t)
, at any instant of
time,
t
, is represented by:
where:
τ is the exponential time constant in seconds for time-weighting F or S
ξ is a dummy variable of time integration from some time in the past, as indicated
by –∞ for the lower limit of the integral, to the time of observation t
p
x
(ξ) is the x frequency-weighted instantaneous sound pressure
p
0
is the reference sound pressure, equal to 20 μPa
The exponential time constants are stated in Table 2.2.