Larson Davis System 824, Техническое справочное руководство

Страница: 542 / 673

B-4 824 Reference Manual 9/29/09
The actual equations used within the analyzers are based on
that for IEC-804 and are implemented according to this
equation:
where:
L
(s)
is the current SPL at sample s (for measurements that
include a threshold, L
(s)
is set to -× if L
(s)
is less than the
Threshold Level L
t
)
k is the exchange rate constant which is equal to:
10.00 for an exchange rate of 3dB (L
eq
)
13.29 for an exchange rate of 4dB (L
DOD
)
16.61 for an exchange rate of 5dB (L
OSHA
)
20.00 for an exchange rate of 6dB (L
Avg
)
n is the total number of samples taken in the measurement;
the sample rate is 32 samples per second, and
L
cal
is the calibration offset that corrects for various sensitiv-
ities of microphones
Accuracy is maintained by having large fixed point variables
for the number of samples and the summation accumulator.
The limit of
s is 4,294,967,295 samples and is stored in a 32
bit integer variable within the System 824. This limit is
greater than 1553 days or greater than 4 years and 4 months.
The value that can be stored in the summation accumulator,
a 96 bit integer, can be the overload level (maximum allow-
able signal amplitude into the System 824) for the maximum
number of samples. Using large fixed point variables pre-
vents the inaccuracies obtained in systems based upon float-
ing point variables.
L
TWA
L
cal
k 10
L
s ( )
k
--------
s 1 =
n
∑
⎠
⎟
⎟
⎞
⎝
⎜
⎜
⎛
log
•
n
( )
log – + =