Larson Davis 831 - Page 516

Model 831 Manual Basic Integrated Level Calculations C-3
FIGURE C-1 TWA
Note that the 831 displays the time
weighted average as TWA[Hr]
where Hr is the time in hours over
which the average is performed.
The actual equations used within the 831 are based on those
for IEC 61672-1 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
)
The currently selected Frequency Weighting (A) and
Detector Response (Slow) for SPL
The currently selected Exchange Rate
Time-weighted average (TWA) based on 8 hours
The Projected Time Weighted Average for 8 hours
Elapsed measurement time shown as h:mm:ss.s
Threshold Level
The numerical value for the TWA and ProjTWA
L
TWA
L
cal
k 10
L
s
k
--------
s 1=
n









log nlog–+=