Larson Davis System 824, Reference Manual

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A-2 824 Reference Manual 9/12/00
If no frequency weighting is used, the quantity is simply
called equivalent continuous sound pressure level.
The Larson•Davis System 824 calculates many time-inte-
grated levels or time-weighted averages (TWA) based on
different parameters and time intervals. They are all
designed and programmed to perform the equation specified
in IEC 804 (above) with allowances for the following:
A, C and Flat frequency weighting characteristics
Various interval times, both fixed interval TWAs and vari-
able interval event TWAs
Exchange-rates, or “doubling rates” can be entered that
effect certain TWA measurements
Certain TWA measurements include a programmable thresh-
old with only levels above this threshold contributing to the
measurement
The titles for the measurements are designed to indicate the
current exchange rate in force: “L
eq
” is used to designate
TWAs based upon a 3dB exchange rate, “L
DOD
” (for United
States Department of Defence) is used to designate TWAs
based upon a 4dB exchange rate, “L
OSHA
” (for the United
States Occupational Safety and Health Association) is used
to designate TWAs based upon a 5dB exchange rate, “L
Avg
”
(for average) is used to designate TWAs based upon a 6 dB
exchange rate.
No attempt is made to meet the IEC-804 requirement to title
the TWA by frequency weighting and time interval within
the analyzer’s display or report system. It is represented
though, by placing the weighting designation following the
“dB” units indication associated with the numerical output.
The actual measurement time is also provided for each TWA
value to fulfil the time interval description requirements.
L
Ceq,T
10lg
1
T
--- p
C
2
t
1
t
2
∫
t
( )
dt
 
 
 
 
p
0
2 ÷





dB





=