Larson Davis System 824, Technical Reference Manual

Page: 540 / 673

B-2 824 Reference Manual 9/29/09
title and the formula of the quantity, for example equivalent
continuous C-weighted sound pressure level:
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 6dB
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
L
Ceq,T
10lg
1
T
---
p
C
2
t
1
t
2
∫
t
( )
dt
⎝ ⎠
⎜ ⎟
⎜ ⎟
⎛ ⎞
p
0
2
÷
⎭
⎪
⎬
⎪
⎫
dB
⎩
⎪
⎨
⎪
⎧
=