Spartan 821/721 Manual
Glossary
C-5
Equivalent Continuous Sound Level (L
eq
)
L
eq
is the level of a constant sound over a specific time period that has the same sound
energy as the actual (unsteady) sound over the same period. It is one way to describe
sound levels that vary over time as a single decibel value, which takes into account the
total sound energy for the period of interest. While not technically correct, it’s often
referred to as the “average” noise level for a measurement.
For sound level meters, an L
eq
value is recorded for two separate intervals. The first
interval records the value of L
eq
for the entire record’s run time. The second records the
value of L
eq
for each individual time history sample. Therefore, the time period of
interest has the same total sound energy as does a sample with varying sound (T = T
2
-
T
1
). The L
eq
is annotated as L
Aeq
for an A-weighted L
eq
, or as L
Ceq
for a C-weighted L
eq
.
Note
: The default exchange rate for L
eq
is 3 dB for ISO and British Standard
measurements. In some countries, other rules may apply. OSHA (U.S.A.) standards use 5
dB as the default exchange rate. These values and others are shown as default values in
the 721/821 Dosimeter Settings.
Equivalent Time-Weighted Average TWA(x)
The level of a constant sound expressed in dB, which, if measured for a time period
equal to the Criterion Duration, produces the currently measured noise dose.
The x in TWA(x) represents the Criterion Duration. For example, suppose a worker is
exposed to a noise environment with L
AVG
of 90 dB. Also, assume that the exchange rate
is 5, the criterion level is 90 dB, and the Criterion Duration is 8 hours.
After 1 hour, the worker’s noise dose is 12.5%, the TWA(8) is 75.0 dB, and the L
avg
is 90.0
dB. A TWA(8) of 75 dB indicates that if the worker is instead exposed to a noise
environment with a L
avg
of 75 dB, the noise dose after 8 hours is 12.5%.
L
eq
10 log
10
1
T
---
p
2
t
p
o
2
------------ t
d
T
1
T
2
dB
=
p(t) = instantaneous, frequency-weighted (A or C), sound pressure in pascals
p
o
= reference sound pressure, 20 µPa
T = measurement period or Run Time (T = T2 – T1)
TWA x
L
avg
q log
10
T
T
c
------ dB
+
=
L
avg
= average sound level in dB, only recording values
higher than the user specified threshold level
T = measurement period or Run Time
T
C
= Criterion Duration in hours (8 hours)
q = exchange rate constant
If exchange rate = 3, q = 10
If exchange rate = 4, q = 4 / log
10
(2) ≈ 13.29
If exchange rate = 5, q = 5 / log
10
(2) ≈ 16.61
