730/730-IS Dosimeter Manual
B-4
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,
will produce 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%.
Eight Hour Time-Weighted Average Sound Level - L,TWA(8)
It is the constant sound level that would expose a person to the same Noise Dose as the actual (unsteady) sound
levels. According to Standard: ANSI S12.19, this definition applies only for a Criterion Duration of 8 hours. D = Dose,
and q is the Exchange Rate factor.
Energy Equivalent Sound Level (L
eq
)
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. In the equation below, p is the sound pressure and the Measurement Duration
(specific time period) T= T2–T1. See also
"Sound Pressure Level (SPL, Lp)."
Exchange Rate (Q)
ANSI S1.25 defines this as “the change in sound level corresponding to a doubling or halving of the duration of a
sound level while a constant percentage of criterion exposure is maintained.” The rate and factors are given in the
table below.
Exchange Rate,
Q
Exchange Rate
Factor, q
Exposure Factor,
k
3.01
10
1
4
13.29
.75
5
16.61
.60
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
If exchange rate = 6, q = 20
