Spartan 821/721 Manual Glossary C-14
Projected Sound Exposure (E
8
or E
40
)
Projected Sound Exposure shows what the actual sound exposure will be for a specific
time period if the current Equivalent-Continuous Sound Level (L
eq
) remains at its
current level. The dosimeters calculate an 8-hour and a 40- hour projected sound
exposure.
AND
Projected Time-Weighted Average, P.TWA(x)
The projected Time-Weighted Average is the Equivalent Time-Weighted Average
assuming that the current rate of noise exposure continues for the duration of a work
shift (Shift Time). This metric displays as “P. TWA” on the dosimeter screen.
RMS Value (Root-Mean-Square Value)
One method of describing the energy in a sampled waveform. A waveform is often
described by the wave’s peak value. However, the peak value occurs only once positively
and once negatively within a cycle. Since the RMS value (root-mean-square) occurs
twice positively and twice negatively, as identified in Figure C-1, the sum of these,
provides a better sample of the total energy of the measured sound. For this reason,
RMS is often applied in the process of calculating noise exposure. Mathematically, the
RMS value is defined as the square root of the mean value of the squared values of the
quantity taken over an interval (time).
E
8
8
T
---
p
2
t td
T
1
T
2
=
E
40
40
T
------
p
2
t td
T
1
T
2
=
p(t) = instantaneous, frequency weighted (A, C, or Z), sound pressure in pascals
T
2
– T
1
= measurement period or Run Time
P.TWA x L
avg
q+ log
10
T
S
T
C
-------
dB
=
L
avg
= average sound level in dB, only
recording values higher than the user
specified threshold level
T
S
= work shift duration or Shift 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
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