Spartan 821/721 Manual
Glossary
C-1
Appendix C
Glossary
This appendix contains technical definitions of key acoustical and vibration terms commonly used with
Larson Davis instruments. The reader is referred to American National Standards Institute document S1.1-
1994 (R2004) for additional definitions. Specific use of the terms defined are in the main body of this manual.
Allowed Exposure Time (T
i
)
The allowed time of exposure to sound that a constant A-weighted sound level in a
chosen Criterion Level, Criterion Duration, and Exchange Rate.
The equation for T
i
is
where L
c
is the Criterion Level, Tc is the Criterion Duration, Q is the Exchange Rate, q is
the Exchange Rate Factor and L
AVG
is the Average Sound Level.
Example: If L
c
= 90, T
c
= 8, Q = 3 and L
AVG
= 95 then
This means that if a person is in this area for 2 hours and 31 minutes he will have
accumulated a Noise Dose of 100%. Standard: ANSI S12.19.
See
Exchange Rate (Q), Exchange Rate Factor (q), Exposure Factor (k)
Average Sound Level
(
L
avg
)
The level of a constant sound, expressed in dB, which in a given time period (T = T
2
– T
1
)
would expose a person to the same noise dose as the actual (unsteady) sound over the
same period. ANSI S1.25-1991 defines L
AVG
or average sound level.
Ti
Tc
Lavg Lc
–
Q
2
------------------------------------------
Tc
Lavg Lc
–
q
10
-------------------------------------------
=
=
Ti
8
2 95 90
–
3
---------------------------------
8
10 95 90
–
10
---------------------------------------
2.52 = 2 hours and 31 minutes
=
=
=
L
avg
q log
10
1
T
--- 10
L
AS
q
---------
t
d
T
1
T
2
dB
=
L
AS
= A-weighting frequency and slow-
exponential weighting time sound level in dB (in
the formula above, if the sound level is less than
the user-specified threshold level, then L
AS
= -∞)
T = measurement period or Run Time (T = T
2
– T
1
)
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
C
