SoundAdvisor Model 831C
C-3
Daily Personal Noise Exposure (L
EP,d
)
It is the level of a constant sound over the Criterion Duration that contains the same sound energy
as the actual, unsteady sound over a specific period. The period is generally shorter, so the sound
energy is spread out over the Criterion Duration period.
Example: If the Criterion Duration = 8 hours and the specific period is 4 hours and the average
level during the 4 hours is 86 dB, then the L
EP,d
= 83 dB.
Day-Night Average Sound Level (DNL, L
dn
)
A rating of community noise exposure to all sources of sound that differentiates between daytime
and nighttime noise exposure. The equation for it is
The continuous equivalent sound level (See definition) is generally calculated on an hourly basis
and is shown in the equation as L.
The values for the hourly periods from midnight to 7 a.m. have 10 added to them to represent less
tolerance for noise during sleeping hours. The same occurs from 10 p.m. to midnight. They are
energy summed and converted to an average noise exposure rating.
Decibel (dB)
A logarithmic form of any measured physical quantity and commonly used in the measurement of
sound and vibration. Whenever the word level is used, this logarithmic form is implied. The deci
-
bel provides us with the possibility of representing a large span of signal levels in a simple manner
as opposed to using the basic unit Pascal for acoustic measurements.
It is not possible to directly add or subtract physical quantities when expressed in decibel form
since the addition of logarithmic values correspond to multiplication of the original quantity.
The word level is normally attached to a physical quantity when expressed in decibels; for exam
-
ple, L
p
represents the sound pressure level.
The difference between the sound pressure for silence versus loud sounds is a factor of
1,000,000:1 or more, and it is very unpractical to use these large numbers. Therefore, a measure
that would relate to “the number of zeros” would help, for example, 100,000 would be equal to 50
and 1000 would be equal to 30 and so on. This is the basic principal of the dB measure.
All dB values are unit free and therefore, the dB value is not the value of the quantity itself, but the
ratio of that quantity to an actual reference quantity used. Thus, for every level in decibels there
must be a well defined reference quantity. Sound versus vibration uses different references, but
the dB principal is the same. When the quantity equals the reference quantity the level is zero. To
keep dB values above zero, the reference is generally set to be the lowest value of the quantity
that we can imagine or normally wish to use. Before explaining the calculation of dB values, it is
useful to remember the following rules of thumb when dB values are used for sound levels:
• Doubling of the Sound Pressure = 6 dB
• Doubling of the Sound Power = 3 dB
L
dn
10
Log
10
1
24
------
10
L
i
10
+
10
0000
0700
10
L
i
10
0700
2200
10
L
i
10
+
10
2200
2400
+
+
=
