Larson Davis SoundAdvisor 831C Reference Manual page 205

Day-Night Average
Sound Level (DNL,
L )
dn
Decibel (dB)
SoundAdvisor Model 831C
A rating of community noise exposure to all sources of sound that differentiates
between daytime and nighttime noise exposure. The equation for it is
L = 10Log
dn
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.
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 decibel 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 example, L
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
• Doubling of the Perceived Sound Level = (approx) 10 dB
0700

1
 
  10 
L + 10
----- -

i
10 +
24
10

0000
represents the sound pressure level.
p
2200 2400

   10 
L 10 L + 10
i i
10 + 10
0700 2200



C-3