C-4
Model 820 Technical Reference Manual
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, typically
used in sound and vibration measurements. Whenever the word
level
is used it implies this logarithmic form. The relationship is
relatively simple, but the mathematics can become complex. It is
widely used and was developed so that the very wide range of any
quantity could be represented more simply. It is not possible to
directly add or subtract physical quantities when expressed in deci-
bel form. The word level is always attached to a physical quantity
when it is expressed in decibels; for example L
p
represents the
sound pressure level. The table below shows the actual value of a
specific item, such as sound pressure, for which the level is to be
determined. First the value is put into exponential form in powers
of ten; the exponent is the Bel. The exponent is then multiplied by
ten to yield the decibel. This procedure converts multiplication
into addition; every time 10 is
added
to the level, the value is
mul-
tiplied
by 10. When the value is not a even multiple of ten the
exponent is more complicated as shown in the table. Every time the
level increases by 3 dB, the value is multiplied by 2 (doubled).
These two rules are worth remembering.
The definition of decibel is intended for power-like quantities (W).
Sometimes power is represented by the square of a measured quan-
tity and this results in a different form of the equation (See Sound
Pressure Level).
The value of the item in the table is not the value of the quantity
itself but the ratio of that quantity to a reference quantity. So for
Linear form
Level form
Ration of Value to Reference
Exponential Form of
Ratio
10•Exponent
1
10
0
0
10
10
1
10
100
10
2
20
200
10
2.3
23
1000
10
3
30
10000
10
4
40
100000
10
5
50
1000000
10
6
60