Introduction to Health and Safety at Work

 
⫽
 
20 log
10
(p/p
0
) dB
It is important to note that since a logarithmic scale to 
the base 10 is used, each increase of 3 dB is a doubling 
in the sound intensity. Thus, if a sound reading changes 
from 75 dB to 81 dB, the sound intensity or loudness has 
increased by four times.
Finally, since the human ear tends to distort its 
sensitivity to the sound it receives by being less sensitive 
to lower frequencies, the scale used by sound meters is 
weighted so that readings mimic the ear. This scale is 
known as the A scale and the readings known as dB(A). 
There are also three other scales known as B, C and D. 
Finally, since the human ear tends to distort its sensitiv-
ity to the sound it receives by being less sensitive to 
lower frequencies, the scale used by sound meters is 
weighted so that readings mimic the ear. This scale is 
known as the A scale and the readings known as dB(A). 
There are also three other scales known as B, C and D.
Originally the A scale was used for sound pressure 
levels up to 55 dB, the B scale for levels between 55 dB 
and 85 
dB and the C scale for values above 85 
dB. 
However today, the A scale is used for nearly all levels 
except for very high sound pressure levels when the C 
scale is used. The B scale is rarely used and the D scale 
is mainly used to monitor jet aircraft engine noise.
Table 15.5 gives some typical dB readings for 
common activities.
Noise is measured using a sound level meter which 
reads sound pressure levels in dB(A) and the 
peak sound 
pressure
in pascals (Pa), which is the highest noise level 
reached by the sound. There are two basic types of sound 
meter – integrated and direct reading meters. Meters 
which integrate the reading provide an average over 
a particular time period which is an essential technique 
when there are large variations in sound levels. This value 
is known as the