Typical sound pressure levels of various
environmental sounds.TYPICAL SOUND PRESSURE LEVELS
To get a feel for decibels, look at the table below
which gives values for the sound pressure levels of
common sounds in our environment. Also shown are the
corresponding sound pressures and sound intensities.
From these you can see that the decibel scale gives
numbers in a much more manageable range.
Table of Sound Pressure Levels and Corresponding
Sound Pressure and Sound Intensity
Examples Sound Pressure
Level dB Sound Pressure
Pa Sound Intensity
watts/m²
30m from jet aircraft 140 200 100
Threshold of pain 130 10
120 20 1
Chainsaw 110 0.1
Disco 100 2 0.01
90 0.001
Kerbside of busy road 80 0.2 0.0001
70 0.00001
Conversational speech 60 0.02 0.000001
50 0.0000001
40 0.002 0.00000001
Quiet bedroom at night 30 0.000000001
Background in TV studio 20 0.0002 0.0000000001
10 0.00000000001
Threshold of hearing 0 0.00002 0.000000000001
From the above you can see that a sound which has a
sound pressure level 10 dB more than another has a
sound intensity 10 times as much which, as we saw in
the first lecture, means that the sound contains 10
times the energy.
A sound 20 dB more than another, has a sound
intensity 100 times as much and contains 100 times
the energy.
A sound which is 3 dB more than another contains
twice the energy.
This is an important concept to remember when we
come to assessing people's noise exposures in later
lectures, for if someone is receiving noise 3 dB
above the exposure standard, they are not merely
'just above' the standard, but are receiving twice
as much potentially damaging sound to their ears.
A different concept is that of loudness, a
psycho-physical sensation caused by sound. Here an
increase of 10 dB in sound pressure level causes the
sound to be perceived as twice as loud. A decrease
of 10 dB is perceived as half as loud. So, if you
are able to introduce noise controls which make the
sound half as loud, you have actually reduced the
energy getting to the ear by a factor of 10 - a very
worthwhile effort. |
|
|
|
|
