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

Curbside 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.