WHAT'S A DECIBEL?
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The human ear is designed to detect pressure changes in the air around us and is incredibly good at it. We can hear the rustling of leaves on a summer's day, stand loud music at a disco and equally cope with the noise of a big jet taking off at a range of 100 metres or so. It is possible to measure these pressure differences but it is more accurate to measure the energy causing them. Scientists using advanced instruments can measure the actual energy involved in Watts per square metre. In these units the quietest sound we can normally hear is 0.000,000,000,01 Watts/rn2 while the loudest is 100,000,000 Watts/m2.
Because it is quite impossible and impracticable to think in terms of one noise being 10 million million times louder than another, engineers use the number of tenfold increases instead. In other words, to get to 10 million million you would have to multipy 10 by itself 1 3 times. Each tenfold increase is called a Bel (after the inventor of the telephone Alexander Graham Bell) but because this a rather large unit it is divided into ten smaller units called decibels. So, instead of having to say the jet plane is 10 million million times louder than the rustling leaves it is rather more convinient to say it is 130 decibels louder. The human ear is not perfect and can detect some frequencies better than others. Scientists have accurately determined these shortcomings and are able to take them into account when dealing with everyday noise by using a frequency weighting known as the "A" scale. This is why the unit dB is nearly always followed by a capital A in brackets in a truck or environmental context: eg 80dB(A).
Unfortunately, this tenfold or logarithmic method of measuring sound level means it is not possible to simply add or subtract sound levels. For example, 80dBIA) plus BOdB(A) does not equal 160dBIA) but 83dB(A) and a halving of 80dB(A) is not 40dB(A) but 77dB(A). Noise is a difficult subject.