Sound and Noise
Measuring sound levels and loudness
Our ears hear sounds by detecting small changes in air pressure caused by sound waves. Sound level measurement instruments work in exactly the same way; microphones detect tiny changes in air pressure and convert them into electrical signals. The louder the sound, the bigger the signal. These signals are then processed to work out just how loud the measured sound was. To do this we need some reference scale, or a way to quantify how loud or quiet a sound source is. For example, we can think of how water freezes at 0°C and boils and 100°C. We use these reference points to define a scale and then gauge how hot something is. Once the scale is clear it is easier to make less technical comparisons like "close to boiling" or "almost freezing" that give us a good sense for temperature without having to use numbers all the time.
Sound levels are a little bit different. Our ears are able to detect a huge range of pressure levels, with the highest being over a million times bigger than the smallest. Using a scale that goes from 1 to 1,000,000 is not very practical, so we use decibels instead.
The decibel scale can be used when the numbers between one end of the scale and the other are too large to deal with. Decibels look at ratios instead of absolute measurements. This makes it easier to look at situations where one number is much larger than another. For measuring sound levels, decibels are referenced to the lowest sound pressure that the human ear can hear, known as the Threshold of Hearing. This is 0dB (decibels). When we measure a sound pressure level, it is a reference of how many times greater the level is, compared to this very low reference.
For example, a sound pressure that is one thousand times bigger than the Threshold of Hearing has a level of 60dB, while a number that is one million times bigger than the Threshold has a level of 120dB. Using decibels gives a manageable scale for the very large range of sound pressures that our ears can detect.
So what counts as noise?
Measuring noise is not always as straightforward as measuring sound levels. The main goal of managing noise pollution is to minimise the effect that is has on people as they go about their daily routines. However, just because a sound source may be quite loud, does not mean that it is a source of noise pollution. Sound levels at the sea front on a windy day can be as high as a busy roadway at rush hour, but the effect of this sound on people is very different. When we try to quantify noise we need to look at some other factors, including the source of the sound and the number of people in the area that are likely to be affected. Understanding noise annoyance means that we factor in some of the personal considerations of how people are affected by sound.