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Noise > Terms > Introduction

After the Introduction, this section consists of

General Introduction
Noise is generally defined as unwanted sound.  So, you now know the answer to the philosophical question "If a tree falls in the forest, does it make a noise, if there is no one to hear it?" - answer "No" - reason "As noise is defined as unwanted sound, if there is no observer, the sound (if any) cannot be unwanted and therefore there can be no noise" - (Enough philosophy for the whole site!)

The next question is "What is sound?" Sound is a series of vibrations in air [or water or metal and so on] which causes pressure changes in the ear; these pressure changes can vibrate the inner workings of the ear very fast [heard as a high tone] or slowly [a low tone] or somewhere in the middle, and we hear these pressure changes as sound.

So, if sound is just air pressure, why isn't it measured in the same way that the weather men measure air pressure, in nice easy units like millibars, or atmospheres? Unfortunately human ears/brains don't respond like barometers. A barometer can detect a one millibar change easily, no matter whether pressure is low or high. On the other hand, human ears can detect small changes in pressure only when the level of sound is low; when the level of sound is high, a very large change in pressure is needed before we notice the change. This is very different to normal ways of measuring pressure; generally speaking the sound pressure needs to be doubled before we notice the change, and increased by a factor of 10 before we think it has doubled!?

This is why decibels [dB] are used because they respond in a similar fashion to the way the human ear. 0 dB has been designed as the threshold of human hearing. On this basis 200 dB is a very loud explosion and some deafness is almost certain to be instantaneously caused.

A 3 dB increase/decrease in the level of noise is a doubling/halving of the sound pressure level (or in the energy contained), but we would only just notice this increase; whereas if we actually perceived a doubling or halving, the noise level would have changed by 10 dB.

A further complication, is that human ears do not respond very well to high tones (frequencies) or low tones (frequencies). Not surprisingly ears respond best at the same frequencies as human voices. Therefore sound measurements are weighted to respond in the same way; this is known as "A" weighting and the most commonly used units are hence dBA, which can also be written as dB(A).

There are other weightings; the most commonly used are dBC or dBLin, which for most purposes are approximately equal; they are mainly used in the assessment of Noise at Work.

A further complication, is that the response time of the meter (how quickly the "needle" is allowed to rise and fall), will affect what the meter reads for any given noise. Although the more steady the noise, the less important the response time is. The sound measuring system can be set to one of several time weightings; 'Peak', 'Impulse', 'Fast', and 'Slow'. Broadly speaking 'Peak' produces the highest readings then 'Impulse', 'Fast' and 'Slow'.  'Fast' is the most common setting for readings.

The next problem in trying to represent the vagaries of human hearing in a scientific unit, is that virtually all sounds vary from moment to moment, and how we respond to a noise depends on how it changes.

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