Acousticians use many different descriptors. Some are simply a different way of expressing the physical measurement (e.g. sound pressure level (SPL) is the sound pressure expressed in decibels relative to 20 µPa), others are much more complicated and include other information (e.g. Noise and Number Index (NNI) was developed to quantify annoyance around an airport, and starts with measurements of sound pressure level but also includes a subjectively determined frequency weighting, a threshold level, and the number of aircraft movements at specific times of the day and year).

The use of the decibel in many descriptors stems from some observations about human reaction to sound, particularly that the ear is a non-linear transducer. For example, it is observed that the minimum detectable increase in sound pressure is not a constant but depends on the existing pressure, and is approximately 1 dB (or 12%). Essentially, the ear reduces its sensitivity as the input increases, behaving as a logarithmic detector, and consequently some users find a logarithmic scale convenient. Other users find the consequences of a decibel scale more confusing (e.g. given several identical noise sources in a room, if a measurement is made of the noise when one source is operating and then a second source is turned on the level will increase by 3 dB. It would take two more sources to be activated to increase the level by a further 3 dB and four more to increase it by yet a further 3 dB). For those concerned with understanding physical systems, rather than human reaction, the decibel representations can be avoided by simply converting all levels to the underlying physical quantities.

In their most basic form the two most common sound measurements are those concerned with quantifying the magnitude of a sound field and the strength of a sound source. The sound field could be described in terms of changes in density or particle velocity or particle displacement, but in practice sound pressure is the easiest field quantity to transduce so it is usually this that is measured and expressed in decibels relative to 20 µPa as sound pressure level. A source is characterised by the sound power that it produces and this is expressed in decibels relative to 1 pW as sound power level. Either of these descriptors could include a standardised frequency weighting designed to represent the frequency response of the human ear when they would become 'A-weighted sound pressure level' and 'A-weighted sound power level'.

The use of the decibel to describe both sound pressure level and sound power level is a common cause of confusion amongst inexperienced users of sound measurements. Many expect it to be a simple step to calculate one from the other (or don't realise the distinction) because they are both expressed in decibels. In an analogous field one would buy a domestic heating appliance rated in Watts and not expect it to be a trivial task to predict the temperature (in Celsius) that it was capable of generating without detailed knowledge of the environment that it was placed in.

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