There are many different ways to measure sound, the most common of which is using a sound pressure level meter/analyser. The sound pressure level may be measured as a single overall value or broken down into different components such as octave frequency bands. Where a single overall measurement is taken, this is often measured as the sound pressure level in decibels dBA to provide an indication of how loud the sound may appear to a listener. There are different ways of quantifying this sound level, such as the (logarithmic) average over a certain time period (LAeq), the maximum (LAmax), minimum (LAmin) or other parameters, such as the level exceeded for 90 per cent of the time (LA90), which is the quietest 10 per cent level or the LA10, which is the level exceed for 10 per cent of the time.
These types of measurements may also be taken with the sound level meter, responding quickly to variations in the sound level using a fast response, or more slowly, which will tend to give a smaller variation as the sound level changes.
It is important to remember that in most cases the sound level being measured varies continuously with time and also with location. This means that any sound pressure level measurement will depend upon many factors, including when and where it was measured, together with what was being measured and what was happening at the time. Although many sound level analysers are relatively precise instruments with a known measurement uncertainty, giving an answer often to 0.1dB precision, this is not the accuracy of the measurement which is considerably less certain than this degree of precision.
Other measurement systems, such as lower quality sound level meters or mobile device apps, are less accurate and do not provide a reliable indication of the actual sound level being measured.
When taking any sound level measurements it is important to identify what is to be measured, what extraneous sound will also be measured at the same time and to be able to identify, minimise and quantify to an appropriate level the various uncertainties that will be involved in taking the measurement(s). In addition to ensuring that appropriate measurement parameters are selected, it is also important to ensure that measurements are taken at a suitable location and time, for an appropriate duration and under representative meteorological conditions. Typically this means that sound-level measurements should not be taken when there is any precipitation or possibly wet ground surfaces, which can have such effects as increasing the level of noise due to passing road traffic, and where the wind speed is not excessive and the wind is in a suitable direction.
Despite taking these precautions, it is often the case that a sound level measurement of the acoustic climate at a particular location will vary by a considerable amount, depending upon the time of day and meteorological conditions, so that even if measurements are repeated at the same time on different days there may be a considerable variation between given measurements, even if taken under “good measurement” conditions.