The LEP,d is the level of personal noise exposure that a worker is exposed to in a day, normalised to an 8-hour work period. The letters in the term LEP,d refer to level, exposure, personal and daily respectively. If a worker’s shift is shorter or longer than 8 hours, the calculation of the LEP,d takes the shift length into account and gives the average sound level for an equivalent 8 hour shift. This means that a worker’s LEP,d can be easily compared to the exposure limits set out by the Control of Noise at Work Regulations, which specify exposure limits for an 8-hour period.
The dose is the noise exposure expressed as a percentage of the total allowable daily noise exposure.
SNR stands for single number rating. It is a single number, given in decibels, that allows a comparison of the attenuation provided by different hearing protectors. The noise level that reaches a user’s ear can be estimated by subtracting the SNR from the external noise level (in dB(A)).
A-weighted decibels, dB(A), are the units of sound exposure limits as specified by the Control of Noise at Work Regulations. Mandatory hearing protection zones are defined by areas where the sound pressure level reaches a daily or weakly average value of 85 dB(A).
Sound pressure level is measured in units of A-weighted decibels, dB(A). Calculating the dB(A) value is a two stage process:
- First, the signal from the microphone is A-weighted. Any noise consists of a range of frequencies, and human hearing is more sensitive to certain frequencies than others. For example, a low-pitched sound will be perceived as being quieter than a high-pitched sound, even if they are played at the same sound level. The A-weighting process takes this into account by emphasising the frequencies in noise that human ears are sensitive to.
- Second, the microphone signal is converted to a decibel (dB) scale. The dB scale is logarithmic, which works differently than a linear scale. A multiplication on a linear scale (e.g. doubling) corresponds to an addition on a logarithmic scale. By converting the microphone signal into decibels, a large range of values can be condensed into a manageable range of values.
The LAeq is effectively the average sound level over a given period of time. It has units of A-weighted decibels, dB(A).
Noise levels can fluctuate a lot over time, and the LAeq
gives the average noise level for a given period of time. However, since the units of the sound level are logarithmic, a couple of extra steps are involved in calculating the average:
- Convert the noise level from decibels, which are logarithmic, to a linear unit of sound pressure.
- Calculate the arithmetic average of the noise values during a certain period of time. This is simply the sum of the values divided by the number of values.
- Convert the results back into decibels to give the LAeq.