Sound Pressure Level and the decibel (dB)

A sound wave is a small fluctuation of atmospheric pressure. The human ear responds to these variations in pressure, producing the sensation of hearing. The ear can detect a very wide range of pressure variations. In order to cope with this wide range of pressure variations, a logarithmic scale is used to convert the values into manageable numbers. Although it might seem unusual to use a logarithmic scale to measure a physical phenomenon, it has been found that human hearing also responds to sound in an approximately logarithmic fashion. The dB (decibel) is the logarithmic unit used to describe sound (or noise) levels. The usual range of sound pressure levels is from 0 dB (threshold of hearing) to 140 dB (threshold of pain).

 

Frequency and Hertz (Hz)

As well as the loudness of a sound, the frequency content of a sound is also very important. Frequency is a measure of the rate of fluctuation of a sound wave. The unit used is cycles per second, or hertz (Hz). Sometimes large frequency values are written as kilohertz (kHz), where 1 kHz = 1000 Hz. Young people with normal hearing can hear frequencies in the range 20 Hz to 20,000 Hz. However, the upper frequency limit gradually reduces as a person gets older.

 

A-weighting

The ear does not respond equally to sound at all frequencies.  It is less sensitive to sound at low and very high frequencies, compared with the frequencies in between. Therefore, when measuring a sound made up of different frequencies, it is often useful to ‘weight’ each frequency appropriately, so that the measurement correlates better with what a person would actually hear. This is usually achieved by using an electronic filter called the ‘A’ weighting, which is built into sound level meters.  Noise levels measured using the ‘A’ weighting are denoted dBA. A change of 3dBA is the minimum perceptible under normal everyday conditions, and a change of 10dBA corresponds roughly to doubling or halving the loudness of sound.

Noise Indices

When a noise level is constant and does not fluctuate over time, it can be described adequately by measuring the dBA level.  However, when the noise level varies with time, the measured dBA level will vary as well.  In this case it is therefore not possible to represent the noise climate with a simple dBA value.  In order to describe noise where the level is continuously varying, a number of other indices, including statistical parameters, are used.  The indices used in this report are described below.

L_Aeq is the A-weighted ‘equivalent continuous sound pressure level’ which is an average of the total sound energy measured over a specified time period. In other words, the L_Aeq is the level of a continuous noise which has the same total energy as the real fluctuating noise, measured over the same time period. It is increasingly being used as the preferred parameter for all forms of environmental noise.

L_Amax is the maximum A-weighted sound pressure level during the monitoring period.

L_A90 is the A-weighted sound pressure level exceeded for 90% of the time period. The L_A90 is used as a measure of background noise.

L_A10 is the A-weighted sound pressure level exceeded for 10% of the time period. It is used for assessment of road noise and indicates the noisiest 10 % of sound produced.

 

Example average noise levels: