Sound Pressure and the Decibel Scale
Prolonged exposure to loud noise causes hearing damage, resulting in hearing loss. Therefore it is important that we can measure the loudness of a noise, to determine if it is loud enough to be harmful. Measuring noise and interpreting the results is not straight forward, and often leads to confusion. This web page on Sound Pressure and the Decibel Scale aims help understanding of noise measurement.
Sound Energy
Sound Energy is defined as the total energy in air minus the energy that would exist in the air with no sound waves present. The
SI unit of energy is the joule (J), however the range of human hearing, 0.000000000001 joules up to 10 joules or more, means the joule is not practical for everyday use.
Sound Pressure
A sound source, e.g. a machine or piece of equipment, creates vibrations in the surrounding air, and the sound travels as a wave through the air. The sound wave causes small changes to the local atmospheric air pressure. This is called Sound Pressure. The
SI unit of pressure is the Pascal, so sound pressure is measured in Pascals. The pressure range to which the human ear is sensitive ranges from approximately 0.00002 Pa to 200 Pa. These are relatively small values compared to normal atmospheric pressure which is around 100000 Pa. This means using Pascals as a measurement of sound is not very practicable.
Sound Pressure Level
To overcome the impracticalities of using SI units to measure sound - the concept of Sound Pressure Level is used. Sound pressure level (SPL) is a logarithmic measure of the pressure level of a sound relative to a reference value, i.e. pressure level of the surrounding air with no sound waves present. This logarithmic measure is expressed in decibels (dB).
The formulae for sound pressure level is:
Sound pressure Level depends on the distance from the source of the noise and the acoustic properties of the surroundings of the source. The further from a noise you are, the lower the Sound Pressure Level.
Decibels
A decibel (abbreviated as dB) is used as the "unit" for sound pressure level. Strictly, the decibel is not a unit, it is the logarithm of a quotient, so it essentially unitless.
Decibels are measured logarithmically. This is a way of counting or measuring something that increases rapidly, or exponentially. For example, every increase of 10 dB on the decibel scale is equal to a 10-fold increase in sound pressure level (SPL).
A decibel is one tenth of a bel. The term bel is derived from the name of Alexander Graham Bell, inventor of the telephone. The unit decibel is used because a one-decibel difference in loudness between two sounds is the smallest difference detectable by human hearing.
Actual Sound and Perceived Sound
Being told that something measures "85 dB" isn't particularly useful unless you know what 85dB sounds like. Hopefully the chart below will help you put SPL readings into context:
A-weighted decibel (dBA or dB(A))
Noise measurements relating to loud noise at work are usualy given in dB(A) – this ia a frequency weighting applied to the decibel measurements, in effect, they are decibel scale readings that attempt to replicate the sensitivity of the human ear to different frequencies of sound. A-weighting gives more value to frequencies in the middle of human hearing and less value to frequencies at the edges as compared to a flat audio decibel measurement. A-weighting is the standard for determining hearing damage and noise pollution.
What is too loud?
Continued exposure to noise above 70 dBA over time will cause hearing loss. The volume (dBA) and the length of exposure to the sound will tell you how harmful the noise is. In general, the louder the noise, the less time required before hearing loss will occur.
Standards for acceptable noise vary, though in general the maximum recommended limit for human exposure is 85 decibels over a period of 8 hours or 70 decibels over a period of 24 hours.
Hearing protection is recommended when you are exposed to noise levels of 85 decibels or above.
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