Decibels (dB) are most commonly used as a measure of sound level, but they are also used in electronics, signals and communications.
Sound is a variation in pressure detectable by the ear, whereas noise is undesired sound, or any sound which causes disturbance or annoyance to the recipient. The unit used to describe sound wave intensity is the bel, named after the inventor Alexander Graham Bell. The human ear is sensitive enough to detect changes of as little as 1/10 of a bel, and so sound intensity levels are described in decibels.
A sound wave’s intensity is the average amount of energy transmitted per unit time through a unit area in a specified direction. The sound intensity level, I, in decibels is 10 times the logarithm of the ratio of the intensity of a sound wave to a reference intensity:
The decibel scale gives an approximation of human perception of relative loudness. This is because the human ear has a logarithmic response to changes in sound level.
On the decibel scale, the smallest audible sound (near total silence) is 0 dB. A sound ten times more powerful is 10 dB. A sound 100 times more powerful than near silence is 20 dB.
The logarithmic nature of the dB scale means that each 10 dB increase represents a 10-fold increase in acoustic power. A 20 dB increase is therefore a 100-fold increase in power, and a 30 dB increase is a 1000-fold increase. However, an increase in acoustic power of ten times does not mean that the sound is perceived as being ten times louder. The ear perceives a 10 dB increase in sound level as only a doubling of sound loudness, and a 10 dB decrease in sound level as a halving of sound loudness.
The lower threshold of human hearing is around 5 dB. Normally speaking voices are around 65 dB. A rock concert can be around 120 dB.
Sounds that are 85 dB or above can cause hearing damage, and the higher the sound pressure, the less time it takes to cause damage. For example, a sound of 85 dB may take 8 hours to cause damage, whereas a sound of 100 dB may start to cause damage after only 30 minutes. A sound of around 150 dB can cause instantaneous hearing damage.
 Sound levels in construction
- Backhoe: 84-93 dB
- Bulldozer: 93-96 dB
- Concrete joint cutter: 99-102 dB
- Crane: 90-96 dB
- Earth tamper: 90-96 dB
- Earthmover: 87-94 dB
- Front-end loader: 86-94 dB
- Hammer: 87-95 dB
- Jackhammer: 102-111 dB
- Pneumatic chip hammer: 103-113 dB
- Portable saw: 88-102 dB
- Stud welder: 101 dB
The Control of Noise at Work Regulations 2005 requires that employers to take steps to reduce the harmful effects of noise on hearing. These relate to the levels of exposure to noise averaged by an employee over a working day or week and the maximum noise (peak sound pressure) to which they are exposed in a working day. There are two main action levels for continuous noise:
- Daily or weekly exposure of 80 dB.
- Peak sound pressure of 135 dB.
Upper exposure action value, above which the employer is required to take reasonably practicable measures to reduce noise exposure, such as engineering controls or other technical measures. If the noise cannot be controlled by these measures, hearing protection is mandatory.
- Daily or weekly exposure of 85 dB.
- Peak sound pressure of 137 dB.
Exposure limit value, levels of noise exposure which must not be exceeded.
- Daily or weekly exposure of 87 dB.
- Peak sound pressure of 140 dB.
 Related articles on Designing Buildings Wiki
- Airborne sound.
- Building acoustics.
- Control of Noise at Work Regulations 2005.
- Environmental health.
- Impact sound.
- Sound absorption.
- Sound frequency.
- Sound v noise.
- Structure-borne sound.
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