Decibel dB

[edit] Introduction

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.

Today it is nearly impossible to find a place in a town or city where there is absolute and total silence. There is simply too much traffic, too many people, factories, plants, railway stations, airports and so on. And yet noise pollution is proven to be detrimental to human health and safety. That is why permissible levels of sound emissions are under permanent control by various regulatory agencies. Any equipment used outside (vehicles, devices, machinery) should meet the requirements of the directives concerning noise and outdoor equipment.

Especially in residential neighborhoods, there are particular time periods when work by means of drills and other noisy equipment can be carried out. Even though these sounds are fleeting, their levels are extremely high for unprotected human ears and must be measured by a sound level meter to assess risk and prevent damaging exposure. Problems with noise levels also are experienced by those living in close proximity to railway stations, airports, busy streets, hospitals and fire and police stations. Excessive long-term exposure to high-level noise negatively impacts human health, resulting in hearing damage, raised nervousness, irritability and unstable psychological conditions. Thus, in some areas noise monitoring and control by use of a sound level meter is an essential and mandated task.

Noise levels between 120 dB and 150 dB (e.g., airplanes taking off and landing, drilling and sawing of roadways) are dangerous to human hearing and their continuous presence has dire consequences. Perfect noise levels are between 30 dB and 50 dB, though these pleasant levels are sometimes unreachable in today's society.

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:

[edit] The decibel scale­

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.

[edit] Sound levels in construction

Those working in the construction industry are often exposed to an increased noise level. Some examples of construction equipment and their approximate decibel levels are shown below:

• 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:

Lower exposure action value, at which the employer must make hearing protection available and provide information and training.

• 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.

[edit] Related articles on Designing Buildings Wiki

• Airborne sound.
• Approved Document E.
• Audio frequency.
• Building acoustics.
• Control of Noise at Work Regulations 2005.
• Environmental health.
• Equivalent continuous sound pressure level.
• Impact sound.
• Robust details certification scheme.
• Room acoustics.
• Sound absorption.
• Sound frequency.
• Sound power level.
• Sound v noise.
• Structure-borne sound.
• Workplace noise exposure estimator.

[edit] External references

• Cirrus Research – Measuring noise in the construction industry.
• Dangerous Decibels.
• HSE – Noise regulations.
• PCE Instruments - Sound Level Meter