Help Topics

Help Topics - A

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One of the standard frequency correction curves (or weighting) applied to sounds in a measurement device to simulate the hearing capability of the human hearing mechanism. The A weighting is the most common frequency weighting used in sound level measurements and is found in virtually every sound level meter. Other frequency weightings include the 'B', 'C' and 'Z' curves.

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Absorption

The property of all materials that allows a reduction in the amount of sound energy reflected from it. Incident sound energy is turned into heat inside the material during the absorption process.

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Absorption coefficient

A measure of the sound-absorbing ability of a surface. It is defined as the fraction of incident sound energy absorbed or otherwise not reflected by a surface. The value of the absorption coefficient varies in the range from about 0.01 for marble to 1.0 for a room covered in foam sound wedges.

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Accuracy

The degree with which a measuring instrument obtains the "true" result. When noise measurements are carried out using a noise meter this will be the dB value representing the true sound pressure plus or minus the error at the time of the measurement. The acceptable limits for the accuracy (or error) of an instrument are usually specified in national and international standards issued by independent bodies such as ANSI or IEC. For noise meters they will cover frequency response, effect of the direction of sound arriving at the instrument and other various environmental effects such as temperature and ambient air pressure.

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ACGIH

The American Conference of Governmental Industrial Hygienists organization in the United States.

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Acoustics

The science of sound, including the generation, transmission and effects of sound waves, both audible and inaudible.

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Acoustic trauma

The damage to the hearing mechanism caused by a sudden burst of high-energy noise such as a blast or gun fire. The term is usually considered to be caused by a single impulsive event with a very high peak sound level.

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Action level (dB)

The 8 hour continuous notional noise level at which the employer must undertake certain duties of care for exposed workers. Typical values are 85 and 90 dB measured for a whole working day with the 'A' frequency weighting.

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AED

The aerodynamic diameter (of particles) usually expressed in microns (millionths of a meter).

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Aerosol

A finely divided material suspended in air or other gaseous environment.

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Airborne sound

Sound energy that reaches the point of interest by propagation through air.

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Air monitoring

Sampling for pollutants in the air using a variety of means such as sorbent tubes, bags, filters and sampling conventions.

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Air pollutant

Any substance in air that could, in high enough concentration, harm man, other animals, vegetation, or material. Pollutants may include almost any natural or artificial composition of airborne matter capable of being airborne. They may be in the form of solid particles, liquid droplets, gases, or in combination.

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Air pollution

The presence of contaminant or pollutant substances in the air that do not disperse properly and interfere with human health or welfare, or produce other harmful environmental effects.

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Air quality monitoring

Sampling, measurement/analysis of airborne pollutants.

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Air quality standards

The level of pollutants prescribed by regulations that are not to be exceeded during a given time in a defined area.

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Airborne particulates

Total suspended particulate matter found in the atmosphere as solid particles or liquid droplets. Chemical composition of particulates varies widely, depending on location and time of year. Airborne particulates include: windblown dust, emissions from industrial processes, smoke from the burning of wood and coal, and motor vehicle or non-road engine exhausts.

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Alveoli

The tiny sacs of air in the lungs where gaseous exchange takes place. Oxygen is taken into the body and carbon dioxide is expelled.

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Ambient air

Any unconfined portion of the atmosphere: open air, surrounding air.

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Ambient sound

The total amount of all noise present at a particular place and time in the environment at the point of investigation.

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Ambient temperature

Temperature of the surrounding air or other medium.

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Amplitude

The “strength” of a sound signal as detected by the ear or as measured by a sound level meter.

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Analog display

A form of readout device that uses a needle moving across a pre marked scale to indicate the instantaneous sound level in decibels. Analog display movements are particularly prone to mechanical damage and are not typically used in higher quality instruments any more. Usually only a single parameter can be displayed on an analog display and so they are only fitted to simple instruments that show the current noise level.

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Anechoic chamber

An enclosed space or room that has walls and floors made from nearly perfect absorbing materials. In this case it is possible to obtain almost free field conditions indoors.

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ANSI

The American National Standards Institute in the United States. This organization publishes a wide variety of documents covering the specifications of the instruments used to make physical measurements of noise under a wide variety of conditions.

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Aromatic

A type of hydrocarbon, such as benzene or toluene, added to gasoline in order to increase octane. Some aromatics are toxic.

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Articulation index

A numerically calculated measurement of the intelligibility of transmitted or processed speech. It takes into account the limitations of the transmission path and the background noise at the point of interest. The index can range between 0 and 1.0. if it is less than about 0.1 speech intelligibility is generally low. If it is above about 0.6 speech intelligibility is generally high.

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