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## Decibel

The decibel (symbol: dB) is a unit of measurement used to express the ratio of one value of a power or field quantity to another on a logarithmic scale, the logarithmic quantity being called the power level or field level, respectively.

## Sound

The sound is defined as a perturbation wavelike that typically propagates as an audible wave of pressure in an elastic medium (such as a gas, liquid or solid) and which generates an auditory sensation. Sound intensity Sound intensity is defined as the sound power carried by sound waves per unit area. The usual context is …

## Absorbance

In chemistry, absorbance or decadic absorbance is the common logarithm of the ratio of incident to transmitted radiant power through a material, and spectral absorbance or decadic spectral absorbance is the common logarithm of the ratio of incident to transmitted spectral radiant power through a material. Absorbance is dimensionless, and in particular is not a …

## Absorptance

Absorptance of the surface of a material is the ratio of the luminous or radiant flux absorbed by a body to the flux falling on it. It is generally represented by the Greek letter $$\alpha$$. By definition, in the case of a blackbody $$\alpha = 1$$. Absorptance was formerly known as absorptivity. This should not …

## Getter

The getter is an inert electrode, coated with magnesium or barium powders, which are good gas absorbers, introduced into the vacuum tubes to increase their useful life. It has the function of counteracting the phenomenon of poisoning of the cathode, due to the bombardment of positive ions of gas, both residual and developed by the …

## Absorber

In high energy physics experiments, an absorber is any material used to absorb some of the energy of an incident particle (efficient at stopping ionizing radiation from passing through). It is sometimes used as a synonym of absorbent, such as for example in the case of the getter. In nuclear techniques, an absorber it is …

## Absolute

Absolute means not limited by exceptions or conditions. What does not depend on another for its existence, therefore opposed to ‘conditioned,’ ‘dependent,’ and does not exclude the relationship for which another would depend on it. The term is used in many different ways in mathematics, physics, philosophy, and everyday speech. Absolute space and time, which, …

## Abrasive

An abrasive is any material (often a mineral) used to cut, grind, or polish a softer material by abrasion – the wearing down of a surface when a harder surface rubs over it. Abrasives are extraordinarily commonplace and are used very extensively in a wide variety of industrial, domestic, and technological applications. Mild abrasives such …

## Ablation

Ablation is the removal of material from the surface of an object by vaporization, chipping, or other erosive processes. Ablation may refer to the removal of the surface layers of a meteorite during its passage through the atmosphere. Ablation may also mean the removal of surface material such as occurs in a combustion chamber, or …

## Semiconductor

A semiconductor is a substance with an electrical conductivity between that of a conductor and an insulator — the conductivity of a semiconductor increases as temperature increases. Adding appropriate impurities also increases conductivity. Amorphous semiconductor An amorphous semiconductor also called “thin film” is a non-crystalline semiconductor material that has no long-range order. Although easier and …