Attenuation coefficient

Of the absorption coefficient α, and: sip ratio indicates which part of the power of an incident wave (e.g., sound or electromagnetic radiation such as light ) absorbed by a surface, that is, is recorded.

Basic relationships of the disturbed propagation

• The emissivity ε is a measure of the ( directed or undirected ) intensity for which the body itself is the source
• The reflectance, usually also ρ, is a measure of the total backscattered intensity, resulting from reflection and emission.

• Of the absorption coefficient α is a measure of the power absorbed by the body intensity.
• The transmittance τ is a measure for the transmitted intensity.
• The grade of dissipation δ is a measure of the converted into thermal energy, ie by dissipation " lost" intensity.
• The reflectance ρ is a measurement of the reflected intensity comes from outside.

In acoustics the transmittance τ is set α as part of the absorption coefficient because it does not matter for the acoustics that the sound energy is lost in a space by conversion to thermal energy, or to the outside, or in an adjacent room.

This results in sound for the following relationships:

From the first equation it can be seen that the proportion of the absorbed intensity of the sum of the shares of transmitted ( of transmitted ) and " lost " ( dissipated ) composed intensity. The second equation indicates that the sum of reflected and absorbed intensity corresponding to the total intensity.

In contrast, the absorption and transmission are in electromagnetic radiation treated separately, since most is the total emission of a body of interest, less the direction. The absorption coefficient in this case is a measure of the " lost" intensity, and it is

That is, the radiated energy (intensity) is partially reflected, transmitted in part, and the rest is absorbed ( "swallowed" by the media).

Sound waves

The absorption coefficient α indicates how large is the absorbed fraction of the total incident sound. α = 0 means that there will be no absorption, the entire incident sound is reflected. When α = 0.5, 50% of the sound energy is absorbed and reflected 50%. Wherein α = 1, the entire incident sound is absorbed, that is, a reflection no longer takes place (for example, open windows, or the ideal " anechoic " space ). Normally the values ​​are depending on the sound absorption system between 0.2 and 0.8. α is dependent on the surface material and the frequency. For sensorineural in a room, the ratio of absorbed and reflected sound energy plays a decisive role. α ρ = 1

Occasionally, values ​​of the sound absorption coefficient α is greater than 1, so given > 100 %. This is determined under practical conditions and reflects the fact that the effective area of an absorber is slightly larger than its geometric area.

Electromagnetic radiation

Of the incident on the surface of a body part of the radiation is reflected in the control, a portion passed through the body and absorbs the rest. The absorbed energy increases the internal energy of the body. The absorption coefficient ( absorption coefficient and spectral absorption coefficient or SAK ) indicates what fraction of the incident radiation is absorbed. It can take values ​​between 0 and 1, the extreme values ​​0 and 1 can only be achieved in practice approximated. The degree of absorption can depend on the direction of incidence and the frequency of the incident radiation. Depending on whether these directional and frequency distributions are to be explicitly taken into account or not, four different levels of absorption can specify.

Directional spectral absorption coefficient

The spectral irradiance (unit: W m- 2 Hz -1 SR-1) which is exposed to a body that specifies the radiation power at the frequency of the given by the polar angle and the azimuth angle direction per unit area, per unit frequency interval, and per unit solid angle is incident on the body.

The directional spectral absorption coefficient of an object indicates what fraction is absorbed at the frequency of the incident by the angle given direction and spectral irradiance of a surface member of the body:

The directional spectral absorption coefficient is a material property and does not depend on the properties of ( derived from external radiation sources) spectral irradiance. He is usually direction- and frequency- dependent and is also on the surface condition of the body ( eg roughness) strongly influenced.

Hemispherical spectral absorption coefficient

The spectral irradiance (unit: W m- 2 Hz -1) which is exposed to a body that specifies the irradiation power meets at the frequency from the entire half-space per unit area and per unit frequency interval in the body:

The cosine factor accounts for the fact that during irradiation from any given direction through, and only the projection of this direction perpendicular to the surface appears as an effective receiving surface. is an element of solid angle:

The hemispherical spectral absorption coefficient of an object indicates what fraction of the incident in the frequency of the half-space the spectral irradiance is absorbed by a surface element of the body:

Facing total absorption coefficient

The irradiance (unit: W m-2 sr -1) which is exposed to a body that specifies the radiant power incident on all frequencies from and represented by the angle of direction per unit area and per unit solid angle in the body:

The targeted total absorption coefficient of a body is to that fraction which is absorbed on all frequencies of the incident by the angle and direction given irradiance of a surface member of the body:

Hemispherical total absorption coefficient

The irradiance (unit: W m -2) which is exposed to a body that specifies the radiation intensity at all frequencies from the entire half-space per unit area exceeds the body:

The hemispherical total absorptance of the body indicates which fraction of the on all frequencies incident from the half-space irradiance is absorbed by a surface element of the body:

All beam sizes and degrees of absorption can of course also be formulated as a function of wavelength rather than the frequency.

Properties

The directional spectral absorption coefficient describes the direction and frequency dependence of the absorption of radiation. The hemispherical spectral absorption coefficient describes only the frequency dependence of the directional total absorptivity only the directional dependence and the total hemispherical absorptance only the total absorbed radiation power.

Only the directional spectral absorption coefficient is a material property of the considered body. The other levels are also dependent on the absorption of the ( by external sources of radiation determined) direction and the frequency distribution of the incident radiation. For example, white paint in the visible frequency range has a low spectral absorption coefficient, so absorbed incident solar radiation only a small fraction: the total absorption coefficient for radiation in this frequency range low. In the long wave infrared, however, he has a high spectral absorption coefficient, so absorbed by incident ( emitted at room temperature) thermal radiation a high proportion: the total absorption coefficient for radiation in this frequency range high.

According to the Kirchhoff's radiation law is for every body the directional spectral absorptance equal to the directional spectral emissivity. For the other absorption and emission levels, the equality holds only under additional assumptions.

A perfect absorbent body which completely absorbs all electromagnetic radiation incident on it, is called black body. He is after Kirchhoff's law of radiation also an ideal emitter which emits the largest physically possible thermal radiation power. The emitted radiation also has a universal frequency distribution that is described by the Planck's law of radiation.

A body having a directional spectral absorbance is not dependent on direction, a diffuse absorber. A body having a directional spectral absorbance does not depend on the frequency, is a gray body. In both cases, there are often considerable simplifications for radiation calculations, so that real bodies often - if possible - be approximately regarded as diffuse absorber and gray body.

Absorbance

The absorbance A ( equivalent absorption area ) of a wall is

Here, S is the absorbing surface in m².

A is thus equal to the equivalent absorption surface with α = 1, also called " surface open window " called.

Since the absorption effect increases in a material with the particle velocity, the absorber d / 4 should effectively in fast - maximum are as distance from the wall or should have a corresponding density. Much easier to measure the sound pressure minimum, which is located at the exact spot of the Fast - maximum at d / 4.