Sound absorption

Slab track on the high-speed route Cologne – Rhine / Main with sound absorbers to reduce the spread of wheel-rail noise .

The sound absorption refers to the reduction of sound energy , in particular by conversion into heat. Absorbing is synonymous with "swallowing" and "sucking up".

Differentiation from similar terms

The term sound absorption must be distinguished from the following terms, which are used very similarly:

• Under Schalldissipation is exclusively the conversion into other forms of energy as sound, in particular heat understood, while the absorption principle, also other types of "disappearance" can be meant by sound (see absorption coefficient ).
• Sound attenuation refers to any type of reduction in sound intensity that does not necessarily have to do with a reduction in sound energy , e.g. B. by divergence , i.e. by distributing the sound energy over a larger area.
• Sound insulation is the shielding of sound from the outside. It can already be brought about via the inertia of a smooth, reverberant wall (made of steel or concrete, for example), which causes maximum sound reflection on the inside . On the other hand, sound absorption serves to reduce reflections (e.g. to suppress reverberation and / or standing waves in indoor spaces).

Quantitative determination

Absorption coefficient

The absorption coefficient is - as in optics - the exponential coefficient of the decrease in the intensity of a flat (i.e. divergence-free) wave . It is a material constant of the transmission medium during the propagation of sound . Because this material constant is characterized only by dissipative absorption, it is in fact identical to the dissipation coefficient .

Degree of absorption

The sound absorption coefficient is defined as the ratio of the absorbed sound intensity to the total incident sound intensity : ${\displaystyle \alpha }$ ${\displaystyle I_{\mathrm {a} }}$ ${\displaystyle I_{0}}$

${\displaystyle \alpha ={\frac {I_{\mathrm {a} }}{I_{0}}}}$

The degree of sound absorption is related to the degree of sound reflection (measure of the reflected sound intensity), the degree of sound transmission (measure of the transmitted sound intensity) and the degree of sound dissipation (measure of the "lost" sound intensity): ${\displaystyle \rho }$ ${\displaystyle \tau }$ ${\displaystyle \delta }$

${\displaystyle {\begin{alignedat}{2}\rho +&\alpha &&=1\\\rho +&\tau +\delta &&=1\\&\alpha =\tau +\delta \end{alignedat}}}$

The first equation says that the sum of reflected and absorbed sound intensity, i.e. of sound reflection and sound absorption, always corresponds to the total sound intensity. The last equation expresses that the absorbed sound intensity is made up of the transmitted (transmitted) and "lost" (dissipated) sound intensity; Sound absorption thus arises through simultaneous sound transmission and dissipation.

Unit of measurement

The unit of measurement for sound absorption is the sabin .

Modes of action

Not all constructions that are treated under the term 'sound absorber' today are actually based on absorption, i.e. on the conversion of sound into thermal energy.

Sound path enlargement

In practice, porous and / or fibrous absorption materials are used to absorb airborne sound . As a result, the surface on which the sound hits is enlarged many times over. Part of the sound energy reduction can therefore already be based on a mere extension of the sound path in that the sound is deflected (reflected) many times before it exits the absorber again. Since sound energy has a quadratic reciprocal relationship to the sound path covered (see dissipation ), the energy level of the sound exiting the absorber is reduced, even with a rigid or reverberant absorption material (e.g. meerschaum or pipes), because the sound already has a longer way behind it than would be the case with a smooth and flat reflective surface.

In the case of porous absorbers, the sound energy is converted into heat by friction between the air molecules in the absorber; this process is known as dissipation . The absorption capacity of porous absorbers is frequency-dependent and is determined by the porosity, the structure factor and the length-related flow resistance. The advantage of porous absorbers is their high absorption in the middle and upper frequency range, whereas low frequencies are only slightly absorbed.

Mechanical damping

In addition, sound energy is also converted into kinetic energy if the damping material is elastic (e.g. wool fibers or rubber molecules), movable (e.g. sand) or deformable (e.g. chips ), whereby the energy level of the sound emitted from the absorber can be additionally reduced.

In the case of porous absorbers, the damping behavior can be approximated mathematically using the theory of quasi-homogeneous absorbers.

Resonators

Another possibility for sound absorption is offered by resonators , especially plate resonators ( membrane absorbers ) and Helmholtz resonators . Here, too, the sound energy is first converted into kinetic energy, namely into vibration energy.

Anti-noise

Sound can also be extinguished electronically ('actively'). This process does not actually refer to absorption (conversion into heat or kinetic energy by a material), but rather to sound cancellation. An anti-phase signal is superimposed on the incoming sound by at least one additional loudspeaker, so that phase-dependent cancellation or attenuation occurs (see counter- sound ).

Combinations

Many constructions for reflection reduction combine the above. Modes of action, so. z. B. a plate oscillator construction (resonator), which is also filled with elastic material.

Sound absorption in room acoustics

The sound generated in a room propagates as an airborne sound wave and hits the boundary surfaces of the room, which partially absorb, let through or reflect the sound.

Carpets are the only floor coverings that allow sound absorption of airborne noises. In the room acoustics calculation, z. B. set the required sound absorption level for carpets . The measurement is carried out in a reverberation room by comparing the reverberation times with and without covering.

Perforated drywall sheets are often attached to the walls and ceilings in larger rooms with little furniture . Some of the sound waves pass through the holes and are already weakened; Any portions that pass through are absorbed by the underlying porous materials (such as melamine resin foam ).

Web links

• Degree of absorption α of various materials and surfaces
• Egg cartons to improve the acoustics? No thanks! (PDF file; 95 kB)
• Absorption coefficient of air
• Active absorbers
• Sound absorber variants
• Acoustic solutions - guidelines for the hörkomm.de project - barrier-free hearing and communication in the working world

Individual evidence

1. ↑ Fridolin P. Mechel: Sound absorption , Chapter 18, in: Manfred Heckl, Helmut A. Müller: Pocket book of technical acoustics , Springer-Verlag, Berlin, Heidelberg, New York, 1975, ISBN 3-6429-7357-4