Absorptionswiderstand

Absorptionswiderstand, in physics and engineering often called absorption impedance, is a property of a boundary or material that describes how it opposes the absorption of energy when waves – acoustic, electromagnetic or other – impinge on it. It characterizes how much incident energy is converted into other forms (typically heat or internal energy) rather than being reflected or transmitted.

In acoustics, the surface impedance Zs at normal incidence relates acoustic pressure p and particle velocity v at the boundary: Zs = p/v. The boundary absorbs energy more effectively when its resistance part is large relative to the surrounding medium’s characteristic impedance Z0. The reflection coefficient for plane waves is Γ = (Zs − Z0) / (Zs + Z0), and the absorption coefficient is α = 1 − |Γ|^2. Maximum absorption (α → 1) occurs when Zs matches Z0, reducing reflections. The real part of Zs represents dissipation (absorption), while the imaginary part represents energy storage (reactance).

Materials with high absorptivity, such as porous foams or fibrous composites, are designed to yield appropriate Absorptionswiderstand values through material choice, thickness, and microstructure. In engineering practice, impedance matching is used to minimize reflections in ducts, vents, or concert halls, and similar concepts apply to electromagnetic absorbers in antennas and radar-absorbing coatings, where surface impedance is engineered to limit backscattering.

Measurement methods include impedance tubes and reverberation techniques to determine Zs or the related absorption coefficient. The concept bridges material science, acoustics, and electromagnetism, providing a framework to quantify and optimize energy dissipation at boundaries.