Acoustic Impedance

Acoustic impedance is the product of the medium’s density and the speed of sound within it, representing the opposition a material presents to the passage of sound waves. For air at standard conditions, it equals approximately 413 Pa·s/m. Specific acoustic impedance (the ratio at a surface) is critical for calculating reflection and absorption coefficients. Per ISO 10534-2, impedance is measured in an impedance tube using the transfer function method, which yields both the real (resistive) and imaginary (reactive) components. The resistive component relates to energy dissipation, while the reactive component relates to energy storage. Understanding surface impedance is fundamental for designing effective absorbers, particularly Helmholtz resonators and micro-perforated panels where tuning depends on the reactive impedance.