Porous Absorber Calculator
Model an absorption-coefficient-vs-frequency curve from flow resistivity, thickness, and air gap using the Delany-Bazley-Miki model.
Delany-Bazley-Miki porous absorber model — flow resistivity, thickness, and an optional air gap behind the material.
Absorber thickness5cm
Air gap behind absorber0cm
Material · flow resistivity σ
Pa·s/m²
Absorption vs. frequencyNRC ≈ 0.70
NRC ≈ 0.70 for this absorber build.
Preview only — open the 3D builder for full placement, treatment, and analysis.
How the model works
The Delany-Bazley-Miki model predicts a porous material's characteristic impedance and complex wavenumber from a single measured property: flow resistivity. From those, a transmission-line transform through the layer (optionally over an air gap) to the rigid wall behind it gives the surface impedance, and from that, the absorption coefficient at any frequency.
- Flow resistivity (σ): higher values mean denser, stiffer material — light fiberglass and rigid mineral wool sit at very different points on the same absorption curve.
- Thickness: thicker absorbers push the useful absorption band lower in frequency.
- Air gap: spacing the absorber off the wall extends low-frequency performance without adding material.
Frequently asked questions
- What is flow resistivity in acoustic absorbers?
- Flow resistivity (σ, in Pa·s/m², also written rayl/m) measures how much a porous material resists airflow through it. It is the single input the Delany-Bazley-Miki model needs to predict a material’s complex characteristic impedance and, from that, its absorption coefficient at any frequency.
- Why does an air gap behind a panel improve low-frequency absorption?
- An air gap moves the absorber away from the rigid wall, where particle velocity (not pressure) is highest. Because porous absorption works by converting particle velocity into heat via viscous friction, a gap shifts the panel’s peak absorption lower in frequency without adding material thickness.
- What is the difference between normal and random incidence absorption?
- Normal incidence is the absorption coefficient for sound arriving straight-on (0°). Random (or field/diffuse) incidence averages absorption over every angle a real reverberant room actually presents to a panel, weighted by solid angle (Paris’ formula) — it is usually a better match for published NRC ratings and real-room behavior.