Input Parameters
Example Data Table
| Case | Thickness (mm) | Air Gap (mm) | Resistivity (Pa·s/m²) | Frequency (Hz) | Predicted Absorption |
|---|---|---|---|---|---|
| Studio Wall | 50 | 50 | 12000 | 1000 | 0.86 |
| Machine Enclosure | 75 | 25 | 18000 | 500 | 0.63 |
| Ceiling Raft | 40 | 100 | 10000 | 250 | 0.58 |
| Duct Liner | 25 | 0 | 22000 | 2000 | 0.79 |
Formula Used
Zc = ρ0c0[1 + 0.0699(ρ0f/σ)-0.632 − j0.1070(ρ0f/σ)-0.632]
kc = (2πf/c0)[1 + 0.1093(ρ0f/σ)-0.618 − j0.1603(ρ0f/σ)-0.618]
Zin = −j(Zcky/kc) cot(ky d), with cavity loading included when an air gap exists.
R = (Zin − Z0)/(Zin + Z0), α = 1 − |R|2
This calculator uses an empirical Delany–Bazley–Miki style equivalent-fluid approach for porous absorbers under normal or near-normal incidence. It is useful for early design, comparisons, and material tuning, but not a substitute for lab impedance-tube testing.
How to Use This Calculator
- Enter absorber thickness in millimeters.
- Enter optional air gap depth behind the absorber.
- Provide measured or published flow resistivity.
- Choose the target design frequency and incidence angle.
- Fill density, porosity, tortuosity, and installed area for practical context.
- Submit the form to view absorption, reflection, impedance, and NRC estimate.
- Review octave-band style rows for broad performance trends.
- Use CSV or PDF export for reports or design comparisons.
Frequently Asked Questions
1. What does a porous absorber do?
It converts sound energy into heat through viscous and thermal losses inside pores. It works best when airflow can enter the material and travel through enough thickness.
2. Why is flow resistivity important?
Flow resistivity controls how easily air moves through the absorber. Very low values under-damp the wave, while very high values can reflect sound before enough energy dissipates.
3. Why does an air gap improve low-frequency absorption?
The gap moves the porous layer away from the rigid wall, placing more particle velocity inside the absorber. That usually improves absorption around lower frequencies.
4. Is NRC the same as broadband absorption?
Not exactly. NRC averages only four octave bands, so it is convenient but incomplete. Always inspect the band-by-band values before selecting a product.
5. Can I use this for foam, mineral wool, or fiberglass?
Yes, when you know reasonable airflow resistivity and thickness values. The method suits many rigid-frame porous materials used in room and enclosure acoustics.
6. Does incidence angle matter?
Yes. Oblique incidence changes the effective wave path and surface impedance. This tool includes angle input, though normal incidence remains the most common quick-check case.
7. Are density, porosity, and tortuosity used directly?
They are included for engineering context and reporting. In this simplified empirical model, flow resistivity and geometry dominate the direct calculations.
8. When should I use measured laboratory data instead?
Use lab data for specifications, compliance, guarantees, and final procurement. Measurements capture real mounting, facings, anisotropy, and manufacturing variation better than simplified models.