Acoustic Resistance Calculator

Result

Enter values and press Calculate to see results here.

Calculator Inputs

Use consistent units. Missing fields are validated.

Reset

Calculation method

Pick the approach that matches your data.

Medium

Air uses engineering approximations.

Area, A (m²)

Cross-sectional area used for volume-velocity resistance.

Air temperature (°C)

Affects both density and sound speed.

Air pressure (kPa)

Used to estimate density by the ideal-gas law.

Density, ρ (kg/m³)

Typical air near 20°C is about 1.2 kg/m³.

Sound speed, c (m/s)

Typical air near 20°C is about 343 m/s.

RMS pressure, p (Pa)

Use RMS values at your test frequency.

RMS volume velocity, U (m³/s)

U equals particle velocity times area.

Quick interpretation

Higher resistance means more pressure for the same flow.
Specific resistance (Rayl) is per unit area.
Volume-velocity resistance (Pa·s/m³) includes area effects.

Example Data Table

Scenario	ρ (kg/m³)	c (m/s)	A (m²)	R_s (Rayl)	R (Pa·s/m³)
Air, 20°C, 101.3 kPa	1.204	343	0.010	413	41,300
Air, 0°C, 101.3 kPa	1.293	331	0.005	428	85,600
Water, typical values	998	1482	0.010	1,479,000	147,900,000

Values are rounded for readability; your results depend on inputs and assumptions.

Formula Used

Plane-wave (medium properties)

R_s = ρ · c (specific acoustic resistance, Rayl)
R = R_s / A (resistance to volume velocity, Pa·s/m³)
Air density uses ρ = p / (R_spec · T) (ideal gas).
Air sound speed uses c ≈ 331.3 + 0.606·T_°C.

Measured pressure-flow

R = p / U using RMS pressure and RMS volume velocity.
If area is given: R_s = p / v = p · A / U.
For reactive effects, impedance is complex; this tool focuses on resistance magnitudes.

How to Use This Calculator

Select a calculation method that matches your available data.
Enter area, then provide medium or measured inputs.
Press Calculate to display results above the form.
Use CSV or PDF export buttons for documentation.
Adjust inputs to compare scenarios and sensitivities.

Acoustic Resistance: Practical Notes

1) Meaning in one sentence

Acoustic resistance describes how much sound-pressure is needed to produce a given particle velocity through a surface or element. It is the real (lossy) part of acoustic impedance, and it models energy dissipation by viscosity, turbulence, and porous friction in air paths.

2) Two common forms

Specific acoustic resistance is written Rₛ = p/u and uses Rayl (Pa·s/m). For a finite opening of area A, the volume-velocity form is Rᵥ = p/U with units Pa·s/m³, where U = u·A. This calculator reports both so you can switch between “per area” and “per device” views.

3) Useful reference numbers

As a sanity check, compare to the characteristic impedance of air Z₀ = ρc. At 20°C, ρ≈1.20 kg/m³ and c≈343 m/s, so ρc≈412 Rayl. Well-damped absorptive surfaces often target Rₛ on the order of 0.2–1.0·ρc (≈80–400 Rayl) for strong absorption near normal incidence.

4) The role of area

Area matters twice: it converts u to U, and it changes how “hard” a fixed pressure drives the flow. If you halve the area while keeping the same u, U halves; if you keep U fixed instead, u doubles. Use the area input to match your real geometry: vents, perforations, panels, or absorber faces.

5) Frequency and level effects

Resistance is rarely constant with frequency. At low levels, viscous boundary layers dominate and resistance can scale roughly with √f in narrow gaps. At higher velocities, turbulence and jetting can raise resistance quickly. That is why the calculator lets you enter either measured values or derive R from pressure and velocity data.

6) Ducts, ports, and damping

In ducts and ports, acoustic resistance shows up as insertion loss and reduced resonance peaks. A small added R can tame ringing in Helmholtz resonators or muffler cavities. Too much R, however, reduces efficiency and can shift tuning. Design by checking a few candidate R values and observing the trend in results.

7) Measurement and unit tips

For measurements, take care with units: pressure in pascals (Pa) and particle velocity in m/s. If you measure SPL, convert to Pa using p = p₀·10^(SPL/20) with p₀ = 20 µPa. For velocity, use a particle-velocity probe or infer from volume flow U and area A (u=U/A).

8) Design workflow checks

Typical workflows are: (1) start from a known surface resistance (Rayl) from a material datasheet, (2) convert to volume-velocity resistance using area, then (3) compare scenarios for different temperatures or assumed air properties. If results look extreme, verify that area is not entered in cm² by mistake.

FAQs

1) What is the difference between acoustic resistance and impedance?

Impedance includes both resistance (real part) and reactance (imaginary part). Resistance represents energy loss, while reactance represents stored energy from mass and compliance. Many practical elements have both, especially ducts and resonators.

2) Which unit should I report: Rayl or Pa·s/m³?

Use Rayl when you want resistance per unit area (surface behavior). Use Pa·s/m³ when you want resistance for a specific opening or device (volume-velocity form). The two are linked by area: Rᵥ = Rₛ / A.

3) Why do my results change a lot when I change area?

Because particle velocity and volume velocity are connected by U = u·A. Changing area alters the conversion and therefore the “effective” resistance for a fixed U or u. Make sure the area matches the real exposed face or opening.

4) How can I convert SPL to pressure for the calculator?

Convert using p = 20×10⁻⁶ · 10^(SPL/20) pascals. For example, 94 dB SPL is about 1 Pa. Then divide by particle velocity to obtain specific resistance.

5) What values are considered ‘high’ resistance?

A common reference is ρc ≈ 412 Rayl at 20°C. Values much smaller than ~0.1ρc behave “leaky,” while values much larger than ~ρc behave “stiff” and can reflect sound strongly. Context and frequency still matter.

6) Does temperature affect acoustic resistance?

It can. Air density and viscosity change with temperature, which affects characteristic impedance and viscous losses in narrow passages. If you are comparing winter and summer conditions, update temperature-related inputs or measured values consistently.

7) Can this calculator model porous absorbers exactly?

It provides useful first-order resistance conversions and checks, but porous materials also have reactance and frequency-dependent behavior. For accurate absorber prediction, you typically need complex impedance models or measured normal-incidence data.