✓ Airflow Resistivity
✓ Characteristic Lenght (current)
✓ Open Porosity and Bulk Density
✓ Static Thermal Permeablity
✓ Thermal and Viscous Lenghts
✓ Tortuosity
✓ Viscoelastic Properties
✓ Young’s Modulus, Poisson’s Ratio and Loss Factor
What is the Characteristic Length?
From the Johnson and al’s theory and the ultrasound tortuosity measurement technique [1], the characteristic length can be defined as shown below:
Where:
- n(ω) is the refraction index (ultrasound tortuosity meter)
- c₀ is the speed of sound in air
- c(ω) is the speed of sound in the material
- γ is the specific heat ratio for air
- B is the Prandtl number for air
The characteristic length is linked to the viscous and thermal characteristic lengths!
How to Measure the Characteristic Lenght?
Equipment: Ultrasound Tortuosity Meter
Method: Evaluation of tortuosity in acoustic porous materials saturated by air [1]
Parameter: Δ∞ – Characteristic length
References
[1] J.F. Allard et al., « Evaluation of tortuosity in acoustic porous materials saturated by air, » Rev. Sci. Instrum. 65, 209-210 (1994)
Want to Learn More?
✓ Airflow Resistivity
✓ Characteristic Lenght (current)
✓ Open Porosity and Bulk Density
✓ Static Thermal Permeablity
✓ Thermal and Viscous Lenghts
✓ Tortuosity
✓ Viscoelastic Properties
✓ Young’s Modulus, Poisson’s Ratio and Loss Factor