What is the Characteristic Length?
From the Johnson and al’s theory and the ultrasound tortuosity measurement technique , the characteristic length can be defined as shown below:
- 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 
Parameter: Δ∞ – Characteristic length
 J.F. Allard et al., « Evaluation of tortuosity in acoustic porous materials saturated by air, » Rev. Sci. Instrum. 65, 209-210 (1994)