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
Did You Know?

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

Transmission Method for Tortuosity Meter
Reflexion Method for Tortuosity Meter

References

[1] J.F. Allard et al., « Evaluation of tortuosity in acoustic porous materials saturated by air, » Rev. Sci. Instrum. 65, 209-210 (1994)

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