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# Characteristic parameter

The characteristic parameter is linked to the viscous and thermal characteristic lengths. If one of the characteristic length is known, or one of their ratio, both characteristic lengths can be determined from the characteristic parameter. From the Johnson and al’s theory and the ultrasound tortuosity measurement technique [1], the characteristic parameter can be defined as shown below.

Equipment: Ultrasound Tortuosity Meter
Standard: None
Method: Evaluation of tortuosity in acoustic porous materials saturated by air [1,2]
Property: Δ – characteristic parameter

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