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Torvith Tortuosity and Characteristic Length Measurement System

Measurement of tortuosity and characteristic lengths

Tortuosity and characteristic length measurement

Nihon Onkyo Engineering provides a system that measures tortuosity, viscous characteristic length, and thermal characteristic length, three of the acoustic model material parameters that characterize the acoustic property of the material. These parameters can be used in STRATI-ARTZ acoustic material modeling software and in various simulation software using SEA or FEM calculations.

What is tortuosity?

Sound propagation in porous material

Tortuosity is a parameter related to the air that fills the porous material. This parameter indicates the complexity of the voids in the poro-elastic material. To be specific, tortuosity is determined as the ratio between the average length of the voids in the poro-elastic material and the thickness of the material. As the inner structure becomes more complex, the tortuosity becomes larger, and the same effect is expected as the thicker material.

Tortuosity measurement

Tortuosity measurement systemMeasuring tortuosity is done by measuring the sound velocity of ultrasonic waves transmitted in the air that fills the porous material. Accordingly, measurement systems include sensors for transmitting and receiving ultrasonic waves, an ultrasonic wave power amplifier, and an oscilloscope. Such systems are different from ordinary acoustic measurement system.

What is characteristic length?

Model image of porous material

There are two kinds of characteristic length, viscous characteristic length (VCL) and thermal characteristic length (TCL). Both are related to the shape of the voids inside the porous material, and both are parameters related to the structure of the fluid (air) that fills the porous material.
Viscous characteristic length is a parameter related to the effective density of the porous material and can be interpreted as representing viscous attenuation caused by the movement of the air in narrow voids.
Thermal characteristic length is related to the effective bulk modulus and represents attenuation caused by heat exchange between the air and the frame consisting material skeletal structure.

Model image of porous material

Microscopic photo of porous materialThe characteristic length measurement system is almost identical to the tortuosity measurement system. A big difference in the measurement compared with tortuosity is the need to perform two measurements, one with the porous material in air and another with the material in a different media, typically Argon. The two characteristic lengths can be determined using the Q-delta method from the results of the two measurements.


Method (Tortuosity) Ultrasonic sound
Method (Characteristic length) Ultrasonic sound (air, helium gas), Q-delta method
Frequency range 100k - 1000kHz
Operating condition Windows 10
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