Among the material parameters of Biot model which characterize the acoustic property of poro-elastic material, we provide the system to measure Tortuosity, Viscous Characteristic Length and Thermal Characteristic Length. These parameters can be used in our acoustic material modeling software, as well as in CAE software such as Auto SEA.
The sound propagation in the air (left) and in a porous material (right).
Tortuosity is related to the airborne sound component.
Tortuosity is a parameter which is related to the fluid that fills the porous material (air), and indicates the complexity of apertures in the poro-elastic material. To be specific, it is determined in the ratio between the averaged length of the apertures in the poro-elastic material and the thickness of the material. As the inner structure becomes more complex and the tortuosity becomes higher, the same effect to the thicker material will be expected.
Tortuosity can be measured by measuring the velocity of sound which is transmitted in the fluid that fills the porous material (air), in ultrasonic domain. Therefore, the measurement system is composed of sensors for transmitting and receiving ultrasound, power amp and oscilloscope. They are different from the ordinary acoustic measurement system.
The energy loss inside the porous material
(the loss by heat occurs in the part where the cells are stretched, and the loss by viscosity occurs in the narrow part)
The characteristic length has two kinds, "Viscous Characteristic Length (VCL)" and "Thermal Characteristic Length (TCL)", that are related to the shape of the apertures inside the porous material.
Each of them is the parameter which is related to the structure related to the fluid that fills the porous material (air).
The measurement system and method for the characteristic lengths are almost same as the measurement system of the tortuosity. The greatly different point during the process of the measurement is that we have to perform two measurements, two cases that the porous material is put in the air and in another gas. Therefore, the holder to place the measurement sample is required to set in the container which is possible to replace the gas inside. From the measurement result of the two cases, the two characteristic length can be determined using Q-delta method.
| Method (Tortuosity) | Ultrasonic sound |
|---|---|
| Method (Characteristic length) | Ultrasonic sound (air, helium gas), Q-delta method |
| Frequency range | 200 - 1000 kHz |
| Operating condition | Windows 2000 / XP |