Currently we are constraining prediction techniques for acoustic characteristics of porous material based on the Biot model. By applying this technique, it is possible to measure or predict the basic parameter of the materials and estimate the acoustic absorption performance and the sound insulation performance of the materials. We have already developed a facility and measurement technology to measure almost all the material parameter used by the Biot model, to characterize the acoustic performance of material. These facilities and technologies have been used for experiment requested by client and consulting. Also, we have the measurement facilities to measure for normal incidence sound absorption coefficient, the reverberation absorption coefficient, and sound transmission loss. We try to develop accuracy improvement for measurement through research and development activity and develop new measurement techniques to satisfy a wide-range of our customer's requirements.
It is not limited to automobile industry, when conducting design of the product, the target spec for the product is determined first and takes a top-down approach which breaks down the overall objectives into each part level. On the other hand, the actual development is made by the bottom-up approach. Depending on the production phase, required measurement technique and prediction technique is different.
Acoustic material development starts from development of single-layered material. It is important to develop single-layered materials which have sufficient absorption and sound insulation performance, and to identify the parameters to characterize acoustic property of them. With the characteristics which represent the acoustic performance accurately, the acoustic performance of the multi-layered material can be predicted. These parameters are called as "Biot parameter", which are used to build the complete acoustic model of poro-elastic materials. We have the facility to be able to measure these parameters in our laboratory. It is rare to have such facility in Japan.
Usually, Biot model uses eight parameters to describe. They are including thickness and density which are general parameters. Other parameters are special for this model, such as porosity, flow resistivity, tortuosity, viscous characteristic length, thermal characteristics length, shear modulus and Poisson's ratio.
Finally the normal incidence sound absorption coefficient is predicted to verify the modeling result based on the Biot model.
The sound absorption and insulation performance of the multi-layered material can be predicted by transfer matrix method, if the Biot parameters for each layer consisting of multilayered structure are obtained. By using this technique, our original software called "CAPMLS" can be used to predict the acoustic performance of both single and multi-layered material. And it is contributed significantly to optimize multi-layered structures. As an experimental approach, it is effective to measure the normal incidence sound absorption coefficient here as well. In addition, if the final purpose of the material is an interior material, it is usual to evaluate the reverberation absorption coefficient, which is close to the actual acoustic wave incidence condition.
Interior material of automobile is pressed finally forming. There are some deficient cases such as there is a hole to put through wire harness or duct, and thickness of material is not uniform, it may not demonstrate the same performance obtained in Phase #2.
In general, sound absorption performance is evaluated by the absorption coefficient in reverberation chamber. However for product performance evaluation, it is difficult to determine the absorption coefficient, so it is generally evaluated by sound absorbing power. Sound insulation performance is evaluated by sound transmission loss. These measurements are very important to obtain the correct assessment result while requiring advanced know-how to measure, but it is not enough to figure out and control the weak parts for sound insulation.
To obtain further detailed information, we evaluate the sound intensity distribution close to the surface of specimen which is installed at the openings in between the reverberation chamber and anechoic chamber. Also, the sound source identification and visualization by "Noise Vision" can be used. Such advanced analysis can be contributed to obtain the information which is directly related to countermeasure.
The final phase of development is performance evaluation of the materials installed to the test vehicle. In the measurement using the test vehicle, various factors are related other than the developed products. Since an automobile has many parts, it is necessary to take out many other parts to install the developed products.
We have to be careful because the measurement result may be affected by changing of the supporting conditions of materials while installing or taking out these parts. At this phase, the contribution analysis (it is sometimes called acoustic sensitivity analysis) and the sound source identification and visualization using "Noise Vision" can be applied. Also, to evaluate the sound insulation performance of the dash panel against the engine noise systematically, automotive cut body is installed to the openings in between the reverberation chamber and anechoic chamber.