Acoustic material characteristics measurement
Quantitative design of acoustic material characteristics
Nihon Onkyo Engineering is currently developing technology for predicting the acoustic characteristics of porous material based on the Biot model. Applying this technology will allow us to measure or predict the basic parameters of materials and to estimate the acoustic absorption performance and sound insulation performance of the materials.
Nihon Onkyo Engineering has already developed equipment and measurement technology capable of measuring most Biot model material parameters that characterize the acoustic performance of material. These equipment and technology have been used for experiments requested by clients and for consulting. We also have measurement equipment capable of measuring the normal incidence sound absorption coefficient, the reverberation absorption coefficient, and sound transmission loss. We strive to improve the accuracy of measurements and to develop new measurement technology through research and development activities in order to satisfy a wide range of customer needs.
Acoustic material design example: Design flow of interior automobile material
Not limited to just the automobile industry, when conducting equipment design, the target specifications for the overall equipment is determined first, after which a top-down approach breaks down the overall objectives into individual component levels. Actual development, however, is done from a bottom-up approach. Depending on the production phase, the required measurement techniques and prediction techniques are different.
Phase 1: Design and evaluation of single-layered material
Acoustic material development starts with the development of single-layered material. Developing single-layered material that has sufficient absorption and sound insulation performance is important, as is identifying the parameters that characterize the material's acoustic properties. Accurately defining these parameters enables prediction of the acoustic performance of the material when organized into multiple layers. These parameters are called Biot model material parameters and are used to build a complete acoustic model of poro-elastic material. Nihon Onkyo Engineering has equipment capable of measuring these parameters in our laboratory, which is a rare accomplishment in Japan. The Biot model includes nine (9) parameters, including the two general parameters of thickness and density. The other parameters--porosity, flow resistivity, tortuosity, viscous characteristic length, thermal characteristics length, shear modulus, and Poisson's ratio--are exclusive to this model. The normal incidence sound absorption coefficient is measured lastly to verify the modeling result based on the Biot model.
Phase 2: Design, evaluation, and optimization of multi-layered structures
The sound absorption and insulation performance of multi-layered material can be predicted using the transfer matrix method if the Biot parameters for each layer of the multi-layered structure is obtained. Using this technique, Nihon Onkyo Engineering's proprietary STRATI-ARTZ software can predict the acoustic performance of both single and multi-layered material and contributes significantly to optimizing multilayered structures. As an experimental approach, this method is also effective at measuring the normal incidence sound absorption coefficient. In addition, if the material will ultimately be used as an interior material, evaluation is generally performed based on the reverberation absorption coefficient, which is close to the actual acoustic wave incidence condition.
Phase 3: Evaluation of molded parts, and optimization of shape
Interior automobile material is press-formed toward the end. During pressing, holes for wire harnesses or ducts are opened, or the thickness of the material may not be even after pressing, preventing the same performance as in Phase 2 from being obtained.
In general, sound absorption performance is evaluated using the absorption coefficient in a reverberation chamber. However, for product performance evaluation, determining the absorption coefficient can be difficult, so evaluation is generally performed using sound absorbing power. Sound insulation performance is evaluated using sound transmission loss. These measurements are very important for obtaining the correct assessment result and require advanced know-how to perform. However, simply figuring out and controlling areas with weak sound insulation abilities is not enough.
To obtain further detailed information, Nihon Onkyo Engineering evaluates the sound intensity distribution close to the surface of a specimen installed in an opening between a reverberation chamber and an anechoic chamber. We also may use Noise Vision for sound source identification and visualization. Such advanced analysis allows us to obtain information that is directly related to appropriate countermeasures.
Phase 4: Performance assessment using an actual vehicle
The final phase of development is performance evaluation of the material installed on the test vehicle. During measurement using a test vehicle, various factors other than the developed products are involved. Because automobiles have many parts, it is first necessary to remove many other parts to install the developed product. Care must be taken, as the measurement results may be affected by inadvertent changes to the supporting conditions of material while installing or removing parts.
At this phase, Nihon Onkyo Engineering may apply contribution analysis (which is sometimes called acoustic sensitivity analysis) and acoustic source identification and visualization (Noise Vision). Also, to evaluate the sound insulation performance of a dash panel against engine noise systematically, we will install a cut automobile body in the opening between the reverberation chamber and anechoic chamber.