The idea that SMC/BMC thermoset can block noise is often misunderstood. Noise travels as waves. The source of the noise vibrates the air around it, and this energy wave then travels through the air to our ears. So to block noise, there are several approaches: Reduce the source that is generating the noise, absorb or redirect the noise, or be far away from the source so that the energy/noise is too weak to be heard by the ears.
The way SMC/BMC thermoset blocks noise is like a good window that shuts out noise from the outside. The majority of the sound energy is reflected away. Because the glass in the window is dense, it absorbs some of the remaining energy. Although the absorbed energy can still be heard, it is quieter. But if three windows are facing the street, all three windows need to be closed—not just one. Similarly, in application, the SMC/BMC thermoset part must completely enclose the noise source and not allow any gaps through which sound waves can pass.
Once the noise source is completely enclosed, the majority of noise reduction is achieved. For further reduction, the next step is to optimize the material itself. There is, however, a trade-off. If noise blocking is optimized with high-density fillers, then the molding performance may be reduced and cost may be affected. In one extreme experiment, metal filings were used to increase the density of the molding material, but the metal filings were so heavy that they dropped out of the resin as the resin moved forward to fill the mold.
An SMC/BMC thermoset part has effective noise-blocking properties. But it has limitations, too—limitations that, along with the trade-off in material properties, must be understood in order to save time and money.
