Last Updated: Jul 20, 2017 Views: 30
Every type of material - glass, metal, plastic, etc. - will reflect some sound and absorb some sound. Harder, stiffer kinds of material, like glass, tends to reflect more sound, while plastic absorbs it. The windshields and side/back windows in cars actually contain both glass and plastic, because safety glass is two pieces of glass stuck together with a special soft transparent adhesive. Automotive engineers have understood for decades that this laminated stack can be tailored to reduce the amount of noise inside a car. But in the past few years, as engines have become quieter and more efficient, the role that the glass/plastic laminate might play to also make the cabin quieter, and to reduce the weight of the car overall, has led to new research.
The glass in your car is a result of that engineering research into optimizing the sound deadening capabilities of the laminated safety glass, likely be playing with the details of the plastic adhesive layer, and the thicknesses of the glass. Of course, all of this is certainly done in a way to still pass safety rules.