It's used for both. Most effective use of the butyl/aluminum stuff is use in patches to reduce vibration & dampen sound in body panels. It's popular to cover the entire surface, but its not necessary for sound deadening. A relative small piece in the middle of a surface does the job. Use of layered sound deadening material such as the acoustical foam matting over the butyl/alum stuff is most effective. Here's a great place to begin the research. When used as designed, you'll require less deadener & have far better sound insulation.