Got this free with new speakers, so opted to use it during the install.
With the factory speakers, when cranking up the volume, I could hear the door outer door panel resonate when outside the car. After installing the new speakers and this Deadener, while I got my music cranked, and it seems like it's louder than I was playing the old factory speakers, I no longer hear the resonance of the outer door skin when outside the car.
So, it seems to be doing its job.
However, time will tell if it stays in place or falls off. Or worse melts and creates a mess.