Dynamat is a sound deadening product, often used in vehicles ...
I have this product in my car. Expectations have to be set properly when using this stuff. It's primarily intended to address sympathetic resonance, such as that arising from hollow body cavities and long, unsupported body panel spans, rather than primary resonance, i.e. the source of the sound. It accomplishes this primarily by just being heavy. Heavy objects have a lower resonant frequency and require more energy to resonate. For a given component, adding dynamat pushes either or both of these parameters outside the range of human detection.
You can pretty much tell if dynamat is going to help or not by just rapping on things with your knuckles. If you get a ping, twang, clang, boing, crash, or boom, slap some on. If you get a click or thud, skip it.
True sound deadening requires highly energy-absorbent materials, like carpet, dense foam, and fiberglass batting, along with complete mechanical isolation including air passageways. So that means no free hot air from the engine compartment in wintertime, or even really any ventilation of any kind.
