Back wave distortion, a common problem with every speaker, occurs when your speaker sends the same sound out the back as it does out the front! If this rear sound wave energy is allowed to bounce back and return to the speaker, it interferes with the … see full wiki
I bought a kit and used them behind in-wall speakers to break up the back wave and tighten the sound. I put a sheet of Dynamat on the drywall behind them, then glued these squares onto it using the provided superglue. The glue came out so fast that I ended up using it all on one square (there are two in the package), so make sure you have some standard superglue on hand before you start.
You can also staple them to the surface if what you are applying them to isn't too hard (like drywall, in my case). This won't work for a standard metal car door, however. In that case you'll want to make sure you've got some good glue to stick it in with, and enough of it to cover both squares in the package.
One thing to be aware of is that these are smaller than you might think. That is a good thing if your speaker is small, like a standard door speaker, but check the measurements and make sure they are a good fit for your application.