I've had my 12" sub and amp in my xB for a little while now, but have yet to soundproof the license plate/hatch.

What is the cheapest and easiest way to do so?

 

spray undercoating

 

the 3m undercoating is a great place to start($4), but the best stuff i've used is peel n seel. Lowe's has some generic that comes in pretty bigg roll for just 13$, no stink all stick. It's the same as the first gen dynamat.. it's in the roofing section and its silver.. 11 rolls n 3 cans in my b, halla

 

sorry to repost but, walmart has polyfill which is whats used to stuff pillows and its nonflammable i got the 5lbs box and stuff the entire thing in the hollow hatch, works great for 7-8 bucks..

 

Or if you have an old poly filled blanket, tear it open and stuff away. I have an entire queen sized comforter in my hatch and my 2 rear compartments where the tails are mounted. I got the idea from Jeff in NJ years ago, and it works great.

 

You can stick some closed foam weather stripping on the back of the license plate. I stuck some strips on there and it really quiets things down. The hatch can be quieted down with e-dead sound deadening and some of that insulation that goes between walls.

 

^^ i just put a rubber grommet behind it.....my problem is roof flex LOL

 

There's a few things you can do that are quite cheap.

1: Closed cell foam behind the license plate
2: Polyfill the back hatch cavity (if it rattles on the outside, who cares... it's your music, not theirs)
3: Peel 'n Seal, it's very cheap and works very well in layers

If you do all three, you will achieve the best results. Don't expect the P&S to be the miracle cure though... the adhesive on it is not designed to withstand 130 degree temps on a vertical surface. If you go with a sound deadener that is made of butyl rather than asphalt, you will pay a pretty penny, but the deadening will be a massive improvement. With the P&S, you should do at least three layers to achieve the best results. You can also pick up some wood floor underlay (1/8" is pretty cheap for a 50' roll at 48" wide). The floor underlay can be used as a closed cell foam for under the carpet and behind panels. This will help eliminate the high pitched noise from the road.

I've done my entire car twice over. Multiple layers of deadening on the critical areas and a single layer everywhere else. The back hatch is filled with polyfill as well as the area behind the taillights. I've put down a layer of floor underlay in every spot I could with the exception of the roof. The result is very dead. I can have my stereo blasting with the doors closed and windows up... you can't much two car lengths away. I open the door/roll down the window and you can feel the bass hit pretty hard.

 

I bought 2 big packages of the poly fill today at walmart, about 5$ total. I stuffed my whole rear hatch behind the plastic interior cover. I can already tell the difference when closing the hatch or locking/unlocking the rear. It doesn't have that hollow loud noise anymore. I had stuffed the panels behind the tail lights and all in the sides when I had all the plastic out redoing all my airride.

 

Awesome guys, thanks for all the ideas.