Making the inside of a car quiet(er)

[cdixon6]

Ok, I have my own ideas but some of you have way more experience than I do so I have to ask.
My dad wants to make the cabin of his '69 firebird quieter. Now he doesn't want to extend his tail pipes(which cut off under the seat basically) so regardless what is done it'll probably be loud. He's begun by laying down a layer of foam insulation from OSH that is about 1/8" thick, one side is metallic so it holds its form. The insulation goes under the carpet, like dynamat, and covers the whole floor of the interior. He's done this because that is what you do with dynamat although I have my doubts as to the effectiveness of it. He thinks a lot of noise comes from the trunk.
Some ideas I've given him are to change the weather seals, try and make it air tight. For the trunk, I told him to put a layer of heavy foam behind the back seat to absorb that energy rather than rigidly seal it. Now that I'm thinking about it, how about a single ply with corrugated foam on either sides? There is no way you could fit a resonator behind the seat that would absorb low frequencies...(right?).
What kind of ways could you absorb that acoustic energy? Any ideas?

[goldanimals]

I'm not a scientist and I know nothing about insulating cars...but I would say loose the foam idea and somehow add a layer of Owens Corning 703 anywhere possible.

[oldguitars]

Dynamat...

[cdixon6]

I guess I forgot to mention we are looking for the cheap solution. Dynamat is way too expensive for this problem. And like most expensive and simple solutions such as dynamat (especially with car products), there is usually an equally effective and cheaper solution.

[Z-Plane]

Maybe try to figure out how much of the sound is due to mechanical coupling, because that might severely limit the effectiveness of any insulation/absorbtion if the whole interior is acting as a resonator. In general, I'd try to pinpoint the sources before trial and error with possible solutions. Here's one idea, perhaps good in theory but results may vary in practice - put the car on ramps and take a small PA speaker or similar, then blast sound at the interior from every angle you can and listen for the most leakage, or similarly from the inside out. That might highlight any areas where insulation could work, but I suspect you are ultimately looking at decoupling anything that vibrates, which is probably most things! Good luck.

[T-rex]

Dynamat...

I know it's not the cheap solution, but it really makes a big difference. I built a 66 Mustang with a hot rodded 302 and 3" pipes. I used Dynamat and fabricated a new baffle between the trunk and the cabin where there wasn't a factory one. That made a huge difference. And a loud ass stereo didn't hurt either!

Also, you could try this company. I can't speak for their pricing but they specialize in car silencing.
http://www.secondskinaudio.com/

I don't know of any DIY solutions for really cheap though. . .

[T-rex]

Honestly I think a solid baffle would probably work best behind the seat if there isn't one. Mine was solid wood covered with material and I used it to block sound from the trunk and for an amp rack. You could try two pieces of thin wood with a layer of green glue between maybe? I don;t know how that would work in a car environment but that's not too expensive.

[kslight]

Best free solution is probably to avoid starting the engine.. Also a long shot but keep the windows shut too...




To me it sounds like something that would be difficult to effectively accomplish on any budget, let alone low budget. But I agree with others that finding the worst offending vibrators and attempt to mechanically decouple, then if that isn't effective enough start looking at insulation. I really doubt the effectiveness of an 1/8" sheet of foam beneath the carpet..

[cdixon6]

Thanks for the tips. The only thing between the back seat and the trunk right now is the original felt like blanket (two holes punched in it for some reason). Luckily, it's not vital that the car be quiet, it is a muscle car anyway, and not a daily driver. It would just be nice to get out after a cruise and not have ringing ears. I imagine a baffle would help a bit.

[T-rex]

Yeah with the pipes cut off, it is going to be really tough. I totally understand the reasoning and coolness factor, but the point of running them to the back of the car is to get the noise (and fumes) away from the passengers. Dynamat (or something) on the floors and a baffle will help. If you can afford any dynamat at all it will do wonders for lowering noise behind the door panels AND will in effect seal the doors giving you better bass respoonse if you have any speakers in the doors.

[timcoalman]

Not sure if you thought of this ?
I installed car stereo systems in my youth and there is a huge industry of also modifying the car to handle the high volumes (front windshields were known to pop their seal and fly out with all that pressure). You might try some message boards of car audio groups. I am sure they will recommend the standard industry products but just like home recordists there is a group of ?shade-tee installers? who have tons of inexpensive/home-made/innovative solutions that do the same thing as industry products.