chipperman

Like some of you, I have been annoyed at the ambient sound level while cruising. I will be taking a long trip next month, and after riding in an M Roadster on the freeway and noting how quiet it was, I decided to do something about it. I always soundproof my cars, and over the years have found ways to maximize the benefits with minimum weight, though not always cost.

First, I had to isolate the source of the most sound, and this was done with the top up, as most of my long distance driving will be done this way. Because of the aerodynamics of the car, very little sound intrusion comes from the doors, a source of irritation in most cars. So I used the rather crude method of cupping hand to ear and adjusting the angle to seek out the source. Not suprisingly, most of the noise comes from the rear, in the area of the top well. There is a cacaphony of sounds blending tire, gear, exhaust, and fuel pump harmonies. This results in almost a sound like amplified white noise.

The first step was to remove the trunk liner panels. It was obvious that Honda sought to address the problem in some areas, but all factory NVH attempts are dollar driven. I also wanted to limit the installed material weight to no more than ten pounds. Since I run without the spare tire, I felt this was not a big detriment. One problem is that the fresh air exhaust passes behind the seats through a crude flap of felt, and then out the rear of the trunk by the license plate. Whatever noise is generated in the trunk is forced back through the vents like a ported subwoofer, so I concentrated on eliminating noise in the trunk area.

A very easy way to diagnose panels prone to resonance it to drum your fingernails along until you hit a loud spot. This turned out to be the fender wells, the inside of the rear fenders, the outer edges of the tool well, and particularly the spare tire storage area floor. If the spare is installed, it acts as a damper, but with it removed, there is a huge amount of ringing, as it amplifies noise from the differential. I covered these areas with a plastic/aluminum damper sheet used in jet aircraft. It is not required to cover the whole area, just with enough material to stop flexing of the substrate, usually by putting some in the middle of the panel. I then stuffed polyester batting in the holes adjacent to the fresh air grilles, and in the areas to the far sides of the bulkhead. This serves to slow down sound waves, and isolates the fresh air from the rear speakers I installed. One immediate benefit is that the bass is significantly boosted from the rear speakers. This material is light and fluffy and dirt cheap at any fabric store.

Where the factory did not cover the metal or the bottom of the rear tray with felt, I applied 1/4 inch adhesive backed open cell foam. I use a special aerospace foam designed to work in conjunction with the damper mat, although neoprene can give almost the same effect.

After reinstalling the liners, I went for a test drive. Impressions:
THIS IS A NOISY CAR! The gear whine, tire , and fuel pump noise are gone. What is now apparent on its own is the exhaust note. It comes through the top, and seems to come in around the glass. Cruising at 60 is much quieter, and now it is easy to hear the wind noise. From about 4000 rpm and up, the induction roar and exhaust note are very distinct. They do not appear to be coming in through the body, but over the top from the outside. A lined top would be helpful.

In summary, it was well worth it, as there is no noise coming through the top well anymore. I will probably install acoustic foam in the door panels next, as it weighs less than 1/2 pound per door, and will eliminate any high frequency noises such as wind whistles.
I have not pulled up the carpet, but it doesn't feel like there is much under there. A vinyl/foam decoupler would kill any resonance present in that area. A side benefit is that I can listen to the radio at a lower setting.

If you think about flaming, consider that that car's noise character hasn't changed; It's not a Lexus, and is plenty loud, especially in VTEC, but fixed throttle crusing is a lot easier on the ears with minimal weight gain. If anyone is interested in a lot of detail, PM me and I'll try to help. Mike

koala

I like what you've done! I've gone to an aftermarket intake and exhaust - and as much as I love them, I'm thinking of selling them to go back to stock, and to possibly do some of the things you've done.

The car sounds AWESOME right now, but it goes get to be a bit much.

Kodokan_4

The description of your project is adequate, but it's a shame that you didn't provide pics of your installation, which would greatly assist others interested in attempting a similar project.

Many of the aftermarket exhaust systems have resonance issues, and further information on how to properly insulate the car, to minimize the unwanted sound levels would be appreciated by many owners.

Further information on where to obtain these special insulation materials would also be extemely helpul.

-Pete

seung

pics please!

chipperman

I've got to get the digital up and running. It's in storage now as I am remodeling, but I can describe the procedure if you want.
Pete, you are fortunate as a lot of these materials can be purchased in small amounts near you. Look for airplane and helicopter upholstery shops. There are a lot of them near LAX. I used the last of bulk rolls I had to do this project. If you have a specific type of noise you need to kill, I can recommend product.
There is a manufacturer of neoprene in Lomita, I think, where I used to get "skins" for free. These were shaved off blocks of neoprene when it came out of the mold, and work great under the carpets for a low budget fix. If you've got the money, EAR Corp. makes a huge variety of dampening products. Look them up on the web and talk to a technical guy. I've found that different types of cars need different things. Sorry again about the pics, I'll work on it. This is a fun 4 hour project.

Kodokan_4

I am definitely going to research this stuff.

I've got a serious resonance problem with my new exhaust system which occurs only between 2000 RPMs and 4700 RPMs.

I'm working on the exhaust system itself, but sound dampening is definitely on my to-do-list.

Thank you very much for the info!!

-Pete

chipperman

Pete and others: This exhaust resonance thing is real tough to deal with. You are getting a big pipe to vibrate at a certain amount of flow, just like a tumpeting elephant. But the trunk area of our cars acts like a guitar soundboard, amplifying the noise many fold. For your situation, you must use a decoupling mat in at least three areas. Under the bottom of the tool well, you can see where the "feet" of the liner contacts the metal of the car body. Honda has left dampening material off this area for some reason, so all the vibration gets transferred to the plastic liner. Rap your fingers on the liner and you will see the affect this has on noise. The second area is on either side of the well. You will see where there is a factory applied damping mat, but it doesn't cover the full area, nor does it decouple the sound. Cover this area fully. The third, and huge area, is the bottom of the spare tire well. When you take the tire out, thump on this and you will hear how loud it is. One of the interesting things about sheet metal is that it will vibrate like a wobble board (listen to Tie Me Kangaroo Down, Sport) when vibrated by sound or air movement. Generally, you won't hear it as such, but the sound it transmits goes everywhere. To stop it at the source, you must damp the movement. The foil backed dampers, and Dynamat is a poor example, will do this. Then you must break up the sound. Decoupling mats do this by allowing some sound through to the heavy vinyl layer, where the low frequencies are absorbed, and the highs reflected back through the foam layer. That sound is attenuated in the same manner as a muffler or silencer for a firearm; that is, noise is dissipated by increasing the path it has to travel. I think for your case, and in small quantities, JC Whitney has laminated sound matting. The 1/4 inch to 3/8 inch stuff will do wonders for you. If you take the trunk liners out, a lot of this will be very obvious. And don't worry about cutting full pieces of material, piecing it works just as well and saves money. You might want to look at the fenderwells, too, and notice that Honda spent more time quieting the passenger side one for some reason. Good Luck.

Kodokan_4

Do I need the foil backed damper, and the decoupling mat, or just the decoupling mat?

I'm going to take some measurements, and order some!

This is great info, and I don't think many of us have knowledge in this area, so thank you for your assistance on this!

-Pete

ninegrand

I like the idea of your project! Quick question though, are you you saying the Dynamat product would NOT be good to use for this? I don't have any local resources for the material you speak of, but I know the audio stores around have the Dynamat product.

Thanks for the idea!

Paul

mas

Great post!

What would you suggest for those with subwoofers in the trunk? The goal would be to somehow seal the trunk but still maintain the sound path between the trunk & the cabin. I was thinking of putting something under the rear deck (where the folded top sits) & on top of the fuel tank area and under the spare.

I'm not sure what would be a good damping material, I've never heard of the stuff you've used.

Thanks!