There was no padding under the hatch area carpet on my 84S USA. I ordered some B-Quiet V-Comp which is a multi-layer foam sound transmission blocking pad about 3/8" thick. It costs about the same as a factory pad approx $65. I also had some left over B-Quiet ultimate (like dynamat) that I applied to the hatch area floor under the pad first.
I used my Radio Shack sound meter that I have for my audio system to measure my sound levels.
Results: Before sound treatments Idle 84db, 3000RPM 90db
After sound treatments Idle 82db, 3000RPM 87db
1992 MB 600SEL (control) Idle 71db, 3000RPM 77db
Measured windows and sunroof closed.
Doesn't sound like much but the scale is logarthmic and I think 3db is half the sound energy (whatever exactly that means.)
FWIW
Dave McK.

 

Several Rennlisters have experimented with sound proofing. This is the first instance I am aware of where the improvements were actually measured. Very interesting work, Dave. Can you provide some details (or a link) re: the Radio Shack tool?

 

logarithmic or linear sound measureing means that every 3db is considered to be twice as loud. So, if your base value is 80 dbA and you decrease that value to 73 dbA it is considered to be half the volume.

Also the weighting on the meter is an important thing to consider, the radio shack meter and all other good meters have two weight settings, A and C. The A weighting is done to measure a sound that is more in the speech to symbols range of the sound spectrum. The C weighting is done for the voice to bass range of the sound spectrum.

Most 928's with the factory exhaust have a very deep rich tone that penetrates all living things if sitting in the garage with the door open and running, this is probably the noise that most try to insulate against. The back end of the car being such a hollow resonance chamber really helps to add to this effect.

You could build a bass trap and set it in the rear quater panels inside the car and it should help to absorb some of the bass frequencies from the car. You could also go to a Guitar Center type store and purchase some Auralex foam, they make an excellent bass trap unit that would not add any weight to the vehicle and are reasonably priced for this type of a project.

 

Hi,
I used a Radio Shack Digital Sound Level Meter No. 33-2055. There is an analog meter No. 33-2050 which some prefer. Either is in the <$50 range and can be purchased at RS stores or their website. I use it to tweak my 5.1 surround sound system but I'm happy to find other uses for it. I used C weighting and slow response for my car measurements.
The observation about the bass exhaust note is accurate; it is penetrating and not entirely objectionable. I just wanted to reduce the overall noise level.

 

AMEN! I am hating my Yoko's more and more each passing mile!
(I am just waiting for the chance to do some good burnouts as I have a set of 275 Fuzion's sitting in the garage ready for mounting!)

 

I used some reflectix under the hatch carpets, it deadened the sound noticeably, is waterproof, and pretty cheap.

I will also be using it under my hood, I think.

 

I have done my entire 78 interior with Raamat. It is the 60mil stuff with the silver face. I will t hen cover that with a product called ensolite, and that with carpet. Should be as quiet as a lexus when I want to hear the radio.

 

Dave,
Where did you order the foam? Sounds like a great improvement. The factory foam on my '83 under the rear carpet is turning into powder and I'd like to replace it.
Thanks
Rick

 

Hi Rick,
Go to B-quiet.com. They have stick-on dampening material called Ultimate and foam-based sound transmission blocking pads call V-comp. One piece will do hatch area. Fast service.
Good Luck.
Dave

 

A slow response db meter is good for determing RMS levels versus the fast response for peak pressure levels. To lower the overall level of the car I would recomend the thicker foam as it will deaden the vibrations caused by the panel that it is attached to.

After reading this first post I went out to the car and noticed that there are a lot of resonate chambers in the back of the car, the rear interior panels, the spare tire area and the nice open hatch area to name a few.

If you do not have any luck with the first attempt just try a thicker application or add more onto the existing. Another thing you could try would be to put different densities and thicknesses to help block certain frequency ranges.