sound proofing 230 SL

[Rodolfo]

Hi,

I hope I can get some advices or experiences.

Since I bought last year a 230 SL from 1966, I did already lots of repairs and improvements on what was basically already a good car.

I have a lot less noise already then those first days.

Here is what I did:

1) front: changed grommets through firewall and here and there I did some black silicone to make sure no holes between motor and cabin.
2) added the 3 under dash panels
3) new cable and reconditioning of rev meter
4) new grommet over gear stick

Recently: 3.25 differential and all bushings and rubbers, bearings  replaced on rear axle + new rubber on differential strut in bonnet

All that made a lot of improvement. Now it is great to drive it open. (up to 140 km/h without a lot of noise)

But:

We all know that with the soft-top on, you have more noise. Although it is much less than in the beginning, now there is a distinctive area where the noise comes from: the back.

It is the noise of the exhaust (yes, I have a stainless steel).

If that would be suppressed, then it will be even ok to drive it 120 km/h closed.

What can I do to isolate that? It seems that the trunk is one big bass reflex. And there are complete openings to the place where the roof folds in for the hinges of the trunk. And even a round open hole vertical in the middle.

Should I isolate in the trunk, or under the car just above the rear axle?



Rudy

[Cees Klumper]

Hey Rudy - I also have a stainless system, but don't hear it nearly as loud, it seems, as you, because it doesn't bother me at all (and I am sensitive to noise). Just a long shot, but are all your fittings are tight, and is the exhaust completely free from the body? I don't recall this ever having been reported as any sort of significant issue, so I'm thinking the issue may have a straightforward fix.

[Rodolfo]

Hi Cees,

I am also sensitive to sound: Last year they measured my hearing for fitting ear plugs for driving my motorcycle: I had the same hearing as a 25 year old. (I am 50). I want to keep it like that.

As a matter a fact I will check the last connection. It was a tight fit, but probably would be better with some paste in between. Will try that.
Is there a heat shield in the area of the rear wheels? Maybe I could also fit a longer pipe-end in chrome to make it slightly longer.

But I would like to know for some sound proofing as well. Maybe I just have to store some foam in the back of the trunk??

regards, Rudy

[DaveB]

You are probably right the empty trunk is like a big speaker. Filling with foam would be a good test. A heavy trunk mat might damp some of the vibration.
Here's a video of a pagoda with, I guess, a non-standard loud exhaust. It sounds good but I imagine you'd tire of it:
http://www.youtube.com/watch?v=7KZMx9EsHT0

[garymand]

Rodolfo,  I have a 250SL.  I can't pinpoint your specific area to attach, but: there is a felt pad (about 8x10") between the trunk sheet metal and the X cross metal inside to dampen vibration.  There is only the bubber mat in the trunk but it is all thick metal that is ribbed and that doesn't vibrate easily.  I'm thinking the sound is just coming in through or around your soft top.  

I just put on a new rear seal and don't notice any reduction in noise.  My top is new and fits well along the back and windows.  My exhaust is loud, not as loud as a Ferrari but it does tell you it is a sports car.  Most of my noise comes in through the softtop in the back and at the hinge areas.  BUT, I get a lot of noise from the top being loose between windshield and the 1st and 2nd ribs.  I have a sag of maybe 3/16"?  I have to un-glue the front lip and pull it tighter than I did the first time.  At speed the top 'drums' slightly with the air flow.  I get a little noise from the front weld-loop at the front edge of the top.  It is 1/4" away from the windshield chrome and that creates a buffiting noise.  I've been working on so many other things on this car, trying to get each thing perfect.  I think we all know how that goes.  

Honestly, I got significant noise reduction by padding the floor and firewall.  I think I dropped the noise by 1/2, but my wife senses no difference.  To her, it is still a noisey ride.  

I have a thought on how to reduce the exhaust sound:  I greatly reduced the exhausdt noise on my 944 turbo by wrapping the exhaust all the way from header to the rear muffler.  That reduced the noise very significantly AND the 944 has an encased exhaust: the exhaust pipe is surrounded by a stainless outer shell with some space and insulation between.  That is to keep heat in more than reduce the noise, but the wrapping with fiberglass hotrod exhaust pipe tape dampens the noise significantly.  try to find the 2 inch stuf.  Thanks for reminding me.  http://www.jcwhitney.com/design-engineering-exhaust-wraps/p2011145.jcwx?filterid=d16999y2003g2u0j1

It is a bit tricky and time consuming on our dual pipes, I think i will wrap both not individual.  If you want to know more about it, let me know.  Ciao

[Garry]

I am with Gary in that I would be looking to sound proof the cabin.

 I put dynamat onthe floors and firewall in a both front as well as the rear and then fitted 1/2” jute  under felt below the carpets as well as cocomats on top in front. Whilst my exhaust is quite noisy when the top is open, it is very quiet when the tops are up or on.

With the lower rear axle yours should be even quieter.

Garry

[garymand]

I drove the W113 into work this morning, the exhaust is minor compared to the cabin and softop noise.  The new cars have very thick floor pads to dampen noise.