From Bristol 603 (Nigel)...
I tried to add this to the articles thread on this topic, but I couldn't get it to work for me.
Dear All,
Inspired by Mitch's research, I decided I would undertake a sound proofing upgrade on my 122S. The sound proofing before the work was fairly normal and not in too bad a state, i.e. the bulkhead holes were all fitted with grommets, the gap around the steering column was filled by foam, bitumin sheets were stuck to some areas of the floor and transmission tunnel, the gear lever rubber was sound and filled with felt and there was some old felt under some parts of the rubber mats. The worst part of the existing sound proofing was the bulkhead under the dash where the felt was loose, thin and had holes in it.
Custom Audio Designs are in Petersfield, which is local to me, and so I sourced my materials from them as I could collect them in person which saved some time and delivery cost. The materials I used were:
FT 55 AL 5.5m x 1.2m roll - "Consists of a 10mm layer of acoustic absorbent felt bonded to our 5kg/mē T50 high performance acoustic membrane. Ideal for lining the inside of vehicles and bulkheads etc, can be used under carpets with the felt side downwards. Aluminium foil faced for extra performance and protection. Very pliable, high performance RW 25dB. Easy to cut and shape."
FR12SA Fire Retardent Acoustic Foam (self adhesive) 2 x 1.2m x 1m sheets - "This foam is totally non-flammable and is ideal for lining the bonnets of all vehicles. It is an excellent sound absorber and will protect other materials from a wide range of dangerous ignition sources. The material is black, soft, pliant, resilient and easy to fit.
SY50 (self adhesive) 6m x1.22m roll - "A polymer-based, asphalt-free, high density synthetic soundproofing membrane, that offers good acoustic insulation in different building elements. It has extremely high visco-elasticity as well as being extremely flexible even in cold temperatures. Another added advantage is that it does not contain any PVC."
I did a cheap and cheerful noise measurement before the work. Driving at 65mph down a flat stretch of local road using a sound meter app on my phone measured the noise level at 82db. I seriously would not trust this as an accurate/absolute measurement, but I believe it gives some sort relative measure for comparing with the measurements after fitting the sound proofing.
The first job I did was to fit a sound damping sheet (approx 50cm x 50cm) and then one of the foam sheets (1.2m x 1m) under the bonnet. This was quite easy with the bonnet taken off and upside down on the dining room table (wife and children out for the day!). I was glad of getting the self adhesive versions of these materials as they stuck strongly to the degreased surface and it was much quicker and less messy/smelly than using contact adhesive. Time will tell if they stay in place, but they do seem very firm at present.
I then repeated the noise measurement on the same stretch of road at the same speed. The result was 80db.
The second part of the work took much longer. I took out the seats, carpets, centre console etc and stripped off the now somewhat brittle bitumin sheets. The floors were sound, dry (despite plenty of driving in the rain) and rust free. I then added sound damping sheets covering about 25% of the area following Mitch's suggestion. The floors and tunnel were covered with the felt/absorber sheet as were the footwell sides (glued to the back of the front kick panels). I glued some felt/absorber over the sections of rear wheel arch that are beside the rear seat. I am unsure whether it is worthwhile, but I also put the felt/absorber under and behind the rear seat as these are big metal sheets and it seemed to me they might be a source of road/diff noise.
The upper part of the bulkhead was tricky as I didn't think the felt/absorber would stay in place here longterm even if glued, i.e. the felt would pull away from the heavier outer membrane. Instead I decided to use a sandwich of the foam and sound damping sheet. The self adhesive sound damping stuck strongly to the top surface of the foam. The sandwich would then be stuck to the bulkhead using the self adhesive backing of the foam. I used the shreds of the old bulkhead sound proofing as a template to get the rough shapes. It was then a case of lying on my back in the front footwells, trimming and easing the sandwich in to place. It took time, but fitted fine and seemed to stick strongly.
Removing the old rubber flooring had showed up some splits/tears in this and so I took the opportunity to patch it with some bits of old inner tube rubber stuck with contact adhesive to the rear. The seats and interior then went back in.
I really like the solid sound the Amazon doors make when shut. The bonnet was now shutting with a similar very solid sounding slam with the sound damping sheet in place. I decided to put a small section of the sound dampening sheet on the inside of the boot lid. This now shuts with a more solid sound. This is adding weight, but I think it must be only about 500g (Colin Chapman certainly would not approve and I am not sure if even I do!).
Driving the car down the same road at the same speed produced 78db. In a subjective way, the noise now seems less mechanical and the car is squeak and rattle free. It would have been interesting to have done a before and after spectral analysis, but sadly it is now too late to measure the before. My subjective impression is that a lot of the high frequency noise is gone, but I have no objective data to back this up.
In total I used about 4m of the felt/absorber and most of the two foam sheets. I think I used about half of the sound dampening sheet. The sound proofing on the bonnet was the biggest gain for the least effort. The rest of the work reduced the noise further, but took me about 10 hours (I am slow and not skilled). This was longer than I thought it would take me. I do a lot of motorway driving and so I hope the extra noise reduction will be worth the effort.
I hope this information is of use to others.
Best Wishes,
Nigel.
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