My journey in improving NVH of the 996

I’ve contributed little to the forum, despite owning my 996 for something like 9 years, as I've not had much to say that other haven't already, but I realised the other day I might have some unique information that might help people. I’ve been bugged by road noise and exhaust drone with my 996, which has lead me try to improve the situation. ResoNix, a US company, has a blog/videos in which they treat a 991, which isn’t so different to the 996, and I ended up using similar materials in similar locations. ResoNix products are not available in the UK, so after some research I ended up purchasing the following products from Acousticshop.co.uk: - Flamex Basic Sound absorption, 25mm thick, grey - Akotherm Eco Basic Sound absorbing polyester wool, 20kg/m^3, 50mm thick, white - Akotherm GG Smooth Finish Sound absorbing acoustic panel, 20mm thick, black The CLD I used was mostly left over from a previous vehicle project. My other main source of information was from an NVH engineer I work with. There are plenty of videos on the internet where the solution is to just plaster the whole car in CLD (Constrained Layer Damping aka ‘Dynamat’) materials, with little apparent engineering involved, which is both expensive and heavy. It might be suitable if you are really bugged by the noise or want to install an audiophile sound system, but this is a sportscar at the end of the day. The best tip I had from my NVH friend was to get a plastic tip hammer and lightly tap the various panels of the vehicle. If any sound undamped, place a finger on the panel to see if sound is becomes dampened. If it does, CLD can help. There are a few places on my 996 where I tapped the panel but the sound didn’t change on touch, as the vibration was passed to another panel. 1) The area behind the rear seats got a large coverage of in CLD, as the panel was so mobile. Even after this it still visibly moved so I got some 15x15mm L shaped steel from Homebase, bent it to an arch by hand in a vice (to follow the panel profile), and riveted it across the middle of the panel. This did not improve the general noise BUT massively stiffened the panel and it made a noticeable different to ‘single events’ such as pot holes and bridge joints. Previously there could be a loud BANG on running over such road inputs, but this was massively reduced afterwards. Note, the CLD in the area where the L steel mounts was removed before installation. 2) The rear ¾ panel I found the noisiest point with the hammer but after apply some CLD discovered the noisiest point just moved, so I ended up applying much more than I originally intended. Also, following the ResoNix example, I put a lot on the inner wheel arch. After this I filled the void with Akotherm polyester wool, being careful to make it tight enough that it won’t move around, but not too tight – it is breathable material, but not if compressed, and I didn’t want to risk humidity issues by sealing areas of the vehicle off that were not originally. I did one side before I tried the other, I could tell the difference in noise. 3) In the doors, I put CLD covering circa 50% of the area of the panel above the reinforcer, plus small amounts in other areas, based on the hammer tapping. In the gap between the reinforcer and the outer panel I pushed some Butyl rope/tape, so the reinforcer is helping support the panel. (Note, not applicable to the 996, but need to take care on vehicles with Aluminum panels, as you can end up with visible deformation of the panel). I also had a ringing in the door, which took me ages to trace and ended up being where the top of the outer panel folds back on itself. Some Butyl rope in the fold stopped the ringing. On top of the CLD and factory deadening material, covering circa 90% of the area above and below the reinforcer, I used the Flamex material. This material has better absorption properties than the Akotherm material but can absorb water, so I made sure the to keep it away from the bottom 10cm of the door (note: not used in rear arches as it is not breathable, not an issue in the doors). It was attached to the door via spray glue bought from Acousticshop.co.uk for this purpose. On the inner door panel trim I used some of the Akotherm GG, held in place to double sided tape. It is more dense than the Eco, but also thinner, so suitable for narrow gaps. 4) I put relatively small areas of CLD on the sunroof panel and the roof area behind the sunroof, maybe only 10~15% of the area but it was enough to dampen them. 5) There are two areas of the car floor that ring if you tap anywhere near by, and tapping the area itself it rings like a Steel Drum! They are highlighted in red below (images stolen from another thread). 6) This is for me the biggest area of improvement, which was unintended. I have the solid rubber engine mounts and Powerflex transmission mount insert. When I installed the Engine mounts I noticed a small increase in vibration, which I was fine with, but noticed on engine start up and shut down (so accelerating/decelerating between 0 and idle rpm) that the engine sounded like a bag of spanners! After the transmission insert was fitted, there was increased vibration and harshness from 4k rpm, which I didn’t like but put up with for the improvement it gave in steering and ride. The areas at the rear in blue (above) I found to make a big difference – still some additional (pleasant) noise but the a lot of the harshness and vibration had gone. Additionally a small area at the front (shown below) also reduced these vibrations a little at higher rpm. For 5+6 only one side shown in the images, CLD was applied on LH ans RH sides! Overall Most reduction in sound isn’t massively noticeable but if you do one side at a time, you can tell a difference in noise coming from each side of the car, so it is doing something. With regards to the floor, it is well worth doing if the vibration introduced by stiffer engine mounts and trans mount insert put you off using them, albeit it won’t solve poor installation issues (see my problems with the trans mount insert walking out). As has been stated in many other threads, it is worth noting that changing my tyres from Pirelli Pzero to Michelin PS5 made a bigger difference in road noise, but even so the car has hardly turning into a Rolls Royce, so I’m still glad of the work I did. Also little improvement was made with regards to exhaust drone. I hope this helps. If you are interested in enhancing the induction noise I would recommend installing a 991.1 ‘sound symposer’.

Good work, my mate who has a C2 and tinnitus has done much the same, he didn't take quite the scientific approach you've done but it did make a noticeable difference to overall road noise. It also made the car feel younger and tighter somehow!

Fascinating stuff - being able to remove some of the crashiness from the ride via modification of the parcel shelf is a really nice change. One of my main complaints about the 996 is the behaviour over expansion joints etc at motorway speeds! I'm going to have a look into doing that although I may see about using structural adhesive as riveting sounds a bit scary. The changes to the floor would also seem to be a nice easy win. Thanks again for the interesting post! I'm interested how you added the 991 sound symposia to your 996? Would love to do something similar...

any more pictures please this is awesome - I have a thing about knocks and noises so I think this thread is amazing - thanks

Regarding doors, remember that rain water comes in from the top and runs inside and out the bottom of the door. Previous owner put foam inside my door and it soaked up all the water, I noticed it when I fixed my window regulator so ripped it all out. Only have butyl sheets in there now.