I did a quick and dirty install of some 3M Thinsulate NVH (Noise, vibration, harshness) insulation for the soft top. Links below go into detail on the insulation itself and an install I made for the floor, doors and trunk.

I used a much thinner variety and ran a piece inside the "pocket" of the soft top that is overhead when closed. Another larger single piece is used to just lay on top of the shelf behind the seats. It absorbs a fair amount of reflected noise and that from the trunk area. When the top is down, there are no issues I have found with it compressing. Its as if its not there. Of course, all you have to do to remove is just pick it up. The two side square panels are thin and I just experimented to find a workable shape. It fits behind the flex strap and I stuff an edge under the rear glass flap. The front edges butt up against the tube frame. Once again, the top just folds down on them. I find these side panels effective on the highway especially taking edge of the harsh noise of semi tractors.

All the inside pictures are from the drivers seat looking aft. I had to use a bit of light to expose the shot, the black scrim cover is quite invisible from inside in normal light.

I am thinking about trying some other attachment method instead of placing it behind the strap, perhaps velcro. Its takes a couple of minutes to put it in and smooth it all out and tuck it in. It stays put well enough with the top going up and down. I forget its there. I notice it when its gone by the increased noise from vehicles "behind" the door glass.


https://www.s2ki.com/s2000/topic/107...#entry23139579



https://www.s2ki.com/s2000/topic/108...#entry23181666

 

I have similar pieces made out of some 2" denim sound insulation. I've been slowly doing some sound proofing and testing each step. I haven't tested the top pieces since I have winter tires on now. I'll test it when I get the summers back on that I took the other readings with so they will be comparable.

After 30-40% coverage of the floor/tunnel/seatback are with sound deadener, 1/4" closed cell foam, and MLV I saw a 2db drop at 70mph.
After doing a similar install in the spare tire/fuel pump area and then filling all the air space in there with denim insulation I got another 2-3db drop at 70mph.


I'm not that impressed so far. My goal was to lower highway cruising noise for long road trips. After a 6-7hr trip I have a headache and feel like crap. Most of that is wind noise and not much is making a difference there. I've thought about making a removable headliner out of sound proofing but I'm not sure that will help much either. I read hardtops kill a lot of wind noise but those are expensive and I'd have to store it somewhere.

At 45-50mph the difference is much more noticeable, but I don't spend very much time doing 45-50mph with the top up. Luckily I had most the materials sitting around so I've only spent about $50 but I did spend several hours of time and added 30lbs to the car. Last time I went on a trip I got a set of earplugs and felt great afterwards.

 

My research indicated that soundproofing the top of the top does not bring about much of a loss in NVH. I added about 8-10 pounds for mine and the difference at highway speeds is significant. The audio system is better and can be played at lower levels. Conversation with the wife is normal. We used to feel the stress after an all day drive (like you do) but now we don't notice it. When you rev up you can hear the scream just fine but in lower gear, the drone is gone. The top side NVH panels do a good job taking the edge or harshness of the side traffic. You can hear it but I don't wince any more. I think that big piece sitting on the plastic deck behind the seats absorbs a lot o ambient noise.