NVH (noise) in a 40 year old van

My day job is designing the metal structure of Nissan cars, most recently doing the engine compartment metal of the Mk 2 Qashqai. Part of my responsibilities on top of the metal are dash insulation, carpets, battery brackets and all sorts of other bits and pieces. Having commented a couple of times on noise concerns on CF's on the forums, I thought i'd make a post explaining how to make it possible to have  a conversation with your passenger without having to shout/resort to megaphones.

When we first bought the van, the noise while driving was deafening. Having spent years just getting it running, the noise was never the priority, but a couple of years ago I took one of our NVH (Noise Vibration and Harshness) experts out for a run around the block to diagnose the cause and suggest a solution. He was immediately clear on both - there are gaps between the engine bay and the cabin allowing engine noise to come through unchecked, and there are gaps from the cabin to the outside world allowing wind, road and engine noise to come in. The main problem as well was a blowing exhaust manifold to down pipe joint, but that's the easiest to solve with a new gasket.

All the leakage to the engine bay is through the engine cover. Around the edge there may have been a seal once, but there's nothing there now, so noise goes straight through the gap. I fitted some sticky backed foam sealer with immediate results. Here's a picture of it in place, it runs all the way round the opening

I've lifter the cover up here and you can see i've got two beads of sealer running around. Before being compressed they were 5mm high, but are compressed to ~1mm when the cover is screwed down.

The difference in engine noise was amazing. From shouting to each other and having the radio on full volume, we now drive along in comparitive peace....

Next was a problem that only really shows up at high speed. At high speed the pressure outside the van reduces as the wind rushes past (like an airplane wing), so the pressure of air inside the van is higher, sucking the doors and windows open. The Sash on the front doors is very weak, and can be sucked outwards so much that a gap opens up between the door sash and the seal on the body side. I used the same sealant in this area, in some places with two beads of seal on top of each other the gap was so big.

That was enough to completely close the gap and eliminate the problem. before, I could actually see daylight through the gap like this:

(i've opened the door a fraction to show the effect). Now that i've made those simple changes, the van is so much better than I could ever have expected. in the future I might go further and add some insulation to the actual dash metal panel. The theory here is that you add a soft layer (foam or felt with air in it) that absorbs higher frequency noise, and you add mass tat blocks and reflects lower frequencies. In Nissan we do that with multiple density felts or Polyurethane foam along with 'heavy layer' - a plasticy  gooey substance that's found in the dash insulator. If I get round to it, i'll make another post, but there are more pressing issues to attend to for now...!