With most of the interior properly insulated, the engine was now the most prominent sound inside the cabin. This is the way it goes, when you reduce one source of noise, then another appears, you solve that one and other appears, and so on. So it's a bit like chasing dragons. And heroin high. So I'm told.
We start by the two first layers, Alubutyl to suppress vibrations in a big piece of metal that is the hood. After that a second layer of foam rubber of 10mm to do some thermal insulation, but above all I was worried the third layer wouldn't stick has well to the alubutyl as the rubber foam would. Anything sticks great to the foam rubber, that one will rip before it unsticks.
Since the motor's sound is mainly mid and low frequencies, I decided that the best material to reduce transmitting noise from the engine bay would be the same I already used in the wheel wells, 10mm fleece.
This material had already been very successful for me on the wheels. and the frequency band is similar, so it would work best in this application too.
For some time now the engine cover was doing this resonance noise when the motor passed the 2000 rpm, and upon removing it and analysing it I found that one of the pins that secure both plastic parts was broken, so with the correct resonance they would just vibrate one against the other and make a lot of ruckus. I had some Sugru left from the mirrors, so I just put some between both panel where they touched, and that took care of that problem.
So I decided to cover the interior of the cover with alutbutyl, so the additional weight would stabilize the cover, and fleece to reduce the noise. Separating the plastic parts made making the paper patterns much easier. On the outside part I just did some moulage, no need for patterns. The inserts that secure both parts are a bitch to remove, so be careful with those.
I had some worries about using these materials in the engine bay, so I researched for the specifications, and both the alubutyl and fleece have a maximum operating temperature of 80º and 90º respectively, so that shouldn't be a problem. The engine the hottest I saw was 113º, and that was oil temperature, so on the outside would be much lower. I rode the car on the highway and it reached 100º, and the cover was barely warm to the touch. Of course this is a location where temperatures are high, and a lot of mechanical vibrations, for now I consider this as experimental. I will check regularly and see how the materials behave and their durability.
I must say that the noise reduction was very pleasant, most of the mid frequencies were lower, still there but a lot less intrusive, and the bass growl was still very much felt. Very nice harmonising, I like it.
Only problem is that the wind noise gained so much more volume on the overall noise, it is now the main source of noise in the cabin. I also have an annoying whistle in the front from somewhere, still have to find out where exactly.
So, with this I think I am very near the end of my odyssey killing those noise dragons and sirens, there isn't much more I can do, I think. I am still researching additional door seals, and maybe some hood seals, let's see how that goes. That is it for now. More to come.
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Interesting 👏
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