I'd like any and all engine noises to be diffused as early and as quickly as possible.

I like some of the steps GM has taken to reduce hard surfaces that reflect sound, by changing to material surfaces that scatter and diffuse sound. Take the wheel well liners as an example. The full size SUVs (Escalade, Yukon, Suburban) have "carpeted" wheel well liners now.

I find a similar experience in engine noise attenuation when I drive from the street onto my unpaved property where I dump wood chips every few years. The engine noise instantly and very noticeably reduces as soon as my front end leaves the pavement and drives over the chips. Obviously, the pavement is reflecting the engine noise back at me, whereas the soft chips are absorbing the sounds, just like GM's carpet in the wheel wells.

 

I don't think you fully understand the terms you're throwing around.

Diffusing and scattering is done by reflection: hard surfaces

Absorbtion is done by soft surfaces. It's easier to absorb higher frequencies, but their wavelengths are short enough they run out of steam if you just bounce them around a bit.

It's much harder to absorb lower frequencies, so the damping combined with a rigid surface might actually work better than the soft surface alone. You need mass to wear out lower wavelengths. This is why people make subwoofer boxes out of thick heavy materials. It's also why you have a couch stuffed in the fenders of some of these trucks. Engine noise is dynamic and covers a lot of spectrum.

Wood chips are anything but soft. You're talking about an extremely refractive surface, and gee, you also noticed how well it works.

 

If neoprene didn't melt at such a low temperature, I'd line my whole engine with the stuff.

In reference to the hard surface combined with a soft surface for noise abatement, I would think that may work better for containment than abatement. Explaining that a little further - once the noise is out of the engine, it's now a game of abatement. If you put a soft coating around an engine (like something foam-ish) and a hard shell around that (like Line-X) - that's containment.

Using a reflective surface for noise attenuation is a very precise science - it works great in a controlled environment like the inside of a muffler or a resonator. Once you add variables like the stylish shape of a hood, the constraints of an engine compartment, all the wildly different reflective shapes, and the dreaded valley that makes a nice loudspeaker - you've completely lost the ability to attenuate noise with reflection. You can even amplify specific noises under the right conditions by way of resonance or compounding.

For the overwhelmed reader:
Resonance - think tuning fork, but really big.
Compounding - think waves from a boat cruising in a tight circle. The wake is the same size no matter which direction traveled, but when they meet in the middle - a wave atop another wave is twice the size.
Attenuation - identical sound waves collide with each other in opposite cycle (valley meets crest), cancelling each other out. This is the opposite of compounding.

 

The problem with what you're arguing is that I don't care how loud it is in the engine bay. I only care about what makes it out. So if I can reflect it indefinitely inside the fenders and under the hood, I don't really care how much it's bouncing around and how it's been amplified or not.

The problem with an un-dampened hood is that it's entirely possible for it to become a passive radiator and actually reinforce the sound that you're trying so hard to not hear. By having a rigid layer, then a fiber or foam backer, then the sheet metal; the initial high frequency waves are being reflected back at the engine, and the longer waves are having to pass from a hard to a soft to a hard surface to escape the confines of the hood. Those transitions are wonderful at wearing out sound. Double walled construction is nothing more than two air spaces with a layer of sheetrock between them and no studs back-to-back to transmit sound from one outside wall to the other. It works remarkably well at isolating the vibrations.

 

Do you still have the writeup? The URL is bad now.

 

Here is a link to Woodnthings LED hood light mode write up in PDF:

Superduty Hood Light LED Upgrade.pdf - Google Drive