Engineers love acoustics questions, it is the time we get to prove that 2 + 2 really does equal 5.
Leaving that fun bit aside, we have sound as a spectrum, lets just say High pitched, down to Low pitched. I emphasise this lot is extensively simplified and generalised here but, before tackling any project
, think what you want to take-out - why....
High pitched - change the direction
Low pitched - needs density
So the lead sheet in foam (or even barium in high tech products) are tackling a generalised, broad spectrum. The dense sheet in the middle is absolutely critical to the low frequency noise. The closing of gaps is critical to all bits of the spectrum. The foam and bubbles perhaps more on the high frequency.
1st, what is the source. Air inlet is one, exhaust
is one, but so is the general vibration from the engine and most critically, vibration from everything being shaken by the engine.
Yes, attenuate the air inlet but all you need is a large tube lined with the foam. Air filters work but a foam tube also works.
1st approach in my view is to check engine mounts, flexible couplings etc and eliminate transmission
of vibration. Next, close up all direct air paths to the cabin
. Note the difficulty here, the engine must still breath and the compartment must ventilate.
With mine (I have a YSE8), I have lined all panels
, closed gaps to the cabin
and maintain the anti-vibration mounts and couplings. More than that, you need to remove the engine to start with.
Be practical in what you can achieve.