Another possible approach here might be to fit a noise cancelling system. I know it has been done of some 'spaces' (I'm just trying to remember where I have read of it), I've no idea if this would be a practical approach or not ?? There's others here who I'm sure would have some experience with such systems (Cooper Mac ?). If you've ever tried Noise Cancelling Headphones, you'll appreciate how effective the system can be.
<Edit: After posting this I just had a google to see if anything existed. There's actually many on the market, some even specific for inside vehicles, eg
https://www.silentium.com/ >
Sorry, guys, I've never got into noise cancelling systems, as the kit I work on is a bit big for that, but it's worth some research into what is available!
