Hearing protection.

I'm really fortunate, was around lots of aircraft. My hearing is mostly intact, except for the frequency that allows me to hear my wife.
I was an Artilleryman, we were issued hearing protection, but you couldn't hear firing commands with it so you didn't use it.
I have tinnitus like you wouldn't believe. If my blood pressure is up, its not a ringing, its a scream.
At night, it's unreal how loud something you can ignore during the day, with ambient noise, can be.
Do yourself a favor, anything is better than nothing.
You don't want this stuff. If I could go back and change one thing, that's right there at the top.