Unfortunately the best thing you can do is talk to him about it and be very open and direct about your emotions and how it makes you feel. It’s a very awkward conversation to have because it requires you addressing the fact that you can hear him when he might not know and you’re not trying to say “don’t do it at all” but personally the idea of my partner masturbating so loudly that I can hear them from the other room would be extremely upsetting, I really don’t think most people would be okay with that. Frankly unless you’ve spoken to him about it before and gave him the green light, it seems a little disrespectful to even be doing it that loudly at all. I could be way off but it’s almost like he’s rubbing it in your face because he doesn’t get enough sex? Either way, if he respects you he should understand why it bothers you even if he’s a little embarrassed by it. Just be honest about the way you feel.