I'm pretty good at tuning things out. I remember seeing my SIL read a book with an entire family reunion going on around her, and being astounded. Then, I had kids. And I learned this is survival. You learn what to listen for: Quiet, toilets randomly flushing, sneaky giggles and HELP MOM! That's about it.
However, when writing in public, you're at the whims of others and what they consider appropriate coffee shop behavior. Warning: It may be different than your definition of appropriate cafe behavior.
Sometimes, I wear headphones, but usually not because I'm nosey, and I might miss something I can use in a book. But I've found over the years, there are three things I cannot tune out.
1. A child being treated poorly by a parent.
If the kid is just screaming because they're upset, have at it. But if the parent is being neglectful or callous, I can't tune that out. I want to run to the phone booth, get my supermom costume on and come to the kid's rescue.
2. A really interesting conversation that suddenly goes quiet. I have TERRIBLE hearing unless something is none of my business. Talk on the down-low, and I've got to know what you're saying!
3. A cackling laugh. There's a cackler here today. This strikes me in my core for some reason. Why? I'm glad the woman is happy. She's enjoying herself. That's a good thing, right? But a cackle just sounds so evil and pointed, it gets me in the gut.
These three things pull me out of hyper-focus mode, and make writing in public impossible. Hence, I'm blogging instead. Okay, in all seriousness. There are TWO! cacklers here today and they're not together! I don't know why it makes me think I'm in an Edgar Allen Poe story, but there you have it.