You should ensure you're within speaking distance rather than shout your conversation across a busy store.
3 women in a large store this morning, at least 20 ft away from each other, continued their conversation despite the fact they all had to shout 
Example included:
"They're over here, what size are you?"
"A 14, have they got them in black?"
"Like fuck you're a 14! Nar, only in white or cream"
" Don't want them then, do they have a 16 in black?" And so on
At no point did either of them take a step towards each other (and before you ask, neither seemed to have any mobility problems as they'd been walking bellowing around the shop quite happily)
What is it that makes some people be totally oblivious to (or just not care) the effect their behaviour has on those around them?