Frequencies of human hearing

What is the name of the curve that represents an "equal" level among the frequencies for human hearing?

It looks something like this:
45248794.jpg


Though I can't remember exactly.
 
Question- How is that applicable? Because aren't everybody's ears different and have a different frequency response? (Not talking about actual range here.)
Just something I've been pondering...
 
the curve diagram is based on the average.

a decent explanation (although i don't agree with some thing this guy says).
 
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