Have you noticed when you turn up music you can hear certain things clearer? How about when music is loud and you hear something too much? That's because our ears hear frequencies differently.
Without a history lesson, Fletcher Munson was discovered by 2 great guys. To simply put it, bass or low frequencies takes more db spl (Decible Sound Pressure Level, in this case we're going to call it db) to be heard. While, it take less db for higher frequencies to be heard.
Mid frequencies don't need much of a boost, because our ears are tuned to it due to the human voice. That's why phones covos sound they way they do, because the other frequencies aren't needed!
Here's a chart based off the Fletcher Munson curve (from Wikipedia). It's represented in db spl and frequency. You'll notice how frequencies can be heard at different db spl. #HappyMixing
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