Ever notice how on your 5.1/7.1 surround setup in a large room makes it impossible to hear voices during movies unless you absolutely crank the volume? That’s what equalizing (or dynamic volume normalization) is for – it balances the speaker output to the space you’re in. If you had a $10,000 speaker layout, with speakers perfectly spaced and angled, you wouldn’t need EQ or normalization. But most of us don’t – so that’s why your stereo receiver has those awesome EQ settings – to adjust for the failings of the space, or for the inadequacies of the equipment given the space it’s in. You can also just use it for fun, or for various occasions (eg, more bass for music at a party).
Analog EQ was originally used for recording and live music / performances to make up for the differences and deficiencies of various environments (eg, amphitheaters, stadiums) and quickly made its way into consumer hi-fi equipment. You can still buy separate, basic graphic analog EQ hardware for your home stereo today.
But what about headphones?
This is where many people are applying EQ today, and it’s largely unnecessary.
First, EQ’ing should almost never be necessary in a headphone application. To say an equalizer is “required” for listening to music on your headphones is like saying you need to adjust your television’s color settings every time you change the channel. If you had a really horrible TV that absolutely could not display CNN (lots of red) after being adjusted for FOX News (lots of blue – funny, right?), there might be an exception.
That is to say, if you’re using $5-15 earbuds (or a phone/tablet with a really horrible headphone amp) then yes, EQ can enhance your listening experience when adjusted for particular genres, because your listening equipment is so bad that it’s probably distorting the crap out of whatever you’re listening to.
The other scenario is something like Pandora set to low quality but very high volume – think of it like a channel on an aerial that comes in ever-so-slightly fuzzy on a big 48″ CRT TV. By adjusting the sharpness, you could reduce the visibility of the interference. It was still there, and in no way was the actual interference lessened, your eyes just noticed it less. Alternatively, you could sit further away from the TV. Adjusting the EQ settings when listening to a really low quality audio file can trick your ears into believing it sounds more like it’s “supposed to.” But it’s still going to sound like crap. You can also, of course, just turn down the volume and achieve a similar effect, with the drawback of quieter sound.
What EQ is not is sound profiles to make your music sound “better.” Setting your EQ to “rock” when you listen to rock does not make the music sound “better.” It adjusts the levels such that, for the “genre” of rock according to whoever wrote the preset, your audio will place an increased emphasis on the frequency ranges that are most important on rock tracks, and de-emphasize, or leave neutral, those that are not.
How can a “rock” EQ preset work for AC/DC, U2, and The Beatles? It can’t. It has no fucking clue what you’re playing. In fact, The Beatles sound awful on most rock EQ settings, because the EQ preset is for much more modern (and loud) rock. No song exists that is perfectly shaped to that preset, because it’s modeled on averages.
Custom-setting your EQ to your headphones’ or device’s particular strengths and weaknesses should involve minute adjustments, if any, unless you’re using the garbage ones that came with said device, or something like Beats that over-amplify low frequencies.
The best way to improve the sound of your music? Buy better headphones (or speakers), and use local audio files of at least 192Kbps (128 even is probably fine, blind tests have shown). You can’t make a 12″ CRT TV from 1975 give the color of a 42″ OLED, and you can’t make $5 earbuds sound like a $100 pair of Klipsch, Shure, or Etymotic.
Otherwise, you’re just doing the audio equivalent of adjusting RGB / brightness / contrast levels – a custom arrangement or preset may look better to your eyes, but the same data is coming through regardless of what you change, you just see it differently. Equalization does not enhance sound – it shapes it.