Earlier I posted a question about compression and t he direct effect it has on the sound of the music and got some good feedback. Now I have the same question about EQ. I can't hear how it's altering the music when playing around with it. Can someone please elaborate on the purpose of EQ and explain to me is the effect it usually have on the music hard to hear like compression? Below I've posted the link to my question about compression if you're interested in some of the answers I got.
There are a few different types of EQ, but they all are used to adjust the frequency response of an audio signal. With the exception of a perfect sine wave, all sounds you hear are made up of multiple frequencies anywhere in the ideal audible spectrum from 20Hz (the lowest sound you might be able to hear) to 20kHz (the highest sound you might be able to hear if you're young and have perfect hearing).
So, if you play an A on a musical instrument at 440 Hz, that will also be producing overtones at 880Hz, 1660Hz, etc. We perceive this blended tone as a single sound -- it's just the way our hearing works, and it's what makes a flute sound different than a saxophone. EQ will raise or lower the bass, mids, or treble of that sound. This is used in a lot of different contexts. As an example, if you're playing in a group, EQ can be used to carve out frequency space for each member of the band -- for instance, making sure the high-end of the bass isn't interfering with the low-end of the guitar by rolling the lows off on the guitar and rolling the highs off on the bass. For mastering a full recording, EQ is used to fine-tune things like the amount of bass in the mix, or reducing piercing cymbal hits, etc.
An EQ is basically just a volume control, except it allows you to adjust the volume selectively for different components of a signal, namely, of different regions of their Fourier spectrum.
In the simplest use case, you might employ this to change the mixing ratio of different instruments in a pre-mixed track. This only works when the instruments occupy very different frequency regimes. An extreme example would be a piccolo flute sitting next to an orchestral bass drum. You could here use a low-cut EQ to eliminate the drum, or a high-cut to eliminate the piccolo, or a shelving EQ to change the loudness of each independently.
In practice, that's not something you should do with an EQ (you'd instead use better mic placement and mixing). It generally just wouldn't work like that, if the instruments aren't quite so unlike: in reality, almost all instruments occupy a somewhat broad frequency range already by themselves. The ranges of different instruments overlap. Where an EQ is really interesting is in regulating different frequency parts of a single instrument. This can only be done with an EQ, since the different components are already pickup up mixed together by the microphone.
Again an extreme example would be if there's a strange distracting resonant tone in the snare drum. You could seek out the precise frequency of that resonance and notch it out completely. Or, if there are rumbling “plop” sounds on the vocal track you might use a low-cut to clear up the signal.
In the other way, it's quite common to boost the low frequencies of bass instruments to make the whole mix nice and fat without making the bass instruments themselves appear overly loud. Or you might boost some higher frequencies in a guitar to make it cut through the mix without clashing with the keyboards.
What EQ to apply to each instrument is a science unto itself. Less is usually more here. You should always first make sure the mic placement is good. Then you might check what frequencies in each instrument are superfluous. Cutting these will often allow a much clearer mix.
But of course it depends on what results you want. In some applications, a few crazy boosted peaks in the midrange give just the right effect, especially when combined with distortion.
The usual way EQ is employed in recordings is per instrument. Global EQ is similar to changing the ambient light under which you view a photograph: generally, your eyes will readily compensate the effects unless the light cannot in good conscience be called white anymore.
So global EQ is mostly useful for addressing serious shortcomings of a recording.
Individual EQ, however, is a completely different thing. It makes it possible to adjust the spectrum of overtone-rich instruments in a manner where they retain their character but leave more room for other instruments to be heard. Used judiciously, it can render a mix more transparent. For example, toning down the high frequencies of a cymbal can reduce its impact on other instruments without affecting its perceived loudness significantly.