I am very new at mixing stuff.
Some tutorials show that they high pass about 30 hz the final mixing, but some don't do that.
My question is - Is it good to high pass and low pass the final mixing? Thanks in advance
Back in the days of analog & vinyl, rolling off the extreme bass frequencies had three primary reasons .
No-one would ever hear it
the needle would tend to get thrown out of the groove with extremely low frequencies, especially if it wasn't centred, or was out of phase.
There is far more energy needed to shift bass than treble, so the overall volume that could be cut into the lacquer, or even the playback length of the record side could be improved by removing the extreme lows.
I would say in a modern digital setup, the first of those still stands as a valid reason, & a modified version of the third - that digital has an absolute maximum level, which is easier to stay inside if you remove some of the higher energy data [ignoring extreme brick-wall compressing as a workaround]
In addition to jdlugosz's comment about its effect when the sound is reproduced (which should be an answer) it also provides additional headroom in the (digital) recording itself. The sub 30Hz content could push higher frequency content above the top of the scale. Since it's not going to get reproduced, there is no harm in removing it, and there is the benefit of added headroom.
Dave and Tetsujin are right. Cut out what you don't need. Except for cinema and maybe (!) for CD chances are very rare that you might need sometimes something below 30 Hz. Additionally and maybe most important: the mix will go through measuring-equipment - at your place and maybe at the broadcaster/station... and you/they may get "wrong" results because of this "loud sound at 15 Hz nobody can hear - and maybe some people might lower the volume because of the wrong measurements... i am sure you get the point... so a cut below 30Hz makes a lot of sense - use your analyzer... a cut above 15,8 kHz makes sense too for TV-mixes. Why? Quite often you have quite a loud "noise" at 16 kHz in the original dialogs or in the ADR... So a cut above 15,8 kHz makes sense just to get the job done a bit faster - and on a TV you cannot tell the difference anyway... most broadcasters cannot/do not transmit frequencies above 15 kHz anyway...
It does however not apply to all music, my buddy asked the same question to one of the grandmasters in our genre (Hardstyle) during a private workshop, and he responded with:
"Well, it's there, and it was there when you created the track, so you build the track with those frequencies in mind right? Then why remove them, causing you to revaluate the whole mixing process again?"
I've found that this rule still applies to most of my tracks; once I start removing sub-bass frequencies, more often than not it starts to reshape the low end, something I do not wish to happen since I build the track with a specific low-end movement in mind.
I can, however, recommend the use of a lowcut at 20Hz with a slope of, let's say anything below 24dB/oct, or maybe higher as long as it doesn't change the way your low end rolls, unless that is what you're trying to achieve.
Also, instead of a lowcut, you could also check out the inverted (/negative gain) lowshelf with a Q of 1.00, see how that goes.
But, in the end, if your individual elements are right (specifically your bass) you shouldn't even have to utilize a lowcut in the end.
On the high end, however, I usually set up a highcut (24-48dB/oct) at 17,5kHz, and another one (96dB/oct) at 20kHz. Same rule applies to TV related content with the arrival of digital television.
P.S. All of the above applies to the post-mixing stage, during mixing you should do whatever needed to clean up sounds.