It's pretty common when recording (or mixing) to add a high pass filter (aka low cut) to every track, with the common exception of kick drum and sometimes bass guitar.
Normally this filter starts around 30/40Hz and has a slope of around 12dB/octave, covering the "extremely low end" band. With certain types of instruments you can even cut from some 60/70Hz, but in general it looks like the figure below.
This shouldn't really be controversial.
However, I recently saw a few online discussions with people stating that they didn't like to do that, and that they were recording the and leaving the very low end on all tracks on purpose (or something along these lines). This was quickly followed by encouraging comments by others, stating that "it's good" and "there are no rules in music" and so on.
As much as the bro attitude can be appreciated, the reality is that no: it's not good. Not good at all.
Not cutting the low end in most tracks is a seriously bad idea, and if you want to know why you just have to read on. Now, a disclaimer: I know that most of these ideas come from a genuine interest in trying to create something new and original and/or improve one's music. The intent is good.
In our quest for originality, it's pretty natural to see everybody doing something and to think "what if I do something different?". Rules are made to be broken, they say and in music more than in other fields, right? Well. There are rules and there are rules.
Some "rules" are conventions, agreements, traditions and can be ignored and broken in the name of originality and innovation. Do I need to put the chorus after the verse? No. Do I need to have a chorus? No. Do I need to use the same sounds throughout all the song? Certainly not! These "rules" can certainly be broken and actually they tend to define a certain musical period. Every new generation of music makers almost always tends to break them in the name of innovation. Other rules are just stuff that, well, works because it's more likely to be liked by a regular audience, kinda regardless of the time, or simply are helpful for society to work. Singing in tune and playing in time rarely goes out of fashion (The Shaggs notwithstanding); playing random notes rarely brings success (it's been tried of course, but the market for dodecaphonic music has never been that huge, and even they have rules, it's not just random notes). Respecting traffic lights keep more people alive than otherwise. These "rules" can be broken, but the result isn't usually so great or notable. Finally, other rules are pretty much like physics - we call something a "law" but we cannot really decide to break it. Take gravity. Applying the "there are no rules" to it is a bit like deciding to jump off a cliff flapping your arms and expecting to fly: it inevitably ends up in a mess and it's often terminal. The "low end cut" idea belong firmly in this last category. It's not a convention nor an artistic decision - it's borne out of physics.
Let's see why.
There's nothing good there..
The basic reason is very simple: with most instruments, there simply isn't any musical content in the low end. For example guitar or a vocal line (both midrange "instruments"), won't produce much below say 50-70 Hz on a good day. In the 0-40 Hz band, there's nothing meaningful. Zilch, nada.
..and what there is random
Worse, whatever is there is utterly random, often depending simply on where and when you happen to have recorded your take. Why? Because sounds from 0 to 30-40 Hz is really vibration.
While we can perceive it, mostly we don't really hear it: it's more felt in one's chest as a push (that's the air moved by that 600W bass amplifier at full guff, and it's a lot of air).
And what is it, that vibrates so low?
Usually stuff that's got absolutely nothing to do with what you are recording: the rumble a truck passing on a road nearby, the subway, someone walking outside, that factory a couple of miles away and so on.
As we know, low frequency sound waves are very long and omnidirectional. Such long waves can travel from far, far away (we're talking miles/kilometers). More, they can (and will) get into your recording/mixing room without much problem, unless you have purposely built room-in-a-room which is completely decoupled from the outside shell, something that's it's hard and very expensive to do.
So, if you don't cut your very low end during recording or mixing, you will simply add random rumble noise to your tracks - due to some truck that happened to pass by your house just then.
Worse, this noise will be quite loud with respect to the musical signal (by virtue of the fact that there's no musical signal at all in that frequency band).
The exception are the few instruments which do have meaningful musical content in that very low band: a kick, or a bass guitar and the likes.
For these, the signal/noise ratio between say the actual kick thump and the random thump of a pedestrian outside the studio will be high.
That means that the outside rumble will in general be irrelevant, pretty much the same way as background noise is irrelevant in a well made recording: it's there, but too quiet to affect things. That's why you don't want to high pass a kick drum or a heavy bass line: you want it to give the listener that sudden pressure in the chest - assuming of course that his playback system can deliver it. Because it makes musical sense.
Beyond this basic reason, there's a bunch of other technical aspects which make a big difference to your mixes. Remember, a great mix is simply the sum of a myriad good small decisions, where a bad mix is the opposite. Each element is tiny, but the total of them results in a big difference.
Let's have a look at some.
Rumble eats headroom, so you can't mix properly
When you're mixing, you have a finite amount of space for handling your signal, called headroom.
Even with modern recording at 24 bits and the insane amount of dynamic range it affords, that space is still very much finite. You need that space so that you can do work on each track without distorting the signal - which in very practical terms means go beyond the limit of voltage that the gear can handle. Every track in the mix adds up, progressively consuming more and more of that space as you add tracks; your headroom reduces with every added track, until - assuming your gain structure is good -the pre-master mix should end up peaking at some -6dbFS or the likes. Since every sound can be thought as a complex pressure wave over time, "adding up" means summing waves - aka interference. These waves are generally not correlated - which means that in the same chunk of time, the various portions of wave from the different tracks will have a different shape and "point" at different directions (some will be crests, some will be valleys, some something in between).
This is why when you add that guitar to the mix which averages at -18 dBFS in its own channel, you don't really see your master bus meter increasing by 18dBFS.
However, low end frequencies have absolutely the most amount of let's say "power", that is, they eat the most headroom. The reason is that low frequency means long wavelength and big amplitude, which in turn means moving a huge volume of air. To move it, you need lots of power.
That's the reason for which your stage-ready guitar amp is say a 100W, while a bass guitar amp must have at least say 5-600W to be heard on the same stage. The bass amp must work much harder, because it has to move much more air than the guitar one - which emits midrange frequencies.
The net result of all this is that if you leave all that low end rumble in for every track, which has no musical meaning, you'll use a lot more headroom than otherwise - meaning you can't use that headroom for stuff that has musical meaning (like that lovely boost on your guitar solo).
You can't hear it
Worse, you won't usually have any idea that this is happening. The reason is simply that most nearfield monitors (even the good ones) cannot reproduce such low frequencies in any meaningful way. The result is that you'll see the loss of headroom on your meters, but you won't hear it - and you'll often struggle to understand why. Sure, there's subwoofers - but to use a subwoofer you need a really, really good room (otherwise the bass will totally mangle the sound in the room way before you get to hear the rumble). More, even if you have a subwoofer or even studio "main" monitors (these huge loud things embedded in the wall of large studios in front of you, behind the large console with all the pretty lights), you will need to blast them to feel the rumble.
If you know anything about mixing, you know that the amount of time you blast the monitors is very, very little (mostly you want to mix at conversational level). So even having that kind of gear and room, the chances you spot the low end crap is little.
It makes your track quieter (yes, in LUFS)
We've seen that the majority of your tracks will have nothing meaningful in the very low end.
And we've seen that if you don't cut that out, every time you mix a track in, its rumble noise will add up, eating a humongous amount of headroom for absolutely no reason (summing inaudible random rumble produces... inaudible random rumble).
An interesting effect of that fact is that you will get a mix that peaks very high on the full scale, but sounds very quiet - because most of the headroom is used by the (inaudible) crap you're left in. Want a puny mix?
Don't cut the very low end!
It eats electricity
For similar reasons, if you leave the low end in on every track, chances are that proper playback systems won't be able to play back your mix very loud, or at least as loud as the competition. The reason is again simple: assume the playback system can reproduce low bass (because remember, we want to hear that kick thump). If you have a lot of low frequency content in your song, the playback amplifiers will spend most of their power on that, and you might make the speakers work very hard.. to reproduce nothing useful. That means that when it comes your track's turn to be played, given the same position of the volume knob it will sound much less loud than others. Again, this is difficult to detect with even good home studio kit, simply because that kit (and the room it's deployed in) usually can't properly play back that low bass, masking the problem.
It might make your audience go "wtf"
And even if that stadium or disco system is exceedingly powerful (and thus can just reproduce your mix at say 70% power while the good ones need only 50%), the result to the audience is that they will suddenly hear pressure in their chest at the totally wrong moment.. because the rumble you've left in is random - completely uncorrelated to the music that's playing.
Even if they're not aware of it, it will leave them with a very strange feeling that something's wrong.
It's gonna be cut out at mastering anyways
But what about mastering, you ask?
Well, sure.
A mastering engineer worth of the title will absolutely cut away that crap if it's excessive (that, of course, assuming that the "mastering engineer" is not a guy with an analogue trackbox and a laptop - or just the laptop). But: in doing so, the engineer will necessarily have to cut also the kick, the bass or anything that should be there (because it had meaningful musical content).
And you still will have made a mix where you had far less headroom to make your mixing moves than you could have. So, as always, why not fixing it at recording, or worse case in the mix?
But, but.. if I hipass, won't I damage my low end?
Sometimes, in that maelstrom of myths and knowledge that is the Internet, you will find people stating with no doubt that if you hi pass your tracks, you'll "damage your low end".
Whatever that means. Among these, the ones who like to use big words for show will also mention stuff like "phase shift" and occasionally even "linear phase equalizer". If you have followed the above, you'll realize that this idea is, to put it mildly, misguided.
The reason for high passing most tracks is because there's nothing meaningful there to damage, just random rumble that depends only on where and when you have recorded the track, which eats headroom and forces your real musical low end to sound more puny that it could.
Leave kick (and bass) alone and you'll be fine.
Cut at recording or mixing?
All this said, it should be now clear that most tracks need to be high passed to keep only the relevant musical content and get rid of the low frequency rumble. However, should we high-pass at the recording stage or when mixing? If you've followed the above, the answer is again simple: many of the same headroom issues you find at mixing are present at recording.
Only, instead of the mixing bus, it's the microphone input stage whose headroom will be wasted. To avoid just that, many microphones come with a built-in switchable high pass filter: it's designed in a way that, if the filter is on, the input stage does not "hear" the rumble and passes on the musical part of the sound. That switch is your best friend, and almost always using it it's the smart thing to do.
The usual exception is if you're recording a kick or a bass cab - and the mic can handle the power without distorting (if not, you're using the wrong microphone). However, if you're not sure how the signal was recorded, or your mic doesn't have a high-pass filter, the next best thing is to add it first when laying out the tracks for mixing.
So, shall we break the rules?
This is why generic statements like "it's hard to put hard rules on anything in music" are, well, plain wrong. They simply show that one hasn't really understood what's going on - at least to the level where it affects decision-making. Certain rules can be broken. Other are like gravity - they're just inherent to how things are in the physical world (and sound is very much a physical entity - pressure waves in a gas..). So next time go ahead and by all means make a song with three choruses and two drops, or with reversed words, or putting dissonant chords in the right place, whatever... and maybe you'll make some original music and even a hit.
But for goodness' sake, hi-pass all the tracks that need be hi-passed! Happy recording!
