In this post I will share some of what I've learned about how to achieve a clear sounding low end.
I find that in general it is the low frequencies of the vocals that mess with the low end. The vocals most often have quite a lot of signal assigned which means that the low frequencies will be fairly loud in the mix. This is also why it pays off to duck the kick snare bass on the attack of the vocals, because now you don't have to hi-pass filter the vocals as aggressively which makes the vocals fatter sounding. But the low frequencies of the vocals will cause modulation noise which will distort some of the harmonic content in the low end. In many cases it is therefore good to apply a hi-pass filter on the vocals. What that will do, with almost 100% certainty, is that if you have some very low sub frequencies in the mix, the hi-pass filter on the vocals might for the very first time make those very low sub frequencies audible. So when you work with the hi-pass filter on the vocals, be aware that you might at first only notice the clearity only on some of the bass tones in the arrangement. But it can sound very sweet when you suddenly get those sub frequencies cutting through the mix that you did not hear before.
50 to 500 Hz is tricky. I find that in general it works better to focus 50 to 250 Hz to the mid component and 250 Hz to 500 Hz to the side component. What I think can work quite well is that if you have electric guitars, piano and organ that compete in the low end, then if you make them active on the side, apply a steep high pass filter near 250 Hz with the Q so that it compensates around 250 Hz with a few dB, it will help clear up the low end in conjunction with the pan faders. What this does is it creates the perception that the electric guitars are active in the low end, although in practice that's mostly due to the bump at around 250 Hz. Now you have cleared much of the low end in the mid component, so that the bass and kick can be clearly heard in the center.
For a clear low end it is also important that the attack portion of the bass guitar is clear in the mix. This makes all chords punchy because the attack of the bass string is clearly separated in the mix. The issue with side chaining only the low end is that whenever the bass guitar plays fills higher up on the fretboard, those tones disappear. So the attack portion should be clear across the whole frequency spectrum. To do this one can side chain the bass guitar with a fast release time to duck all sound sources except kick, snare, lead vocals. This will make the pluck of the bass guitar sit on top of the mix and can sound very nice.
When it comes to the harmonization of the bass guitar, that's maybe the most important thing about the low end. This is only possible when the rumble/modulation noise of the low end has been removed so that you can separate the saturation on the bass guitar in the perception. This means that besides hi passing the lead vocals, piano, organ, guitars, you might want to add a hi-pass filter on the remaining sound sources as well, but not so high because that will thin out the mix too much. If you for instance have background vocals and strings, you might want to set the hi-pass filter steeply somewhere in the 125Hz region so that the 75 to 125 Hz area is free and the 125 to 175 Hz is not. Because when you also have the hi-pass filters on the master bus, this will focus the compression to the 75 to 125 Hz area and it is nice if that area is relatively airy/low in density, in other words that it does not contain too many masking frequencies from various sound sources so that the master bus comps and saturators act specifically on those, mostly the kick and bass. This makes the low end less muddy because the saturation is applied on a less complex set of frequencies.
When it comes to the harmonization of the low end, there are several things you can do. I like to apply something like a MaxBass lightly, then pitch shifters on top to enhance the harmonic multiples of that and then tape saturation on top.
Now, to get an "even" bass melody this is to a great degree caused by having an "equal" amount of space for the bass on the tones it plays. Naturally due to the hi-pass filters and because the mix becomes more dense towards the mids, the lower bass tones will be separated a bit more clearly than the higher ones, this is what causes some of the bass tones on some of the chords to drop out a little, it's not nice sounding. To combat this issue you can split the low end of the bass guitar into a low and high track, I like to use 300 Hz split in half, so that the low bass track is 150Hz and below and the high bass track is 150 to 300 Hz. Then you can apply a vocal or bass volume rider on each of these tracks, so that you keep the rms level of the high bass track a bit louder than the rms level of the low track. It can be nice to spread out the high bass track a little more towards the side, and the low bass track a little more towards the center, this can help separate the bass guitar in the mix, so that you can lower its signal and hence free up signal to the mix. But when you do that, do it against stereo widening applied, not afterwards, always pan into stereo widening. This also helps to lower the density of the mix also, because in order to make all of the bass notes cut through the mix you don't have to get the low frequencies gained with it, you free up some mix headroom as well because now you have available signal left that you can focus on the mids and highs.
When it comes to the actual "softness" of the low end, that is to a great degree the result of having done the steps above to allow for less gain and combining that with having lowered the peak-to-rms ratio of the low end frequencies, which is achieved using dynamic side-chaining in conjunction with two stage compression and multi-band compression.
For warmth you need to tune the overall frequency response of the bass guitar snare and maybe vocals, so that although they naturally lack some of the warm frequencies, you bring them in with processes. For this I like to do a complex combo of multi-band comp into EQ frequency matching into multi-band comp into EQ frequency matching, as many stages as needed until it is warm enough, often in conjunction with a little high frequency roll off and some modulation effect and warm ambience. Each stage gently applies a shift in the frequency response across the whole frequency range. Sometimes I call this frequency morphing, because it alters the entire signature of the sound source. When you apply additional ambience for warmth, do it so that it is the most reverb noisy track in the mix that dictates the wetness of the reverb (not only on a single spot in the mix but across the song), else you will overdo the reverb. It is better to do it like this than individually in solo, because in solo you will not notice when the reverb gets too noisy in the context. A good sign that you have applied the right amount of reverb is when you lack some reverb on some sound sources in solo. This is maybe especially important when adding reverb on bass guitar and kick, which can sound awesome but the amount of the reverb is the critical part.
I think this is it. The rest is in my view a matter of taste. Some genres require certain dynamics in the low end, so from here it will vary what you do, but I would say that in general the above mixing decisions will be of help to achieve a better sounding low end.
I find that in general it is the low frequencies of the vocals that mess with the low end. The vocals most often have quite a lot of signal assigned which means that the low frequencies will be fairly loud in the mix. This is also why it pays off to duck the kick snare bass on the attack of the vocals, because now you don't have to hi-pass filter the vocals as aggressively which makes the vocals fatter sounding. But the low frequencies of the vocals will cause modulation noise which will distort some of the harmonic content in the low end. In many cases it is therefore good to apply a hi-pass filter on the vocals. What that will do, with almost 100% certainty, is that if you have some very low sub frequencies in the mix, the hi-pass filter on the vocals might for the very first time make those very low sub frequencies audible. So when you work with the hi-pass filter on the vocals, be aware that you might at first only notice the clearity only on some of the bass tones in the arrangement. But it can sound very sweet when you suddenly get those sub frequencies cutting through the mix that you did not hear before.
50 to 500 Hz is tricky. I find that in general it works better to focus 50 to 250 Hz to the mid component and 250 Hz to 500 Hz to the side component. What I think can work quite well is that if you have electric guitars, piano and organ that compete in the low end, then if you make them active on the side, apply a steep high pass filter near 250 Hz with the Q so that it compensates around 250 Hz with a few dB, it will help clear up the low end in conjunction with the pan faders. What this does is it creates the perception that the electric guitars are active in the low end, although in practice that's mostly due to the bump at around 250 Hz. Now you have cleared much of the low end in the mid component, so that the bass and kick can be clearly heard in the center.
For a clear low end it is also important that the attack portion of the bass guitar is clear in the mix. This makes all chords punchy because the attack of the bass string is clearly separated in the mix. The issue with side chaining only the low end is that whenever the bass guitar plays fills higher up on the fretboard, those tones disappear. So the attack portion should be clear across the whole frequency spectrum. To do this one can side chain the bass guitar with a fast release time to duck all sound sources except kick, snare, lead vocals. This will make the pluck of the bass guitar sit on top of the mix and can sound very nice.
When it comes to the harmonization of the bass guitar, that's maybe the most important thing about the low end. This is only possible when the rumble/modulation noise of the low end has been removed so that you can separate the saturation on the bass guitar in the perception. This means that besides hi passing the lead vocals, piano, organ, guitars, you might want to add a hi-pass filter on the remaining sound sources as well, but not so high because that will thin out the mix too much. If you for instance have background vocals and strings, you might want to set the hi-pass filter steeply somewhere in the 125Hz region so that the 75 to 125 Hz area is free and the 125 to 175 Hz is not. Because when you also have the hi-pass filters on the master bus, this will focus the compression to the 75 to 125 Hz area and it is nice if that area is relatively airy/low in density, in other words that it does not contain too many masking frequencies from various sound sources so that the master bus comps and saturators act specifically on those, mostly the kick and bass. This makes the low end less muddy because the saturation is applied on a less complex set of frequencies.
When it comes to the harmonization of the low end, there are several things you can do. I like to apply something like a MaxBass lightly, then pitch shifters on top to enhance the harmonic multiples of that and then tape saturation on top.
Now, to get an "even" bass melody this is to a great degree caused by having an "equal" amount of space for the bass on the tones it plays. Naturally due to the hi-pass filters and because the mix becomes more dense towards the mids, the lower bass tones will be separated a bit more clearly than the higher ones, this is what causes some of the bass tones on some of the chords to drop out a little, it's not nice sounding. To combat this issue you can split the low end of the bass guitar into a low and high track, I like to use 300 Hz split in half, so that the low bass track is 150Hz and below and the high bass track is 150 to 300 Hz. Then you can apply a vocal or bass volume rider on each of these tracks, so that you keep the rms level of the high bass track a bit louder than the rms level of the low track. It can be nice to spread out the high bass track a little more towards the side, and the low bass track a little more towards the center, this can help separate the bass guitar in the mix, so that you can lower its signal and hence free up signal to the mix. But when you do that, do it against stereo widening applied, not afterwards, always pan into stereo widening. This also helps to lower the density of the mix also, because in order to make all of the bass notes cut through the mix you don't have to get the low frequencies gained with it, you free up some mix headroom as well because now you have available signal left that you can focus on the mids and highs.
When it comes to the actual "softness" of the low end, that is to a great degree the result of having done the steps above to allow for less gain and combining that with having lowered the peak-to-rms ratio of the low end frequencies, which is achieved using dynamic side-chaining in conjunction with two stage compression and multi-band compression.
For warmth you need to tune the overall frequency response of the bass guitar snare and maybe vocals, so that although they naturally lack some of the warm frequencies, you bring them in with processes. For this I like to do a complex combo of multi-band comp into EQ frequency matching into multi-band comp into EQ frequency matching, as many stages as needed until it is warm enough, often in conjunction with a little high frequency roll off and some modulation effect and warm ambience. Each stage gently applies a shift in the frequency response across the whole frequency range. Sometimes I call this frequency morphing, because it alters the entire signature of the sound source. When you apply additional ambience for warmth, do it so that it is the most reverb noisy track in the mix that dictates the wetness of the reverb (not only on a single spot in the mix but across the song), else you will overdo the reverb. It is better to do it like this than individually in solo, because in solo you will not notice when the reverb gets too noisy in the context. A good sign that you have applied the right amount of reverb is when you lack some reverb on some sound sources in solo. This is maybe especially important when adding reverb on bass guitar and kick, which can sound awesome but the amount of the reverb is the critical part.
I think this is it. The rest is in my view a matter of taste. Some genres require certain dynamics in the low end, so from here it will vary what you do, but I would say that in general the above mixing decisions will be of help to achieve a better sounding low end.
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