"Sound mixing" is the process of combining the different audio sources in a song and making sure everything is balanced. In reality, this is a very technical process that requires professional grade software. Here's a simple look at what's happening: each different sound within a song has a waveform. These waveforms are all different heights, so the volume of each waveform is different. Your audio mixing software, such as Audacity, has a volume slider. In order to get the waveforms of various sounds to be of equal height, you must adjust the volume on your software until they are each the same height. When a song is put together from various sources, the waveforms of those sources aren't in the same place in the song. The chorus of a song might have louder drums than the verses, for example. So to make the waveforms from different sources even, you'll need to do some careful adjustments with your software. After everything is mixed, there may be some sounds that you want to be louder or quieter overall. If your snare drum is too loud in comparison to your voice, you can adjust the volume of each track until you're happy with the sound of the song.
Most songs you listen to probably have the vocals mixed in with the rest of the song. In other words, if you turn the vocal track up and the music down, you can hear the singer just fine. This is called "mono" mixing. Mono mixing has a lot of advantages for both listeners and the band. Listeners can hear everything clearly, and bands don't need to hire additional singers who may not be as good as them. If you want more of an "experience," where the music feels more like a movie and the singer is the star, the vocals can be placed in one speaker (or headphones) and the instruments in the other. This is called "stereo" mixing. This type of mixing takes much more work. To create such a mix, every sound source must be separated and sent to different speakers or headphones. That means hiring an audio engineer to do this for you.
If someone tells you they’ve never had trouble getting the low end of a mix right, they’re either lying or deaf! It seems hard because it is hard, and there are several factors that can lead to bass-induced headaches. Here are some of the most common:
One of the biggest problems is that most of us can’t properly hear what’s happening down there. This can be because our loudspeakers aren’t capable of reproducing low bass notes accurately. More often, though, it’s due to the acoustics of the mix room. Most cheap acoustic treatment is only effective in the mid-range and at high frequencies. To control low frequencies properly, you need large-scale treatment designed by someone who knows what they’re doing, and customized to the environment it’s installed in. It’s tempting to splash cash on those wedge-shaped bits of foam that eBay sellers call bass traps, but they’re unlikely to have any effect below 200Hz or so. The real problems in most domestic rooms are further down the frequency spectrum, and they arise because the dimensions of the room lead to standing waves. These cause some frequencies in the bass range to be massively reinforced, while others are cancelled almost completely. If you do have loudspeakers that can put out decent bass, try playing a selection of sine waves at different frequencies between 40Hz and 120Hz. It’s very likely you’ll hear some as being much louder than others, and that the balance will change depending on where you stand within the room.
The very best way to deal with this is to install proper acoustic treatment. With enough time and knowledge — all of it available on the Internet — it’s possible to build effective bass traps using everyday DIY skills, at a cost comparable to that shiny new microphone or guitar you’ve had your eye on. If you can, do — but bear in mind that bass problems are worst in small rooms, and bass traps only work if they’re big. In a small room with unfortunate dimensions, the chances are you won’t get any useful control over low frequencies until you’ve stuffed it so full of acoustic treatment that there’s no room for you to work! And, of course, if you’re in a rented apartment or house, or you share the use of your ‘studio’ room, there may be other reasons why you can’t build a Taj Mahal of acoustic panels in there.
If you know that your mix room has low-frequency issues and treatment isn’t an option, the last thing you should do is rush out and buy a huge subwoofer. It’s better not to put those 30Hz frequencies into the room in the first place than to have them completely misrepresented!
Though they won’t give you the same physical experience as listening to bass over loudspeakers, a good pair of headphones will eliminate the room as a factor, and will be a much better investment than a half-baked acoustic treatment solution. You could also trial a room acoustic correction system, such as Sonarworks’ Reference or IK Multimedia’s ARC. These work by measuring the response at the listening position and applying corrective equalization to what’s coming out of your monitors. They can’t compensate for the most serious problems related to standing waves, but they can definitely make things better. It’s also worth simply firing up a few well-mixed commercial tracks and listening at different points in the room; you might find that there are spots where the bass seems to come together better, and it’s worth knowing where those are.
Being able to hear what you’re doing is vital, but it’s only part of the battle. We’ve all been in that situation where we continually compare our mix-in-progress to a favorite track, applying more and more processing each time and feeling like we’re never getting any closer. But as with nearly all mix problems, difficulties at the low end are often the fault of the arrangement and the choice of sounds. If we define the bass region as being what lies below 100Hz, then almost everything in a typical metal track could be fighting for space in this region: not only the bass guitar and bass drum, but also floor toms, snare, down-tuned electric guitars and even vocals. Between them, the band, tracking engineer and producer should have decided which instruments have bragging rights in which frequency ranges, but if they’ve left it to us — the mix engineer — to sort out, we often need to be pretty assertive with EQ.
It can be helpful to mentally carve up the bass frequency spectrum into smaller bands and arrange things so that each band is filled mainly by a single instrument. Try running each of the competing elements through a spectrum analyzer, and note where their energy is naturally concentrated. This will indicate where conflicts are occurring. For instance, you might decide to give the bass guitar free rein in the 50Hz region, but cut at 100Hz to make way for the ‘chug’ from the bottom end of the guitars. The guitars, in turn, might need a high-pass filter to remove unwanted noise below this point. If the kick, toms or snare are fighting with the bass and guitars in the same regions, you could gate them so that any unwanted ringing after the initial impact is cut off. Another worthwhile trick is to set up a compressor so that the guitars are momentarily ducked whenever the relevant drum is struck. The classic way to do this is to mix the kick and snare a bit louder than you want them, then put the compressor over the mix bus.
In comparison with your average metal track, techno music is usually incredibly sparse, but arrangement problems can still get in the way of an effective mix. Kick drum sounds from classic drum machines are often based around a low-frequency sine wave with a noise burst at the start. If this sine wave corresponds to an E-flat and the rest of the track is in C-major, it’s not going to sound right at any level in the mix, so experiment with the pitch control on your drum machine or sampler before you resort to complicated EQ and compression setups. A good tip is to pitch the kick drum up by two octaves and run it through a tuner plug-in to find out what note it’s playing.
Another thing that can cause major headaches at the low end is sloppy musicianship. The classic problem when working with sub-par musicians is that whatever level you set the bass guitar or kick drum in the mix, the signal ends up coming and going because the notes are not played consistently — one note will stick out and the next will be completely inaudible. Standard compression can help, but very often you need to go further than that. The tone of a bass or drum changes significantly depending on how hard it’s played, so even if you can bring all the notes to the same peak level with a compressor, some will have much more low end on them than others.
One thing you can try is to insert a low-pass filter into the compressor’s side-chain so that its detector only responds to low frequencies. That way, the compressor will set the level for each note depending on how much bass it contains, so that the bottom end gets evened up. A more powerful approach is to use a dynamic equalizer or multiband dynamics plug-in to compress just the low frequencies. If that doesn’t work, you can try using automating the fader on the problem signals.
Finally, the easy way out is to use a trigger plug-in to add a sampled kick drum alongside the real one, or an audio-to-MIDI processor to generate a synthesized bass track to layer with the bass guitar. The use of sampled drums in particular is very common in modern rock and metal music, so if you’re not using samples, you may well be laboring under an unfair disadvantage!