The low end of a kick drum can make a song feel big, loose, tight, fast or slow. Finding the right frequency balance is crucial.
I'm throwing around a couple of terms here that are neither scientific nor "correct". But they are commonly used and can be helpful for understanding the concept and thinking behind shaping the low frequency content of a kick drum.
So what the heck am I talking about? We're basically dividing the low end (bass) into three parts.
The very low part of the frequency spectrum, let's say 80Hz and below (again, there's no rule for this) is what we call "sub bass". This is where the fundamental of your kick drum typically sits, unless you've tuned it very high. A lot of energy in this part of the spectrum typically results in a rather "slow" feel, but it also feels big and adds a pulse and a "rumble" to the song that you can not only hear but feel (on big enough speakers). This is what I call the "BOOM". Can you feel it?
"Mid bass" could be the area between 80Hz and 150Hz, for example. This is where you'll find harmonics. Let's say your kick drum fundamental is at 50Hz. The octave of that will be at 100Hz. This part typically feels much more punchy and tight. It makes the song feel faster and more aggressive. A lot of energy here will sound like the drummer hit harder. But it will also sound smaller if you shift the balance from lots of "sub bass" to more "mid bass". Think "more punch, less rumble".
"Upper bass" could mean 150Hz-250Hz. Depending on how you look at it, you could also say this is low midrange. Whatever you want to call it, this is where you'll probably hear a "knock" in the kick drum that will jump out of the mix even on small speakers. This can be cool, especially when combined with a lot of "sub bass", while carving out some "mid bass" to make room for the bass guitar and to keep the kick drum feeling big. But it's also dangerous, as it can clash with lots of other elements, like guitars, bass and even the lower part of the vocals. It can also quickly overload crappy speakers and jump out too much if not controlled properly. The "knock" can also turn into an audible "note", depending on the tuning and dampening of the kick drum. This note can be distracting or simply annoying.
So, the key is to figure out the right balance for the song
And this is not only a mixing decision. It all starts with the drum selection, drum head selection, the tuning and the player. Then you need to choose a microphone that features (or hides) certain frequencies just right. While making those decisions it helps immensely to use a frequency analyzer to confirm what you think you're hearing. Judging the low end can be very difficult, especially in less-then-ideal rooms or on headphones with an uneven low end response. Watching the changes on an analyzer as you go helps you learn really quickly. You'll get a feel for the different parts of the low end and learn which tweaks really make a difference.
Two popular examples of kick drum mics with completely different low end characteristics:
An AKG D112 will give you just enough "sub bass" (you'll probably want more), a very punchy "mid bass" and a lot of "knock" and "wood". It sounds fast and tight, but also "boxy".
An Audix D6 will give you tons of huge "sub bass", just enough punchy "mid bass" to make it audible on smaller speakers and very little "knock". It sounds huge and modern but also slow and "loose".
You can choose or combine. Whatever serves the song. Once you've chosen the right combination of drum selection, drum head selection, tuning and mic to enhance the feel of the song, you need to make those same decisions about the bass and every other low end element in your arrangement.
Yes, getting the low end right is difficult
But it's the key to making people move and making people feel the energy and impact of your song. If you get it right it's magical. Get it wrong and the groove falls apart.
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