As previously mentioned, the bass portion of the audible spectrum runs from 20 Hz to about 300 Hz.  Setting aside the previously discussed sub-bass portion of this frequency band (frequencies 45 Hz and below), we can say that the bass portion of the spectrum should be reserved primarily for the fundamental frequencies of the roots of the chord changes in the song insofar as tonal content is concerned.  Of course this range should also incorporate low frequency sounds such as kick drums, toms, and even room tones.

Many of the biggest problems people encounter in tracking, mixing, and mastering occur squarely in this region.  Terms like muddy, boomy, and woofy all deal explicitly with the bass region.  We all want “big bass” with lots of thunderous kick drums and thumpin’ bass lines, but unfortunately the arithmetic is not so simple as “turn them all up.”  As many of you following along at home might have already experienced, turning up all the bass instruments in your mix is a recipe for a muddy, distorted mess.

So how do we properly address these issues to get a decent sounding mix?  Well, first we need to take note of the frequency band that encompasses the bass portion and see how it compares to the other bandwidths:

Bass from 25-300 Hz

Treble 2.4-20 kHz.

Look at that again.  That says that the bass range has a bandwidth of about 275 Hz while the treble range has a bandwidth of almost 18,000 Hz!  No wonder we run into problems of indistinct bass but not indistinct top end.

In composition, there is something called the Lower Interval Limit.  This is a commonly held set of rules that say, based on the frequency of the first note, how big the interval must be in order for that interval to sound clear and distinct.  For instance, if we were to use the 440 Hz A as our base note and play the C above that to form a harmonic interval of a minor third, we’d have a difference in frequencies 83.25 cycles per second (C5 is 523.25 Hz so 523.25-440=83.25 Hz).  This is a difference that our ears can distinctly hear without hesitation and we perceive as a pleasant albeit sad sonority.

Now imagine that we started with A four octaves down.  This A has a fundamental frequency of 27.5 Hz.  The minor third above that is a C with a fundamental frequency of 32.70 Hz.  This provides a much tougher to distinguish difference of only 5.2 Hz.

Furthermore, the difference between that A and it’s next closest upper neighbor A# is only 1.64 Hz.  So even in a melodic context, it can sometimes be difficult to properly distinguish the two notes.

As an aside, here is a handy-dandy list of the lowest notes generally accepted in order to have a properly sounding interval.  Bear in mind that these are only commonly held compositional standards and are free to be broken at any time:

The first column is the desired interval.  The second column is the lowest note from which you can build the desired interval.  The third column is the co-responding note needed above the lowest pitch to complete the desired interval.

In my mind, muddiness occurs when we have too much bass information going on simultaneously that creates a big mess of sounds too close in frequency content.  This contributes to a washy indistinct bass. 

Generally speaking, the most common problem is figuring out how to separate the kick drum from the bass.  It is important to remember that even though the kick drum is often regarded as an atonal instrument, it still produces tonal frequencies and especially distinct fundamentals.  So if the bass and the kick drum are sounding in roughly the same range, our ears will be unable to distinguish the two sonorities.

One way to address this is by making sure that each instrument emphasizes different portions of the bass frequency band.  Ideally, these portions would follow the Lower Interval Limit.  For example, if the kick drum is tuned so that its fundamental sounds at about 60 Hz (which is roughly a B1), the bass should play no lower than D#2.  This way the fundamentals adhere to the lower interval limit theory and are reasonably sure to be clear and distinct sounds.

While the Lower Interval Limit theory  is not explicitly intended for this purpose and often it is meant to be used for harmonic intervals (notes sounding at the same time, generally on the same instrument but not necessarily), the point is that creating distinguishable sonorities is all about being able to distinctly hear differences between sounds.  We want to refrain from confusing our ears with sounds that are too close together that muddle distinction.

This will obviously not solve all the problems.  If you’ve ever seen a waveform for a kick drum, you know that its spectral content is very broad and not relegated simply to its fundamental frequency.  As such, it is further beneficial to deal with frequencies beyond the fundamental, however most of that will be dealt with as we move up the audible spectrum into the mid ranges.  For now the focus is on what we can do specifically in the bass register to prevent problems.

That aside, there is always a big collection of frequencies that sound in the bass register on a kick drum.  This is due to many resonations that aren’t perfectly in tune:  the beater head, the shell, the resonant head, not to mention all the nodes between the lugs on the head that yield very dense and complex waveforms.  These frequencies can be so broad that they can encompass a very large portion of the bass register and make the aforementioned solution pretty impossible. 

One way to address this strictly in the bass register is to eq out whole sections of the kick drum so that it creates space for the bass guitar.  If you know the key of the song, you can determine the lowest note the bass player might play.  Your job then would be to carve out a nice chunk of the kick drum sound, not only cover the register that the bass plays, but also below it to create enough space that the two sounds are distinguishable from each other by the Lower Interval Limit theory.

Another aside:  doing these things may seem drastic, but bear in mind that your ultimate goal is NOT to create the best kick drum sound possible and the best bass guitar sound possible and add them together.  Instead, your goal is to create the best sounds for each instrument that work together so that they sound good together in the mix as a whole.

Another issue that crops up is when there are a whole bunch of overdubs that play in the same area.  It is difficult in the bass range for a bass guitar to sound distinct from a bass synth and then have both of them stand out from the kick because you only have about 225 Hz to work with.  Layering a bunch off overdubs can lead to muddling if you do too much layering in the bass region.  One example that comes immediately to mind is an 808 kick drum and trying to make that audible against a bed of kick drum and bass guitar.  The easy thing about 808s is that they are basically sine waves.  So it is easy to determine the note that the 808 is sounding, and game plan the kick and bass around it.

But for more dense things like synthesizers, it can be more problematic.  As I mentioned in the harmonics primer, there are a whole bunch of other frequencies that sound at any given time from any instrument.  Some of these occur in the harmonic series, but many others like formants and native resonances do not.  Sometimes these can occur in clusters around the fundamental note and when that occurs, the extracurricular frequencies from the bass and the synth and kick all roll up together and make a big muddy mess.

The best way to address this is to avoid excessive overdubs in the bass register.  Another way to deal with it is to find out what you can change easily, like the kick drum since it is static, and treat it in a way that keeps it out of the way of the bass and other instruments.  For instance you might EQ to emphasize the fundamental below the key of the song, and then eq out portions from the root up about an octave and a half to keep it out of the way of the bass and other bass instruments.

Next week, I’ll post some common terms associated with bass problems with some quick tips on how to address them.

Then, I’ll examine some issues in the mid-range and further delve into how to mitigate problems associated with the bass as well as those unique to the mid-range.