Welcome to the Sea of Echoes

Working on the album for the last umpteen months has taught me the importance of great tone especially for bass. We all know great tone when we hear it - it’s the essential element of music that distinguishes the various instruments in a band and yet it is hard to control because tone is not described with the traditional pitch/time notation of music.  Good tone sounds good when you play a single note.

So what makes great bass tone?  Bass is a tricky instrument because great bass is more often felt than heard.  And although loudness is important for this, there is a crucial element of not stomping on other instruments in the band.  A lot of concerts I go to see to solve this problem by pumping the bass through a hard lowpass filter set around 100Hz or so, so that all the punch and definition of the bass is rounded off.  This is great in a big band because the bass stays out of the way of the guitars.  Also if the attack of the bass is rolled off then the kick drum will totally dominate.

Still the recordings that I like the best in small band settings the bass plays an essential role, because it needs tone to fill in so the guitar can be versatile.

Some Examples of Great Tone

First let’s consider a couple recordings with what I consider great bass tone.  For each I’ll describe the bass tone with words and then look at it from a signal processing perspective to see what else we can learn.

Alice in Chains - Would

This is a good song to start with because it opens with a repeating 2-bar bass figure.  The notes used are low in the register of a four string bass with an sixteenth-note rhythm and melody that emphasizes first downbeats and then upbeats.

This bass tone is thick, with a fair amount of breathy or puffy sounding highs which give it an explosive, percusive quality even without the kick or toms in.  The overall emotional quality of the sound is urgent, tense, and growly. There is also a fair amount of reverb to give it fullness.  What does this look like in signal processing land?

“Would” - Time Domain View - First Note

The figure shows us the following basic characteristics of the sound:

• Attack:
  • peaks at +/-0.35 (-10db)
  • there is plenty of high frequency (2-4kHz) from the pick attack (-20 to -30db)
  • the pick attack precedes the low frequencies sound by about 1k samples (20msec!?)
  • the bass turns on with an impossibly steep step around 1k samples
  • initial bass risetime is 110 samples (2.5msec)
  • the compressor lets through only about 25msec of bass attack
• Sustained level is at +/-0.2
  • the compressor is on here and there is almost no decay here until the next note begins
  • there is a fundamental at about 40Hz, which matches the pitch of the low E string
  • there is quite a bit of second and third harmonic
• There are subtle differences in stereo imagning for the bass tracks

“Would” - Frequency Domain View - First Note

• fundamental of 40Hz is 15db down from the 2nd and third harmonics which are about even
• everything about 250Hz is 40db below the primary
• general signal rolls off around 40db per decade - some of this looks a lot like a reverb decay curve
• noise floor is around 80db down from main signal
• it appears that the band from 2.2-3.1kHz has about 10db of heat

In general the waveforms look a lot less regular than the comb structures I would expect to see for an single note from a stringed instrument.  In particular none of the peaks stand more than 20dB above their neighboring frequencies.

Let’s look at another song.

Deftones - Change - Thick and Throaty

This song is a little harder to pick apart because the bass line is with the drums and guitars, but its a good example.  The song begins with a guitar riff at -12db(peak), picks up to -6db(peak) from a synth and some noise on the second repetition, and then is at 0db peak when the bass and drums come in.  When the whole band gets going in the chorus, the subjective sound is much much louder.

Note that the drums play a big role in the sound of the bass.  In particular the kick drum in the spectrum analyzer looks like a Mauna Kea with a loudness of 10dB at 60Hz and a 10db bw of about 60Hz.  Almost all of the apparent attack of the bass is actually the kick, since the bass notes that are played between the kick have a totally different quality.  Listen to this short wave file to hear what I’m talking about:

ref-deftones-change-note2

Sweeping a pretty steep filter through various frequencies shows the following sweet spots:

• 30-40Hz - (the heart) this is the ultimate feel good spot for the bass and where the bass gets its thickness.  Even through headphones I experience these frequencies in my chest with a tickling sensation that is really great.  This seems to be the magic missing ingredient.
• 70-80HZ - (upper chest, lower) I can just begin to make these sounds if hum really low, but I’m really digging in the bottom of my range.  On this track this is where most of the sound energy is and it gives the bass a deep growly, thunderous quality.
• 110Hz - (the throat) - We’re firmly at the bottom of the male vocal range and the tone here sort of a low throaty hum.   There is enough definition here to bring out a little distortion and gravel, but it can start to get boomy.
• 140Hz - the mouth - This particular track has this frequency rolled off pretty well.
• 2-4kHz - the breath - This range appears to be boosted and is where the bass gets its consonanty breathyness, which mostly a “ba ba” sound.

Deftones - Change - Time Domain View

This riff is mixed with all the instruments, but the bass is the loudest element in this section of the song except the kick.   Guitars are pulled back about 10-15db in comparison until the chorus.

There are three notes played here but notice that there is very little attack.  The peaks at the attack are only 30% higher than the rest of the peaks and the attack lasts only about 30msec.

Deftones - Change - Frequency Analysis

Notice the following features:

• Fundamental is loudest with second harmonic close behind (only 3db less).  Third and fourth harmonics drop 10-13db. (This could even be guitar).
• Note the lift in the 2-4kHz band again (though this is not necessarily bass), though it is still 25db below the bass frequencies.
• Overall rolloff is around 15-20db/decade
• Lots of structure here.  The peaks of the sound stand about 20db above the nearby noise floor (some of this could be the analysis window though).

So this is a very different sound from the Alice in Chains recording, but still quite good.

Tool - 46 and 2

In my mind Tool sets the standard for bass tone.  All the parts work together, but Tool’s bass lines are so tight they are unworldly.

The intro for 42and2 starts with a pedal bass on the high D string (D2: 73Hz) and fingering intervals around the D an octave above (D3: 146Hz).  There is a phaser effect going and some reverb.

The character of the bass is very crisp on attack, and yet the sustain is quite rich because of the effects.

Tool -46 and 2 - Time Domain

• Notice the two very high frequency spurs on the opening attack - these impulses are only 20samples wide, which gives them a bandwidth of about 2kHz.  This is a very fast risetime on the pick attack.
• Unlike some of the previous waveforms the high frequency (2-3kHz) components here are quite pronounced, perhaps only 12db below the bass frequencies.
• The compressor closes after about 4.6k samples (50msec).
• Attack peaks are 6dB above the floor maintained by the compressor.

Tool - 46 and 2 - Frequency Domain

• Here we see the first three harmonics virtually equal in size, and towering over the noise floor in that frequency range for a nice clear bell like tone.
• Everything from 300-400Hz is at least 35dB down.
• As expected the consonants at 2.2kHz are very hot and only 16db below the fundamental.
• After 5kHz everything is rolled off.

Our recordings

Most of my bass tracks were recorded direct and then reamped and are a combination of the following:

• the DI sound which is pretty clear and bright with little distortion or growl
• an fx sound with an Amplitube guitar patch intended to give the bass some dirt on the high end
• reamped through a Mesa 2×12 cab in a room, which gives the track a little more growl, rattle and reverb

• The tone is very clean.  The harmonics stick out 30db above any nearby noise!
• The breathyness at 2kHz is 50dB below the fundamental.
• The second harmonic is 24db below the first harmonic.

This section is in a high octave so the tone here is probably ok as it is.  The real problem is these are guitar notes and not bass notes.  But it is interesting to consider what things could sound like with a different eq and filtering arrangement.