When a string or whatever object vibrates at a constant rate you assign a pitch to it, but there is never only one pitch. The string produces overtones with various mathematical relationships to the fundamental, and the relative volume of the overtones is one of the major determinants of your tone. You probably know of these overtones as “harmonics”. This exercise assumes you know how to play natural harmonics, which you should. Here is the best chart I’ve found for harmonics on the bass, which I recommend printing out and keeping around. There are probably a few on here you didn’t know. As a general rule, the larger the denominator of the fraction, the harder it is to play. It helps to have a relatively fresh set of strings, play close to the bridge, and turn your passive tone knob up all the way.
The purpose of this exercise is to help you hear the overtones and understand how they respond to your EQ. Generally you produce harmonics by touching the string with your left hand in just the right place (called a “node”) and plucking it with your right hand. For this exercise, we’re going to do things a little differently.
As much as possible, make your surroundings silent. If you can plug headphones into your amp, do so. You will be listening to very subtle features of your sound, and it’s a lot easier without distraction.
Turn your amp up as loud as you can without hurting your ears or pissing off the neighbors.
Play your open E string, and allow the note to decay. Listen to what happens as it decays. The overtones are not all decaying at the same rate.
Keep playing the string until you start hearing some overtones drop out. Try playing it hard and soft, and note the differences in tone. In general, playing harder excites higher harmonics.
Try and identify the overtone you hear most prominently at the end of the note.
Repeat with the A, D, and G strings. How does their decay differ? Does one string take more or less time to decay? Are the overtones dropping out in the same order, or is it different? Even on a single instrument, each string has its own character.
Your ears are now primed and ready to really focus on hearing those overtones. Play an open E, let it ring out for a short time (just 1/2 a second or so), and touch the string at the 12th fret without pressing down to produce a harmonic. By hitting the string initially, you are setting a whole bunch of overtones in motion. By touching the string to produce the harmonic, you are isolating a single one.
Repeat until you can hear the harmonic within the sound of the open string. Sing it, if that helps. It was always there, but you weren’t paying attention to it because the fundamental was so much louder.
Now do the same thing for every other harmonic you can play on the E string – hit the string, and then touch the node. At very least, you should be able to produce the one just behind the 4th fret, which is an untempered major third. But keep going up until you’re not hearing any more overtones. You might find that allowing the string to vibrate for a shorter time before touching it makes the harmonics louder.
Repeat on the A, D, and G strings.
Now that you know how high you can realistically go, start messing with your EQ (on both your instrument and your amp) and playing different harmonics. If you have multiple pickups, try switching among them, too. In general, turning up your highs makes harmonics louder, because they’re high. But pay attention to the nuances – does your EQ favor particular harmonics? Is there a setting that makes certain ones inaudible? How does the effect of the EQ vary from one string to the next? If you have a graphic EQ you can get pretty deep with this.
Before you stop, hit each string and allow it to fully decay one last time. What do you hear now that you didn’t hear before?
This exercise is necessarily sort of incomplete, because there are many overtones in your sound that are too high to play as harmonics. It is merely intended to get you thinking about the elements that make up your tone. Of course, there are a lot of other factors that determine tone. Stay tuned for another exercise focusing on attack.