As previously stated, tonality can be broken up into two parts: melody and harmony. The bass line of a song provides a direct link to the harmonic progression because it focuses on the chord roots of each chord. If you listen to the bass line, you will dramatically improve your ability to hear harmonic changes.
Procedures for Singing Bass Lines
Listen to the melody of a familiar tune. This first video uses the folk chestnut, “Skip To My Lou.” If the song is unfamiliar, you may wish to listen a few times to familiarize yourself with the melody.
Listen again, and hum along with the tune. Ask yourself if you hear the harmonic changes that accompany the tune. Do you hear where the I and V should go?
Hum along again. This time, the bass line will be added. Tune your ears to both parts. When the bass line changes notes, that indicates that there is a harmonic change.
After you are comfortable with that, “bum” along with the bass line in whatever octave feels comfortable. You will be hearing two parts simultaneously - the melody and the bass line. The bass line provides the chord root for the harmonic function.
Next, add a verbal association. This gives your brain a label for the sound. I use the words “one” and “five” on the pitches DO and SO. (For minor, sing the words “one” and “five” on the pitches LA and MI.)
Finally, alternate between singing the melody while listening to the bass line, and singing the bass line while listening to the melody. Do each one three times. Begin by singing the melody.