Changed Bass Notes
When constructing chord sequences, root position Triads and Quadriads would provide the strongest indication of the chord function, since the easily identifiable, lowest-sounding bass note would also coincide with the function-defining degree of the key scale. See Chords (basic)
However, inversions of Triads and Quadriads do not, interestingly, alter the function of the chord. This result is due to the combined harmonics of the notes being sounded. On the one hand, the changed bass note urges the ear to hear a major triad over that bass note (since the first 3 harmonics of the bass note spell the octave, the 5th , the next octave and the maj.3rd intervals). On the other hand, the real notes being played above the bass note completely cover the harmonics (they are all a 2nd apart from the harmonics). Furthermore, these played notes above the changed bass note correspond to the upper harmonics of a fundamental bass note which is not being played ! The overall ‘mix’ of notes is more reminiscent of the root position chord than any other chord found on the scale.
This “reminiscence” is harder to achieve the further the bass note of an inverted chord is below the upper 3 notes, because its harmonics will be stronger, and less challenged by the upper notes.
So what is the purpose of changing the bass note anyway? To create smoother and more interesting melodic motion in the bass. If the changed bass note substitutes a clumsy or monotonous note in another part, it may also serve to render the other part smoother and more interesting. Furthermore, changed bass notes may serve to avoid consecutive and hidden fifths and octaves.
For example, the following chord progression will have a smoother bass line when the bass notes are changed. Compare the progression before and after changing the bass notes.
Progression with fundamental bass notes: Listen
Progression with changed bass notes: Listen
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