Music: Practice & Theory Stack Exchange is a question and answer site for musicians, students, and enthusiasts. It only takes a minute to sign up.
Sign up to join this community
There are a lot of songs that have a melody, usually a simple one or even a two-note pattern, that is played during many bars, but the bass note for each bar changes, implying chord changes?
One example is the intro to Sweet Child O'Mine.
UPDATE: Other is the two-note guitar melody in U2 "With or Without You".
UPDATE2: A perfect example is the guitar melody in Steve Vai's "Junkie", which keeps unaltered even after full strummed chord progression begins.
Is there a "formula" do achieve this?
A repeating phrase is usually called ostinato, regardless of whether or not the harmony underneath changes. It is common that an ostinato line changes a few notes to fit changing chords, but that's not necessary. Note that your example of Sweet Child O'Mine does exactly that, it doesn't repeat unaltered but there is always one note (the first one) that is changed to fit the current chord.
It is quite simple to find fixed phrases that fit well over diatonic changes. Especially if you use pentatonic ideas, the melodies will fit almost any chord of the key. As an example, use a simple two-tone motif with the notes A and G, and play it over the changes ||: Am | G | F | G :||.
A similar concept is found in "Cry me a River" by Davey Graham in 1959. In the intro, he arpeggiates an Am chord four times, and each time the arpeggio starts with a bass note that descends chromatically-- first A (matching the root of the chord), then Ab, G, and Gb.
Similar to this is the intro to "Stairway to Heaven," which features successive forms of an Am then D chord, with the same descending bass notes as Cry Me a River.
I believe this technique is called the "line cliche," and goes all the way back to Bach. So it may help to search for that phrase.
