re: min maj 6ths A min 6th is the inversion of a maj 3rd, maj...

re: min maj 6ths A min 6th is the inversion of a maj 3rd, maj 6th inversion of min 3rd. These refer to intervals between two notes - eg c - a is a maj 6th and a - c is a min 3rd.

Vibrating systems, such as a string, are described as a harmonic series. In fact a string vibrating at say 440 hz has a whole series of subsequent and concurrent vibrations, demonstrated by lightly touching the string at the exact halfway point which will give you the octave of the fundamental, at a third of the length a fifth and so on.

Amplitude refers to the displacement of vibration - ie how far the string moves in either direction - which we hear as volume, it is independent of frequency (pitch)