Which musical concept involves changing the time signature?

The concept of changing the time signature is best represented by mixed metre. Mixed metre refers to the use of more than one time signature within a piece of music or even within a single phrase. This might involve alternating between different time signatures, creating a complex rhythmic feel that allows for varied accents and articulations, which can enhance the expressiveness and interest of the music.

In contrast, monophonic refers to a texture with a single melodic line without any accompaniment, which does not inherently involve changes in time signature. Modulation refers to the process of changing keys within a piece, focusing on pitch rather than rhythm. A motif is a short, recurring musical idea or phrase, typically melodic or rhythmic in nature, but does not entail changes in time signature on its own. Thus, mixed metre most directly relates to the concept of altering time signatures within a musical context.