What is a musical form where two or more voices sing the same melody starting at different times?

The musical form where two or more voices sing the same melody starting at different times is known as a round. A round is characterized by its simple, repetitive structure, where each voice enters successively at different points in the melody. This results in harmonious overlapping of melodies as each voice completes the cycle of the song.

In a round, as one voice finishes the melody, another voice begins, creating a layering effect that can be both complex and beautiful. This technique is often used in choral and vocal music as a means of building harmony and texture within the piece.

While the term "canon" also describes a similar practice of voices entering at different times, it typically follows a more strict compositional rule regarding the intervals and structure, often involving variations or specific counterpoint. A round is more informal and accessible, making it easier for singers to join in at different times without the need for precise counterpoint techniques.

The other options, acapella and vibrato, do not pertain to the concept of singing the same melody in succession. Acapella refers to singing without instrumental accompaniment, while vibrato is a technique used to add expression to the sound by oscillating the pitch of the notes sung.