What does tremolo refer to in music?

Tremolo in music refers specifically to a rapid and regular variation in the amplitude of a sound. It creates a trembling or quivering effect, which is achieved by oscillating the volume level. This modulation of amplitude enhances the expressiveness of the music and is often used in both instrumental and vocal performances.

In contrast, the other choices do not accurately describe tremolo. Frequency modulation involves changing the pitch or frequency of a sound wave, which is distinct from the amplitude changes associated with tremolo. Resonance effects pertain to the amplification or sustaining of sound due to the natural frequencies of an object, while pitch modulation refers to variations in pitch rather than volume. Therefore, the correct identification of tremolo as a method of amplitude modulation provides clarity on its role and application in music.