Vibrato modulates frequency while tremolo modulates amplitude

Evidence

For some musical instruments, such as those in the orchestral string section (violin, viola, cello, double bass), the human voice (particularly in opera solo singing) and to some extent the woodwinds (oboe, clarinet, saxophone, bassoon), an undulation during the steady-state part of the notes is manifest naturally in the acoustic output. This is usually in the form of vibrato and/or tremolo. Vibrato is a variation in fundamental frequency, or frequency modulation, and tremolo is a variation in amplitude, or amplitude modulation. — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 1462

Categorization

• DEFINE: Cleanly separates the two technical terms: vibrato is frequency modulation, tremolo is amplitude modulation.
• SHARE: The kind of tidbit to send a friend who sings in a choir and always conflates vibrato and tremolo.
• corroborated: 2 independent passages support this claim — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 1462