The loudness of music is a relatively straightforward concept. Composers and musicians play music loudly or softly to evoke certain moods and feelings. Broadly, the term dynamics refers to the loudness and/or softness of music. Composers use Italian words to convey their wishes for the dynamics for specific segments of music. Forte means loud, and piano means soft. The prefix mezzo (medium), or suffix -issimo (extremely) can be used to modify the base dynamic.
F and p are used to abbreviate forte and piano, m is used, as in mp (medium soft), for mezzo and the base dynamic is doubled to indicate the –issimo effect, as in pp or ff for pianissimo or fortissimo. Rarely, composers go on p and f binges similar to the way people get carried away with exclamation points in social media posts. Wikipedia notes several examples of such dynamic aggrandizement, enacted even by such greats as Tchaikovsky.
Dynamics can change suddenly or gradually over the course of a piece of music. A gradual loudening or softening is called crescendo, or diminuendo/decrescendo, respectively.
Finally, loudness expressed in this way is relative; it’s not about specific decibel levels. Piano in a tiny room played for two people might be different than piano in an outdoor amphitheater for thousands.
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