In sheet music, 'loud' and 'soft' never mean a fixed volume
A tuba playing moderately soft will usually drown out a guitar playing loud, and a piccolo can seem piercing at a lower decibel level than its neighbours. Musical dynamics markings are relative instructions, not measurements, which is why Tchaikovsky could ask one bassoonist to play with six ps, softer than almost anything on paper.
Dynamics describe how loudness changes across notes and phrases, and they carry much of music's emotional weight. The two basic signs are p for piano, soft, and f for forte, loud. Mezzo versions sit between them, doubling and tripling the letters pushes further toward either extreme, and ffff asks for as much volume and force as possible. Performers still shape every passage: the middle of a phrase normally swells even when the whole thing is marked soft, and melody and bass lines are brought forward within a single marking.
Gradual change has its own vocabulary. Crescendo means growing, while decrescendo and diminuendo both mean fading. Written out, these words can stretch across several pages; drawn as widening or narrowing wedges, nicknamed hairpins, they usually cover only a few bars. Molto makes a change dramatic and poco makes it gentle, and morendo, meaning dying, asks the music to slow and fade together.
Sudden effects are marked separately. Subito, meaning suddenly, turns a new level into an abrupt switch, and subito piano is often used to catch listeners off guard with a sudden intimacy. Sforzando marks a single beat struck suddenly hard, fortepiano means loud then instantly soft, and niente asks for nothing at all. Brahms used pf, a little loud, to suggest a forte character played with a piano sound, though the abbreviation is easily mistaken for the instrument's name.
Composers from the late nineteenth century onward began stretching the scale, usually backing the ink with orchestration such as massed brass and percussion. Holst wrote ffff twice in 'Mars' from The Planets, Stravinsky used it for the strings and woodwinds closing The Firebird Suite, and Grieg scattered it through the cadenza of his Piano Concerto. At the quiet end, Verdi's Otello has a baritone passage marked pppp in one source.
Source: Dynamics (music)
