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Get loud—or get soft! Play it piano, or fortissimo, or somewhere in between. Get louder and louder and louder, or quieter and quieter and quieter.
When you listen to music, you have probably noticed that it's not all at the same volume, all the time. Some parts get really loud, and some parts get really soft. These variations are known as "dynamics."  If the rhythm and meter are the heart of the music, and notes and keys are the brains, then dynamics are surely the voice of the music. Consider the first version above. On your table, tap out: 1 and 2 and 3 and 4 and 5 and 6 and 7 and 8, etc. (the and is how musicians "say" eighth notes). Make sure every beat is tapped at the same loudness, so that it sounds a bit like a helicopter. Now take a look at the second version. Notice the accent mark (>) above every F note. Tap that out, only this time, accent every beat that you see the accent mark. Now, instead of a helicopter, it should sound more like a train. With just a subtle shift in accent, we completely change the character of the music! Just like you don't always talk at the same level—you modulate your voice louder or softer, depending on the situation—music modulates in level too. The way the composer tells the musician what is intended is by using dynamic markings.  There are dozens of dynamic markings you may see on a piece of music, but some of the most common ones you'll find will be the letters f, m, and p.  p means "piano," or "softly."  f means "forte," or "loud."  m means "mezzo," or "medium." This modifies the dynamic after it, as in mf  which means "medium loud", or mp, which means "medium soft." The more ps or fs you have, the softer or louder the music is to be played. Try singing the example above (using solfège—the first note in this example is the tonic, or "do"), and use the dynamic markings to notice the difference. Another very common dynamic notation is the crescendo, and it's corollary, the decrescendo or "diminuendo". They are a visual representations of a gradual change in volume which look like stretched-out "<" and ">" symbols.  A crescendo gradually gets louder, and a decrescendo gradually decreases the volume. You'll notice that, with these two symbols, the "open" end of the symbol represents the louder dynamic and the closed end represents the quieter dynamic. For example, if the music directs you to gradually go from forte to piano, you'll see an f', then a stretched out ">", then a 'p'.  Sometimes a crescendo or diminuendo will be represented as the shortened words cresc." (crescendo) or dim. (diminuendo).