If you were to write a piece of music, you could use some dynamic marks to inform any future performers (including yourself) of how it was you imagined--or intended those notes played.

Dynamics (music)
Relative loudness Teacher. "And what does ff mean?"
Pupil (after mature deliberation). "Fump-Fump." Cartoon from Punch magazine October 6, 1920
The two basic dynamic indications in music are:
- p or piano, meaning "soft" and
- f or forte, meaning "loud" or "strong".
More subtle degrees of loudness or softness are indicated by:
- mp, standing for mezzo-piano, and meaning "medium-quiet" or "moderately-quiet" and
- mf, standing for mezzo-forte, and meaning "medium-loud" or "moderately-loud".
Beyond f and p, there are also
- ff, standing for "fortissimo", and meaning "very loud" and
- pp, standing for "pianissimo", and meaning "very quiet".
To indicate even more extreme degrees of intensity, more ps or fs are added as required. fff and ppp are found in sheet music quite frequently. No standard names for fff and ppp exist, but musicians have invented a variety of neologisms for these designations, including fortissimissimo pianissimissimo, forte fortissimo piano pianissimo, and more simply triple forte triple piano or molto fortissimo molto pianissimo (although in Italian the last expression is not correct). ppp has also been designated "pianissimo possibile".
A few pieces contain dynamic designations with more than three fs (sometimes called "fortondoando") or ps. The Norman Dello Joio Suite for Piano ends with a crescendo to a ffff, and Tchaikovsky indicated a bassoon solo pppppp in his Pathétique symphony and ffff in passages of his 1812 Overture and the 2nd movement of his 5th symphony. ffff is also found in a prelude by Rachmaninof, op.3-2. Shostakovich even went as loud as fffff in his fourth symphony. Gustav Mahler, in the second movement of his Seventh Symphony, gives the violins a marking of fffff, along with a footnote directing 'pluck so hard that the strings hit the wood.' On another extreme, Carl Nielsen, in the second movement of his Symphony No. 5, marked a passage for woodwinds a decrescendo to ppppp. Another more extreme dynamic is in György Ligeti's Devil's Staircase Etude, which has at one point a ffffff and progresses to a fffffff.
And then there are the single note accents

and the crecendoes.
Getting louder

loud then soft:

Then there are the written instructions:
Strong and loud are balanced with soft and quiet.
Debussy Prelude Book 1 No.7 (3:06)
Link: http://www.youtube.com/watch?v=RNcvnOwxFrA
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Loud Music
Mozart - Requiem - Dies irae (1:57)
Link: http://www.youtube.com/watch?v=M_YSEbAWA0Y
Motorhead - Ace of Spades (2:50)
Link: http://www.youtube.com/watch?v=saSWh6PXoug
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Soft Music
Leo Brouwer "Hika" (7:01)
Link: http://www.youtube.com/watch?v=Bl05pZK2-3I
MEDITATION 3 Pour 4 flûte et biwa (2:34)
Link: http://www.youtube.com/watch?v=xAbLYB9ImBw
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Loud and Soft
All music is to some degree loud and soft. They are relative terms. If everything is LOUD ALL THE TIME then our ears grow dull, our nerve endings curl up and ignore the racket. Soft becomes so tiny we no longer hear it... Soft is relative to loud, the size of a wave is relative to the other waves around it, though it can also be measured if we have a scale.
Decibel - Wikipedia, the free encyclopedia
The decibel is commonly used in acoustics to quantify sound levels relative to some 0 dB reference. The reference level is typically set at the threshold of perception of an average human and there are common comparisons used to illustrate different levels of sound pressure.
A reason for using the decibel is that the ear is capable of detecting a very large range of sound pressures. The ratio of the sound pressure that causes permanent damage from short exposure to the limit that (undamaged) ears can hear is above a million. Because the power in a sound wave is proportional to the square of the pressure, the ratio of the maximum power to the minimum power is above one (short scale) trillion. To deal with such a range, logarithmic units are useful: the log of a trillion is 12, so this ratio represents a difference of 120 dB. Since the human ear is not equally sensitive to all the frequencies of sound within the entire spectrum, noise levels at maximum human sensitivity -- for example, the higher harmonics of middle A (between 2 and 4 kHz) -- are factored more heavily into sound descriptions using a process called frequency weighting.
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Anything that is very loud is also very intense, blocking all other signals; and this intensity can be created with waves of different sizes, so there's the high pitched whine of feedback and the gut-shaking roar of a jet engine.
Jet engine test (1:48)
Note how the sound changes at 1:23
Link: http://www.youtube.com/watch?v=L3b5fWlYJK4
And it stops. Ahhhh!
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Silence
Absolute silence is possible, but we can never experience it. In a completely silent room designed in such a way that the walls eat every sound, there's no bounce back, no reverb; you will hear the strange effect of 'no external noise', and then you will hear a hissing and a pumping. The hissing is your nervous system, and the pumping is your heart.
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Sforzando

Italian-- 'ando' - ing - forcing - with effort
Excellent sforzandi in the next piece. Watch in particular for movements of the musicians as they emphasise certain key notes, sforzando, over and over, as we move from p to fff, round and round--fantastic piece!
Edward Grieg - In the Hall of the Mountain King (2:28)
Link: http://www.youtube.com/watch?v=fzyi3C4gNnE
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4'33
What I'd like is for as many of you as possible to listen to 4'33 all the way through. It's famous, it's a conversational point--is it music? It's as much music and the opposite of music as that jet engine.
Here's the thing, though: I found my first time through humorous, engaging, exuberant--it's in the giving in to the idea--you don't have to listen to it; it's just another piece of music.
Like these:
Aretha Franklin I Say a Little Prayer (3:23)
Link: http://www.youtube.com/watch?v=FsP74Mr4WPE
Sxip Shirey playing bowls with red marbles (5:59)
Link: http://www.youtube.com/watch?v=6FsT_lnVppw
Szymanowski - "Stabat mater" (6:12)
Link: http://www.youtube.com/watch?v=2KrOUTp8UUo
The Doors - Break on Through (2:33)
Link: http://www.youtube.com/watch?v=HXPUXc46nWI
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If you've been following through, listening, then pause a moment...
...
listen...
That's the sound of the orchestra warming up. In 4'33 no notes will be played. An orchestra will appear. A conductor will move his baton after observing his stopwatch. Silence. Except--comlete silence doesn't exist, only relative silence, so: you watch the video; you can close your eyes or look around--an audience would have to watch the conductor to see exactly when each movement started (cough breaks occur between movements; if you cough during a movement it will be part of the performance)...you might hear your computer humming, or cars in the street outside. The tick of a clock or the noise of people in the room--each performance is different and your experience of it will be unique--in the exact sounds you notice--because what you don't notice you don't hear.
Unlike the audience at Woodstock, where the piece was premiered in 1952, you're not about to be caught unawares--your ears are primed for the concert. There's an introduction, the piece starts at 01:00.
Link: http://www.youtube.com/watch?v=hUJagb7hL0E
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And now, loud soft and everything in between:
Beethoven Symphony No. 5 (6:06)
Link: http://www.youtube.com/watch?v=N6K_IuBsRM4
!
And finally
Robert Johnson "CROSS ROAD BLUES" (2:31)
Link: http://www.youtube.com/watch?v=LQHYcYq3zPM
Duke Ellington Orchestra and dancers - Going Up (3:42)
Link: http://www.youtube.com/watch?v=fZYT5edrf28
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Okay, I hope you enjoyed the music! Please add your own versions of loud, soft, softloud loudsoftloudsoftstrongquietloudloudsoft. Instructions on how to post a video are here.