Set aside a period of time, like 10-15 minutes and play anything that comes to mind. By anything, I mean anything. This is scary, though. You might be thinking: if I can play anything, where do I start?
First of all, by free improvisation I don't mean you should play random notes and see what happens (although you could try and see if you like it). Instead, your improvisation should have and idea or a theme.
Do not let your fingers dictate your playing but let your mind always come first. In other words, your improvisation should only be as fast or as slow as your musical thinking is. This skill of course can be trained and developed so you can think faster. It just comes from experience - the more you improvise, the faster you will think.
So going back to the question of your free improvisation, choose a theme or a melody of 4 measures long. It is better if the theme is incomplete - the melody should end on the different pitch than the tonic note. We will call it "the question".
Now think of the form of your improvisation. This point is crucial, if you want your composition be understandable (yes, improvising is composing while you play) and it should have beginning, middle, and end. Otherwise, your playing will be like endless speech without limits and meaning.
One of the easiest musical forms to understand and master is the ternary or ABA form. Part A is the exposition of the theme, Part B is the exposition of a secondary theme and Part A is the recapitulation of the first exposition.
In Part A, you can play your question and provide an answer to it (measures 5-8). One of the easiest ways to achieve this is by repeating the question and ending it with a perfect cadence (on the tonic note of the scale).
Then take the theme and play it in the Dominant key by placing it in the tenor voice. Again, use the question and answer technique (measures 9-16). After that return to the home key by playing exact or varied recapitulation of the first segment (measures 17-24).
In Part B (measures 25-48), choose a contrasting but closely related key and take another theme, perhaps in another mode. Repeat the process you have done in Part A as well.
In recapitulation (measures 49-72), you can repeat Part A exactly as it is, or you can alter it a little for more variety. Some of the best ways to achieve this is by taking the rhythmical figures from the accompaniment of part B and using them in your recapitulation.
You can also remind the listeners of the secondary theme by playing it with the answer at the very end (measures 73-80). If you like contrapuntal techniques, try playing your theme in canon at the interval of an octave, fourth or fifth.
Always think in terms of chords when you create the other voices in your improvisation. In fact, you can even harmonize your melody in 4 parts throughout the piece for starters. Observe proper voice leading and avoid parallel octaves and fifths.
In order to make things simple in the beginning, use only 3 or 4 note chords at first: Tonic, Subdominant, Dominant triads and their inversions, Dominant seventh chord and its inversions. If you know other more complex chords, feel free to add them in your improvisation as well.
Use the above points and try to improvise on the organ today. Don't forget to study real compositions of your favorite composers. Do not be afraid to imitate them. As you improvise, I'm sure you will have much fun in the process.
While analyzing your improvisations, you will also discover your weak points. Remember these, go back, and correct your difficult places. You will also find some spots which were successful. Remember these because you can use similar techniques in your future improvisations. You can also write down your improvisations and convert them into real compositions.
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