One of the best ways for organists to integrate ear training exercises into their organ practice is to try to play polyphonic music, such as chorale preludes while singing one part and playing the others. For best results, do not double the voice that you are singing on the instrument.
If you are new to such practice, take a really slow tempo at first. Aim for at least 3 correct repetitions of each version. If you make a mistake, stop and go back a few measures, and correct them 3 times in a row.
Remember that you don't have to play and sing all parts together right away. To make this practice fun and easy, you can first sing each line of the piece without the help of an organ. Then practice 2 voices (one singing and the other playing). Later proceed to 3 parts and finally, learn all 4 parts (singing each line and playing 3 others).
Singing separate parts while playing is a strenuous exercise but quite indispensable for a real education of musician. In fact, students sing this way in ear training classes. Of course, at the beginning they only sing one voice but from about 3rd year they start to practice exercises in two voices which are notated on one staff.
They sing one voice and play the other and vice versa. With time the exercises get more advanced, melodies are notated in two staves, and the bass clef is introduced. The musical language gets more chromatic, with tonicizations, modulations, complex rhythms and time signatures.
From about 9th year into ear training students start to sing in various C clefs. From the 10th year 3-part and 4-part writing is introduced. At the end of air training course, people start to sing polyphonic 3-part and 4-part compositions, which are basically excerpts from fugues.
It probably seems like a huge amount of exercise material and it really is. The best way not to get overwhelmed by the complexity of music education is to aim low and set manageable goals. Focus on small achievements but practice regularly. And remember that with each step you master you move closer to your goal one step at a time.
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