Next to the square are waves with seven different points. You are to repeat the sequence of the notes by clicking under the corresponding points of the wave. If you start out correctly and click on the top bar again the same sequence will play again. If you make a mistake however you will get a whole new sequence of notes. When you click on the shaft the notes of the waves will be played.
Some sequences are obviously easier then others.
I always wait until one starts at the very top or very bottom. Still it took me quite a while. If anyone has figured out the pattern, please share it. When you have successfully played back the seven notes, the puzzle repeats them. This opens the door to the next chamber.