When performing, pianists are in direct contact with the source of sound because they are the ones who are physically playing the piano. They use their hands to press on the keys, which causes the hammers to strike the strings, which in turn produces sound. The pianists can feel the vibrations of the strings through the keys, and they can also hear the sound that they are producing. This direct contact with the source of sound gives pianists a unique perspective on the music they are playing, and it allows them to control the sound in a very precise way.