Bach's son, C. P. E. Bach wrote a number of keyboard fantasias that capture the sense of someone improvising just for themselves. When played on clavichord this is particularly apt because it is often called the most intimate of keyboard instruments. The key activates a small metal blade called a tangent that is held against the string. This means that the performer can feel the string tension through the key, unlike with all other keyboard instruments. Thus the performer has control of both dynamics and pitch--by pressing and releasing, vibrato is possible. Here is a piece by C. P. E. Bach on clavichord where you can hear quite clearly the vibrato:
The clavichord is so quiet that performance in a concert hall is really not possible without amplification. Pianists also wrote music largely intended for private use, such as this Fantasia by Mozart:
This feeling of the intimacy of music took on new significance in the 19th century as it was an important element in Romanticism. The most important composer of the romantic variety of private, or nearly private music was Chopin. People forget that while he made a big splash early on with brilliantly virtuosic compositions and with his concertos for piano and orchestra, for most of his career he only played in small salons. He played less than a dozen public concerts in his whole life. The nocturne, with its dreamy, floating character, is a good example of private music, whether it is just the pianist who is listening or a small salon gathering of a few others:
Individual subjectivity was a very big element of romanticism and even though it began with the piano in an intimate setting, they succeeded in transferring the feeling to the concert hall where perhaps a thousand listeners might be entranced by a solo piano piece.
The 20th century saw a new development in the story of private music. Now a performer goes into a studio and plays entirely alone, except for a microphone or two. In an adjoining room, there is a recording engineer and producer. Eventually, the music is heard by thousands or millions, but they too may be entirely alone, listening at home, or with an iPod. The intimate, private music now is becoming more important than the public music of the concert hall. I suspect that the consequences of this are still playing themselves out. One performer who responded to this early on was Glenn Gould who, at an early stage in his career, simply stopped playing concerts and performed thereafter only in the recording studio:
Thus the old tradition of the individual musician expressing something essentially private and intimate gains new life as it is shared through recording technology. Public music, though often glorious and splendid and for much of history the only kind of music most people had access to, has some potential weaknesses. Sometimes the performer concentrates on playing to the cheap seats with grandiose, excessive gestures. Subtlety can be lost entirely. The spectacle tends to overshadow the music. All these things also seem to apply to the music video, by the way.
While not always realized, the hope with private music is that it is the genuine expression of an individual, direct and intimate. This is one of those things that music has unique powers to convey.