Newbee, I just re-read what I posted, and that does sound crazy (I really should stop making these posts so late at night). If the whole orchestra is playing, and all of the violins are playing in unison, then no, it should be nearly impossible to distinguish an individual member of the violin section, unless, as you say, there is someone very out of tune or playing a wrong note, or an open E string when they shouldn't be. But if it is a very quiet passage, and say someone is vibrating differently than the rest of the section, or is playing too loudly compared to the rest, this can be audible. This is the type of thing I was thinking of when I wrote that.
Timbre does also refer to the tone of an individual instrument, it is not just a general term, though you would usually use the word "tone" when speaking of it in that way. From the back of the hall, an experienced listener should be able to pick out the timbre of the violins as opposed to the violas, even if they are playing in unison and at the same volume, which certainly does happen often. Picking out the tone of an individual violin within the section, however, would be extremely difficult, as you say, and identifying the make of a specific violin would be quite a feat indeed. If one was a violinist, perhaps this could be done, if the violin section was playing by itself. Being a horn player, I could come reasonably close to identifying the make of instrument if I heard it live. With wind instruments, this is quite a bit easier, as our tones are quite a bit more individual than those of a string instrument. I could certainly identify any of my wind-playing colleagues tones in my orchestra instantly upon hearing them, whereas it would be much more difficult for me to do so with my string-playing colleagues, who I almost never hear playing by themselves, except for the principals.
However, I hope it goes without saying that it would be silly for a concertgoer to even attempt to try to identify individual violinst's tones. Why I went there, I have no idea, re-reading that. I plead guilty, with a side of sleep-deprivation. Even to concentrate on a single section would be detrimental to your overall experience of the music. Only music students need to do this. The main thing one should try to do, besides enjoying the whole texture of the music, is to try to isolate the main melody or thematic components from the accompanying ideas, and follow these things throughout the orchestra. The composer assumes his listeners are trying to do this, and of course helps by carefully balancing the sonorities he creates. There is a great book on the subject actually, written by the famous American composer Aaron Copland, called "What To Listen For In Music." I highly recommend it to all who are interested, which I hope anyone perusing this site would be. He asks two questions - Are you hearing everything that is going on, and are you really being sensitive to it. Whatever sort of music you like to listen to, this book will really deepen your understanding and enjoyment of it. Goodnight, and enjoy the music!
