I only discovered this right now. Great piece! Really fascinating to listen to the changes in sound, which I hear almost as a "harmonic progression". I also love the concept of using a single sound as the material for a whole piece. That reminds me of some composition by someone (I forgot who, but I think it was someone famous) who made a piece out of orchestrating a single, slowed-down tone of an instrument (I also forgot what instrument it was). Could have been Stockhausen, maybe. I got to look that up.
I'd really love to listen to a longer version of a piece like this too. In that case however it would be nice to extend the pitch range a bit (looking at a sonogram it seems to pretty much stay between about 80 Hz and 1300 Hz, so you'd easily have two more octaves at either end of the spectrum at your disposal, even more at the upper end). I think opening it to a greater pitch range (perhaps only for a small section in the piece) would make it sound even richer and more colourful than it already is. That is of course just my personal opinion and may not be what you intended. I'm just a sucker for opulence
The other thing I would personally wish from a longer version would be not to keep the formal structure
quite as rounded with build-up, "climax" and decrease, but have it slightly move to something different. But that again is only my personal preference.
Both my points also go against the "piece built out of a single sound" idea and may completely go against your musical intent. I guess the only reason I named them is because I like the piece so much and it comes close to things I would like to compose myself, so its natural for me to bring in my own concepts.
But maybe I'm also mentioning these things because I hear the piece quite "absolutely" and not primarily as an extended piano chord. I have no clue to what degree you directly influenced this chord, respectively the piece, but I imagine that if the piece referenced more directly to the piano, I would be less tempted to apply other musical parameters (such as formal structures) to it and hear it more as a voyage through a single tone. This is especially the case if you don't -know- how the piece was made. I suppose if I hadn't known that it was built out of a piano chord, the lack of an actual formal structure might have somewhat bugged me, but knowing it, I can listen to it from a totally different perspective. I wonder if this knowledge could be brought more out directly through the music, or at least hinted at. What about very faintly including the original piano sound somewhere for example? But of course that may easily come out too "didactic", so it's a difficult task.
But what do you actually want? Do you want it to be heard as a zoom on a piano sound, an exploration of its overtones, or do you want it to be heard as an absolute piece of music that is entirely independant of how it was created? I think those two perspectives ask for a slightly different musical approach for composing such a piece. Do you want to accentuate the origin and concept of the piece, or do you want to shape it according to general musical ideas?
But I'm mainly just babbling to myself, because I too don't really know answers to these questions. It's just stuff that interests me personally.
In any case, it's a piece I really enjoy listening to and a really nice discovery on these forums.
P.S. I also thought "what a pity!" when you mentioned that you got rid of unpleasantly piercing frequencies. I just like rough edges in music! Of course I don't know just how unpleasant they actually were.