My teacher always is very strict with the quality of sound. I am playing Chopin's valse, and she says when I am playing forte, my sound was too rigid, harsh and lacked imagination, that my speed of attacking the keyboard was too fast. She also says that the quality of sound is more elusive than the anything else in piano playing. Does playing Chopin drastically differ from playing Prokofiev or Rachmaninoff? She told me that my "awfully rigid sound" could potentially be of use with Prokofiev, but not with Chopin. When I was watching the Chopin competition, however, I noticed that there were times where the contestants had to bang really hard too. Also, from my knowledge, in order to play forte, you'd have to reach a minimum level of speed when attacking the keyboard. It's about speed and weight, and it wouldn't work without either. So my question is, with Chopin's music, how to play forte appropriately? Does it differ that much from playing Prokofiev or Rachmaninoff? How am I supposed to know if I am doing it wrong, where can I get a point of reference?