hi, anyone knows why sometimes when you depress a key for acoustic piano, sometimes there is no sound. What are the mechanics?

If you don't depress the key all the way down then it will not sound as the hammer wouldn't come into contact with the strings...similarily if you depress the key really slowly when the hammer hits the strings it will hit them that soft that no sound is produced.

When you depress the key the hammer is thrown against the strings - there is what is called an escapement so that the hammer travels the last 3mm free and then bounces back, ready to play again. If you depress the key really gently the escapement will trip but the hammer won't have enough momentum to travel onwards to hit the strings.

If the action is old and/or worn, it doesn't trip and you get a dull clunk, because the key is holding it against the strings, thus dampening them immediately.

Sorry this is not a very good explanation but there is bound to be an animated demo somewhere on the web...