After a lot of reading, watching, and thinking on this topic of piano tone quality, I am going out on a limb and say: I believe this is the same with acoustic pianos.
Now, most of you know I am a beginner beginner. I also have never laid hands on an acoustic piano. So am I totally clueless and ignorant making a claim like that? Well, bear with me. I was born in a family of engineers, trained in natural science, and worked with technology. So there is a part of my brain for logics that always must be reconciled with what I perceive. In my experience, more often than not, art and science agree. Only when science is based on incomplete information or understanding do they falter. Acoustic piano actions, based on widely available information online, does not appear to have much mystery on how it works. So the science should be able to explain it well.
As far as I understand, the double escapement causes the hammer to be out of the players control in its final travel to hit the strings. The hammer also travels in a single direction, which means it will always make contact with the string in a single way (assuming no pedal is depressed). Therefore, aside from tuning and regulation changes (hammer hardness, friction, etc), the only variable at the time of playing is velocity. As long as the velocity at which the hammer leaves escapement (or hit the string) is the same, it will produce exactly the same sound and the same tonal quality.
I have watched and read many teachings on play in beautiful tone, what to do or not to do: hit the key straight down, or with an angle, or with finger only, or with arm weight, or push the key, or pull the key, or tip of the finger, or with the fleshy finger pad, or land from high above the keys, or lay finger on the key surface before pressing, etc. I am not doubting these things don’t make any difference. On the contrary, I firmly believe they do. Yet, the disconnect that bothers me is the reasoning behind. Few of those teaching explains why they work, leaving it more to vague concepts as more often in artistry.
That is until I realized the tonal quality, although being solely based on velocity, can be perceived quite differently because of its surrounding notes. Then it makes sense that bad tonal quality actually comes from imprecise control of the dynamics (velocity) of the notes that are interacting. Therefore, all those ways of varying the body’s interaction with the keys is just a quest of achieve ever more precise control of the key velocity.
I have come across a few piano teachers elucidating exactly this concept. Aleksander Woronicki in one of his articles clearly stated velocity is the only target a pianist controls. Shijun Wang in one of his videos said the same - but he added on more dimension which is a perfect illustration of what I mentioned how science falters with incomplete understanding (on my part). Dr. Wang said the second aspect a pianist can control the tone quality is the depth of playing the keys. I realized this can be explained if one considers the involvement of the damper. When the key is played very close of the surface, the damper is not lift as high, and at softer/slower velocity, the damper could even come back into contact with the string by the time the hammer meet the string, affecting to tonal quality.
