Upper and Lower Noise?

Do forgive me if this has been covered (to death) elsewhere in this forum. I'm new here and have tried searching the forum for these terms but with no success.

Many moons ago I had a book by a one Jozseph Gat called 'The Technique of Piano Playing'. Unfortunately I no longer have the book (I note there's a second-hand one for sale on Amazon at £250!) but I do remember a number of things he talked about including what he termed, if I remember correctly, 'upper' and 'lower' noise. The former is the noise that your finger makes as it hits a key and the latter the noise the key makes as it hits the key bed.

He was of the view, and I think had made some measurements to the effect, that these noises can constitute a very large part of the overall sound you hear when a piano is played. I remember that around the time I first read about this I had recently tried the keyboard of a concert grand that was being restored and had no action. I was amazed at what a racket just playing the keyboard made.

Again, if I remember correctly, Gat was of the view that these upper and lower noises were one of the ways that pianists changed the 'tone' of their playing, for example to suit different pieces or different passages. They were also one of the ways in which different pianists' 'tone' seemed to be so different, even when playing on the same piano.

I tested this out a little myself and convinced myself, for example, that a lot of the brightness of an energetic finger staccato is indeed due to the noise the fingers make as they strike the tops of the keys. You can I think vary the 'tone' of such a staccato considerably just by changing the distance (and therefore speed?) the fingers travel before they hit the note. I think it may be possible to change the 'tone' in this way without necessarily changing the volume (hammer velocity?) much.

I've been surprised in the intervening years not to have heard mention of these effects again. This may of course be because I haven't been reading the right things or talking to the right people. But when recently I read a couple of papers about physical modelling of the piano I was again struck that no mention was made of 'finger on key' or 'key on key-bed' noises. They just weren't included in the model which, on the face of it, seems like a bit of a glaring omission.

Can anyone shed any light on this?

Cheers, Ian.