Just about every piano can play mezzo forte or mf. It’s not all that difficult to get a piano to sound as loud as the resonant body will allow. It’s all in the power you can put behind the articulation. The challenge with a piano is creating a soft, intimate sound.
Particularly with vertical style pianos, we must create enough space for the hammer to fall away from the strings. This is the point of “let-off.” If the hammer doesn’t let-off then it blocks against the string producing only the sound of impact. The inverse problem of let-off is if the hammer falls away too early so that it must traverse the distance by the force of momentum- this forces the musician to put more force into the note just to guarantee the note is heard which results in louder playing. The ideal position for let-off to occur is as close to the strings as possible allowing for seasonal changes in humidity, humidity which can warp the wooden rails ever so slightly but more than enough to change let-off.
One of my customers has an instrument where either the quality of the wood or the grain alignment is so incredibly responsive to humidity change that I have to adjust let-off regularly throughout the year so that they are able to make music. In one season there’s far too much left to inertia that it only plays loudly, so it gets adjusted, the next season he finds the hammers creating that percussive blocking sound.
There is another option of course, don’t use wood! I service many pianos where the wooden rail has been replaced with aluminum. Problem solved? Nearly. the let-off buttons are threaded like a screw, also made of metal. Metal isn’t susceptible to humidity changes but it is susceptible to temperature change. In the Southeastern Pennsylvania Summer we are very fond of air-conditioning which shrinks the screw and the aluminum rail. The holes don’t shrink, they expand because the metal shrinks away which under extreme situation can generate a clicking sound but mostly just allows the regulating screws to change position.
The most reliable way to maintain let-off at the closest point possible is to marry the two materials- quality wood rail fit perfectly inside an aluminum U-channel. The aluminum prevents the wood from warping during humidity change and the wood holds the regulation screws nicely through temperature change.
This allows for very stable regulating, liberating your piano technician to comfortably place let-off much closer to the strings. What this means for the performer is that you can always be sure that your most delicate repertoire will sing in the whisper it deserves.