Acoustic problems

You haven't provided enough data to answer you... The most obvious attempts are:

1) If you haven't already done so, move the piano away from the wall (even 10 or 20 cm if possible, at least to test how the sound changes)

2) get a hygrometer and find out what humidity you have in your house... the best thing would be to place the piano against the driest wall you have, and anyway since the room is full of 'stuff', perhaps it's time to remove any potentially damp objects from near the piano (plant pots, rugs, cloths, sheets, and so on..)

You should tell us where your windows, glass panes, and large windows are, or any particular paintings; then, if everything is in order, we can hypothesize if the problem is caused by the piano: has it always sounded like this? Did you just buy it? Did you just tune it? Did you move it?

I don't think it's possible to give directions without even knowing the basic information... Room dimensions? (Length, Width, Height) RT60 reverberation time? (in ms). Without knowing at least these two parameters, every piece of advice is simply a random guess.

You are right, sorry... so my room is square and on one wall two upright pianos would fit... one next to the other... so it is very small... the window is near the piano... and being in Liguria it is also quite humid, plus I have a very large bed... the piano is there, I've had it since April, it's from 1976 and since it has been in the house it has always sounded like this

I need dimensions in meters at least to calculate the modes and give me an idea of what can happen inside that room (the more precise the measurement, the more precise the calculation of the modes; essentially, it is necessary to be precise to the millimeter with the measurements). Saying it is square can mean everything and nothing. A square room of 3 meters per side and one of 6 meters per side have very different acoustics, even though both can be defined as small, and if solutions can be applied in one case, others must be applied in the other. If in general the problem is excessive acoustic insulation, there will certainly be a need to decrease the acoustic absorption coefficient, but this solution is very simplistic without having access to other data. A small room is not "suitable" for the sound power of the piano, whatever its absorption coefficient may be. A small room in which a high-power instrument like a piano is placed can only make noise. The problem could therefore also be the excessive dynamics of the instrument in relation to an environment that is too "cramped". Not by chance, drummers who play in 3 sqm rooms are accustomed to covering the entire surface with sound absorbers to extinguish reflections as much as possible and use earplugs specifically to lower the perceived sound. In any case, in very small environments, it is always advisable to absorb the sound. Probably if you reflected [the sound], you would only hear a great din, so not only would you not solve the problem but you would make it worse. Reverberation is generated by the reflections of the environment at certain distances. The larger the hall, the greater the decay time of the reflections and thus the more lasting the reverberation effect (given the same absorption coefficient of the walls, called alpha). Unfortunately, there are limits imposed by acoustic physics... Just as it is not possible to make a studio upright piano sound like a Steinway concert grand, in the same way it is not possible to make a room of a few square meters sound like a large concert hall.

Thank you for the lesson... there were many things I didn't know!! The width of the room is 3 and a half meters....very small... I believe then there is nothing to be done

To generate good reverb, I really don't think so. The physical conditions are not there. In these cases, the only thing is to absorb, but since you already hear it very dry, I don't think there is much else that can be done.