My question is, is it better (or at least useful) to add a nuance at the end of the decrescendo like in the following image ?
Thank you for your time and your help !
If you want it to end very quietly then, yes a ppp marking is appropriate. Although it is common to decrescendo to a very low volume at the end of a piece, it is still best to make your intention absolutely clear.
However, you could also use the markings al niente (lit. to nothing) or morendo (lit. dying) to indicate that you want the volume to “die away” to nothing (or almost nothing). You can also use a specific kind of decrescendo hairpin that has a “o” attached to the pointy end to indicate that the music decrescendos to silence:
In general I would recommend a closing closing indication, since
Yes, the player has to know how far down the decresc. (whether notated as a hairpin or not) goes. Not all decrescendi are al niente.
To extend the topic to crecendi as well as diminuendi, there is a great difference between these two examples.
Some would suggest the first would be ok without the central dynamic. Others (me included) would say that it merely indicated normal expressive playing - is you DIDN'T want a natural cresc. and decresc. senza cresc. might be needed! We see a lot more of this (over-)notation than we used to, presumably the result of trying to make notation software play back 'expressively'.