Agree 100% with you on this alebonau,, it is hard to measure or even get dedicated dB measurement devices that start as low as 17-20db, My dedicated sound level meter starts at 30dB and goes to 120dB. The only other sound pressure meters I have seen that go down in the area between 10-20dB is from different sound pressure apps on my iPhone....
One other thing when talking about noise from projectors.
dB measurements are often combined with a distance to the source, and projectors often give out different sound levels depending on where around them the measurement is done.
Here is an example from a Barco Cine9
View attachment 1501416
Taking this into account, I guess it is a little strange to read someone here choose projectors out of dB data because it will be hard to know for sure before the projector is installed in the HT.