Framing for Ceiling - how tightly to attach the joists to prevent sagging?

[Dr.SiC]

I have had carpenters who have done a bit of remodeling at home (new floor on the house). In one area, we tore down everything, both the outer and inner ceiling. A new inner ceiling was therefore necessary to build. They constructed this with 4.5 m long 2"4" cc120 (the area is 8.8*4.5m), with each beam then attached to the joists at two points (and at the ends, of course).

I am quite concerned about sound between the floors and would ideally like to decouple the inner ceiling from the joists above. It is these attachments to the joists that I would like to remove. The question is how close these must be placed? The weight in the ceiling, aside from the beams' own weight, is slat panel, drywall, and insulation (30cm). Can I remove the attachments entirely, or will the ceiling sag over time? Perhaps one attachment per beam would suffice?

 

[Mikael_L]

Without having any evidence for what I'm writing, my gut feeling is that the two fasteners per beam the carpenter added are just right. With fewer than one fastener, I can promise it won't work. Because a 2"4" at 4.5 meters in length will noticeably sag under its own weight.

edit:
Can't we instead consider methods to "dampen" vibrations that are about to be transmitted through the hangers? For example, if they were made of rubber or something like that...

 

[Dr.SiC]

Also feel intuitively that at least one suspension point is needed. Have also considered some rubber variant but it should be something that lasts for 50 years as well. Haven't come up with anything good yet, suggestions are gratefully received.