The recommended values generally apply to unoccupied, furnished spaces with heating and ventilation on.
Any conversation taking place in a space produces sound waves. The sound waves spread out towards walls, ceilings and floors or any other items in the space. Some of these waves are absorbed by these surfaces and the rest is reflected. In a small room the sound is absorbed by all these surfaces and will tend to be quiet and have a short reverberation time. While is the room is large the sound will not be absorbed so fully and the room will be louder with a longer reverberation time. Whatever use of the space the reverberation time should be optimised to be neither too long nor too short. Too much sound absorption where it is not needed is just as unacceptable as too little when it is.
Sound insulation is the reduction of sound being transmitted from room to room. In relation to suspended ceilings the plenum is an important path for sound transmission. The plenum is the space between the structural soffit and the suspended ceiling.
The sound reduction of suspended ceilings depends on the thickness and density of the elements, protecting against the transmission of sound waves. The thicker the element is, the lower the sound transmitted through it and so the better the sound reduction ability. The air tightness of the ceiling joints also influence the quality of the sound insulation. If the element has gaps or cracks the sound transmission will be enhanced and the sound reduction acutely reduced.
The level of sound attenuation depends on the need for speech privacy as well as dampening of the background noise; by using a suspended ceiling below a lightweight roof disturbance from the rain impacting into the room below can be significantly reduced.
The best acoustic design approach is to create a good balance between the performances of the walls and the ceilings.