In my personal opinion, for most fast-casual or even mildly casual restaurants, there should be a faint murmur of noise that will allow for certain activities to be overlooked, such as the clatter of tables being cleared and communication among the staff. There really is no "ideal design", the design needs to be amorphous, or able to adjust to the restaurant's architecture and demographic. If you have a dining room with high-vaulted ceilings that causes reverberation and amplifies noise, you will probably be trying to mitigate the noise through the use of tapestries or roof tiling. If the dining room is so quiet customers feel compelled to whisper, they probably feel awkward as well, so try to supplement it with non-lyrical music. The purpose of ambient noise is to create a gentle lull so that customers can focus on what matters most, the conversation at their table and not be distracted by everything surrounding them.
