In an eating space, building services, background music, and most importantly, human-related activities such as talking, make the greatest contribution to the ambient noise level. The amount by which a source contributes to the ambient noise level is controlled by two factors: reverberation time (RT), and loudness of the source.
Reverberation time is a measure of the time it takes for sound to decay by a fixed amount (60 dB). If reverberation time is high, it takes a relatively long time for an unwanted sound to dissipate from a space. As a result, in spaces with high RTs, high levels of ambient noise can be expected if there are enough noisy activities taking place in the room. Moreover, louder noise sources create more ‘unwanted’ sounds that also result in high levels of ambient noise.
It is the ratio of a desired sound (the conversation to be conveyed) to all unwanted sounds that specifies how easily a conversation is audible. If the level of unwanted sounds is high, then to have a relatively acceptable audible conversation, one must speak louder. However, in a shared environment like a restaurant, speaking louder introduces more unwanted sounds for people who are not part of that conversation.