Noddy: you are quite right that dictionaries are useful, but for guidance only. Some dictionaries (eg OED) provide excellent definitions of how words were used in the past: it is continuously updated, but the meanings of certain words(eg acoustic and folk) mutate extremely fast, and their multiple shades of meaning vary far too quickly for the OED to keep up with.
   Other dictionaries(particularly in France I am told) try to be prescriptive: to say what words ought to mean, rather than defining them by usage. That approach is equally useless here. We are discussing just how "acoustic" is used now: to figure out just what it might mean if the poster outside the pub says "acoustic music night" for example. My guess, in those circumstances, is that the music would probably be people singing with guitars, amplified. But I wouldnt be sure. It might mean people having a session, acoustically(ie unamplified). Dictionaries are no help here, we need to establish usage. And if the current usage is too ambiguous to be informative, consider changing the usage, which is of course nearly impossible. Basically, one side or the other has to give in. The recent conroversy on the BBC's use of the word "gay" is a good case in point. Anone aged 50 or 60 plus in England will have experienced two conroversies about the meaning of this word, as it channge from "lighthearted, insouciant" to "homesexual", and is currently making the transition(among the young) to meaning "rubbish".