Thanks, Randy, for the response. No, they didn't insulate the cavity because I thought we already had adequate soundproofing with the old ceiling. We thought if anything a second ceiling of metal furring strips and 1/2" sheet rock (installed for cosmetic reasons) would only improve matters. But did leaving the cavities make it worse than otherwise? Maybe we created an echo chamber with nothing but air between the two ceilings?
Were you thinking that fiberglas batting would have been the way to go between metal strips?
Too late for removing the new sheet rock (they're already taping), but I could use the cellulose option. A company that infused the front part of the store a couple of months ago by cutting small holes in the plaster and then covering them over. We did notice an improvement there.
I am struggling to keep calm. Faced with a minimum of $1500 for professional testing awhile back, we tried to conduct our own test using a "boom box", sadly undersized by comparison with the workers' own noisemaker and maybe disastrously inadequate. If I had the option back I certainly would have hired the professional for initial tests. $1500 is nothing if it provides us with foreknowledge.
Please let me know if you think the cellulose option would or wouldn't be a waste of time at this point. Now would be the time to hire them rather than later.