I know that Noise cancellation never works on scales larger than the sound's wavelength.but,at underground stations during the train arrival time we experience a lot of disturbance which sometimes make announcements and random talk to be noisy.assuming a free field condition, which is valid locally in a large open space.ignoring the reverberant field . The focus is on reducing the SPL near the tunnel opening, say at the platform on 10m distance.so my question is,is it possible to reduce the tunnel boom and added ambiend noise using a combined active and passive cotrollers?
Tunnels should have easy to calculate resonant frequencys so you could try helmholtz absobers.
active won't do much good for you over a big space - too many variables. Acoustic damping is likely to be the best solution. Kill it at the source.