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 ambient noise using combined active and passive controllers?
First: Classical measures like sound absorption in the tunnel, and in the platform area can mean an important improvement. This also allows the use of sophisticated PA-systems for good intelligibility of announcements.
Second: The fact that the noise source (the train) is moving, unpredictable and composite, complicates active noise control very much.
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