Siren Silencer – listens and plays back exact inverse of the waveform to quieten things down
Siren Silencer – listens and plays back exact inverse of the waveform to silence sirens, or loudmouths – like a handheld tannoy with a mic and speaker, point it at the noise to make it go away
Muzo removes unwanted sounds, stick it on your window, looks great.
Active noise control (ANC) system based on adaptive algorithm is an effective technique for acoustic noise reduction.
Active Noise Control (Cancellation)
Ford use this to quieten things down in cars. Not sure if it works for sirens though.