Check with the supplier the STC rating if they claim that their windows are soundproof/acoustic. Basically, high performance sound proof window system, like those being installed at DJ rooms, recording studios, etc., are much thicker and also more expensive. Some ordinary windows may also cut down a bit of noise, sometimes by adding an additional glass pane (two panes, instead of one). They only help to reduce the noise, will definitely not cut off the noise completely.