How much does it cost to soundproof 4 windows 5 ft*5 ft?

Question

we have our factory in India in a residential area with the adjoining house only 2 ft from our place. they complain about the noise and we want to soundproof the windows. please tell if theres another cheaper way, and the materials needed, with cost.

Answer 1

Install double pane, and hang curtains with a 1/16" plastic sheet between the curtain and an outside white layer. The plastic acts as a barrier and absorbent.