Noise-cancelling headphones have improved a lot and become affordable for a wide range of people in the last few years. They seem like a luxury or novelty product but I wonder if they could end up being quite important in various ways:
i) They allow people to be packed more tightly into offices, apartments and subways without it becoming unpleasant, making higher city density compatible with personal comfort. Given the importance of city density in shaping all aspects of cultural and economic life, even small effects here matter a great deal.
ii) Psych studies suggest that annoying sounds like traffic outside a house are one of the few things that people can't adapt to. If these studies are to be believed (which maybe they shouldn't) background noise continues to make people's lives worse forever. Noise-cancelling headphones greatly reduce this problem.
iii) They might especially improve the welfare of people with general anxiety, social anxiety, ADHD and similar conditions.