Startups race to make delivery drones quieter

Growing drone delivery fleets from Amazon and Walmart are drawing noise complaints. Startup Greenjets claims its enclosed-rotor system cuts drone noise by at least two thirds, while Sharrow Engineering develops loop-shaped tipless blades; Amazon says its latest propellers produce almost half the perceived sound of the prior generation.

• Greenjets claims at least two-thirds reduction in drone noise
• Amazon says new propellers halve perceived sound versus prior generation
• Amazon Prime Air is expanding to nearly 500 US cities and towns
• FAA says operators and local leaders are responsible for noise concerns