Drone industry defenders have often bristled at the suggestion that drones are any louder than other sources of noise, especially vehicle traffic.   In one study funded by NASA, the authors suggested that consumers are simply more accustomed to hearing traffic noise.  But most noise experts disagree.  The problem isn’t really the decibel level of the noise but its frequency.  Drone noise is “broadband,” not “tonal,” and can pierce through other sources of noise.  Large package delivery drones – which are expected to come on line over the next couple of years – will be even noisier than common recreational drones, these experts say.   A real solution needs to be found.

To date, drone manufacturers and consumers have come up with a variety of means to lessen their drone noise.  For example, retrofitting drones with larger slower-spinning propellers that are less noisy is one solution.  Another is to cover drone propellers with shrouds that muffle their noise.  In addition, the design and shape of the drone as well as its propellers can greatly affect the noise level.  More silent acoustic signature propellers can reduce the noise by 60% or more.  For instance, the DJI Mavic Pro deploys a propeller known as the “raked wingtip” to reduce drone noise.

A similar effort is now underway at the MIT Lincoln Laboratory, in conjunction with the US-based technology design company Sharrow Marine.  The two groups have developed a twisted toroid-shaped propeller that dramatically reduces noise while maximizing performance.  The propeller is designed as a twisted loop instead of traditional rotor blade, which tests show can distribute the noise generated by propeller whirring – which normally emanates from the blade tip – across the full length of the twisted loop, reducing its decibel level substantially.

How substantially?  The firm claims that drones equipped with its patented propeller technology are practically “noiseless.” Rather than a persistent and nagging loud buzz, the noise from the drone’s twisted loop propellers now sounds like a “distant whoosh.”

Dr. Thomas Sebastian, a senior member of the MIT lab’s Structural and Thermal-Fluids Engineering Group says the idea for toroidal propellers is not entirely new.  In fact, the idea dates to early designs of World War II aircraft that relied on “ringed” wings.  Sebastian has also designed a modern fixed wing aircraft without moving parts that is practically noiseless and saw creative ways to adapt the design technology to drone propellers.  It took several iterations to arrive at different toroidal shapes for testing but eventually the group found one that maximized the noise reduction compared to traditional propeller blades.

Sebastian says his toroidal propellers could go a long way to minimizing current concerns that low-flying drones, especially those for remote package delivery that fly close to residential areas, are too noisy and intrusive to be introduced on a mass scale.  Additional concerns over potential noise injury to farm workers and to workers on construction and movie sites might also be mitigated.

“By enabling a drone that is less of an acoustic annoyance, this propeller may accelerate the acceptance of such aircraft for a wide range of uses—for example, aerial deliveries, cinematography, industrial or infrastructure inspections, and agricultural monitoring,” he argues.

Sebastian’s new toroidal propeller prototype has already found favor.  Last August, it won a 2022 R&D 100 award in the “Mechanical/Materials” category, one of just three dozen innovations so recognized.  The annual R&D 100 competition, now in its 60th year, is the world’s most renowned science and innovation competition. Entries were received from a dozen different countries and regions, and this year’s esteemed judging panel included 50 well-respected industry professionals.