Drones could soon drone less. A redesign of the rotor blades could reduce the noise made by drones and make them less irritating.
The buzzing sound from drones comes mostly from the way the fast-rotating blades slice through the air. Researchers at Australia鈥檚 Royal Melbourne Institute of Technology, working with Melbourne-based aerospace company XROTOR, are looking to suppress the buzz by redesigning the blades.
The researchers focus on inter-frequency modulation, the sum of many different frequencies produced by rotor blades and which is responsible for the buzzing noise. By quantifying this effect, they could base their calculations on something more useful than the absolute number of decibels measured by a microphone, says lead researcher Abdulghani Mohamed.
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The team developed algorithms to generate new rotor designs and assess them for thrust, sound level in different directions and other factors to produce quieter propellers. The researchers then confirmed the technique by using a 3D printer to test one of the new designs, which they found was 15 decibels quieter but still provided the same thrust as the original rotor blades.
Human hearing doesn鈥檛 have a consistent response to sound and some frequencies appear louder to us than others. Mohamed says the algorithms have been tailored to take this into account. In addition, they aim to engineer rotor noise that merges into the sound landscape.
鈥淚n a nutshell, some frequency ranges can be more natural and especially blend into the ambient noise, making them pleasant,鈥� says Mohamed.
The rotor design algorithms will work at any scale from hobby drones up to air taxis, as well as working at different rotor speeds, so the technique should be widely applicable across the range of future air vehicles.
Antonio Torija Martinez, who works in acoustic engineering at the University of Salford in the UK, says the drone industry is taking noise issues seriously. He notes that any rotor will create sound, but this approach of making the noise less intrusive should be beneficial.
鈥淗umans are very sensitive to high-frequency sounds, so it makes sense to reduce those,鈥� says Martinez.
Mohamed says the 15-decibel reduction is just the start and further improvement is expected. Once the algorithms have been optimised the technology will be made available to commercial manufacturers. This should improve the chances for less annoying buzz even as drones proliferate.
