Watching the Winter Olympics this year was a lot of fun, but there was something different that the world had not experienced in years past: drone footage. The drones were everywhere and provided exciting video from above skiers and snowboarders as they whizzed down mountains. In addition to the very cool footage, sound from the drones could be heard – that distinct buzzing sound. Read on to learn about the noise drones make and possibilities to make them quieter.
While electric motors inside the drones produce a high-pitched hum, and vibrations in the frame from motor and propeller activity further amplify the sound, the majority of sound from drones is produced by the propellers. The high-speed propellers chop through the air and create aerodynamic turbulence and pressure waves at the tips. These small propellers can rotate at thousands of RPMs to generate lift, often producing sound levels of 70-90 dB up close. As a drone flies higher, these levels reduce and blend into the background eventually, but low altitude drones like those used at the Olympics are very audible.
The sound from drones is characterized as a very tonal buzzing or whining sound. We can predict the frequency of this tonal sound by calculating the blade pass frequency (BPF). As each propeller blade tip passes a single point in the air, it creates a pressure fluctuation. The frequency of this pressure fluctuation in Hz (cycles per second) is calculated by the number of blades in a propellor and the RPM (rotations per minute) divided by 60 to get it back to seconds.
Smaller drones, like hobby or video drones, generally operate at higher RPMs, while bigger drones have slower rotational speeds; that is why small drones have a higher pitched sound while larger drones have lower pitched sounds…it all has to do with the BPF. For example, small hobby drones can operate at 30,000 RPM. With two blades, that equates to a BPF of 1000 Hz, which is that high frequency buzz you hear. Larger drones, like Amazon delivery drones that have to lift heavy items, rotate at around 6000 RPM; with three blades, that equates to a frequency of 300 Hz, which is less noticeable and annoying.
So, is there anything that can be done to reduce this tonal buzzing from drones that pierce through the ambient sound so effectively? The answer is yes. Toroidal propellers are loop-shaped blades that create a quieter flight by distributing turbulence around the entire blade instead of at the tip. Swept-back blade tips, much like an airplane, delay the onset of turbulence, smoothing out the airflow. Taking another cue from airplanes, shaping the blades with a depth to them like an airplane wing instead of a flat blade also reduces turbulence and subsequently noise. Finding inspiration from the silent flight of owls, serrations added to the blade’s leading or trailing edges spreads sound energy across multiple frequencies reducing the tonal quality that is so noticeable.
As drones become more prevalent in everyday life, flying the friendly skies should take a more friendly turn as the industry transitions to quieter operation. Let’s leave the buzzing sound to the bees!