October 21, 2021 | How Loud is an eVTOL?

One of the common concerns about urban air mobility (UAM) is that eVTOL aircraft flying overhead will significantly increase community noise levels. That couldn’t be further from the truth. 

Peer through the window of any plane on any tarmac, and you’ll see the ground crew wearing ear protection. Board a helicopter, and you’ll be handed a headset to protect your ears and to enable you to hear your fellow passengers and pilot. When you’re waiting for a subway or commuter train, there’s no mistaking the rumble on the tracks and the high-pitched screeching halt as the cars approach the platform. Living near a highway isn’t much better, diesel engines, horns, and brakes drive up the decibel level. We’ve been conditioned to believe that transportation is loud, but Archer’s electric vertical takeoff and landing (eVTOL) aircraft is designed to be a notable exception to that rule. 

Let’s first talk about helicopters, which are, of course, very noisy. The majority of noise from helicopters is generated by high rotor tip speeds (they’re moving at over 500 mph in flight) The high tip speed, combined with a low rate of rotation, causes the ‘wop-wop’ sound that most of us associate with helicopters. This sound is most pronounced in forward flight because the rotors are spinning "edge-wise," or into the oncoming air. 

Archer’s eVTOL aircraft should not be compared to a helicopter because the two designs are very different. Our two-seater demonstrator eVTOL aircraft, Maker, features 12 propellers (6 dedicated “lift propellers” that allow the aircraft to ascend, hover, and descend, and 6 “tilt propellers” that provide both lift during hover and thrust horizontally during forward flight). During forward flight, these tilt propellers spin on axes that are aligned with the oncoming air flow, rather than edge-wise to the flow (i.e., like a helicopter), meaning they won't generate the similar "wop-wop" sound. Since we are spinning 12 small propellers rather than one large rotor, we can also spin them at significantly lower tip speeds, resulting in much lower noise levels. 

Designed to cruise at approximately 2,000 feet, the design of Maker is such that the noise that reaches the ground should measure around 45 A-weighted decibels (dBA), almost 100 times quieter than that of a helicopter.

Sound is measured logarithmically in decibels (dB). As How Stuff Works explains:

"On the decibel scale, the smallest audible sound (near total silence) is 0 dB. A sound 10 times more powerful is 10 dB. A sound 100 times more powerful than near total silence is 20 dB. A sound 1,000 times more powerful than near total silence is 30 dB."

In a standard unit of measurement, 30 is three times more than 10. In the logarithmic measurement of sound, 30 dB is 1,000 times—or three orders of magnitude—more powerful than 10 dB. That’s important to understand in comparing the relative difference of the sound generated by an eVTOL aircraft to other noises, like from a car or helicopter.

A-weighting is a frequency-based adjustment to the “raw” noise levels measured by a microphone, intended to account for the sensitivity of the human ear, which is less sensitive to low frequencies.

According to University of Michigan Health, soft, pleasant noises like whispers and rustling leaves are 30 dBA, or 100 times more powerful than the smallest audible sound. Standard household noises are 40 dBA, and normal conversation levels or background music register around 60 dBA. Yale Environmental Health and Safety says a household refrigerator registers 50 dBA, while a vacuum cleaner is 75 dBA when you’re about 10 feet away. Sounds above 80 dBA are considered potentially harmful under certain conditions.

Now let’s compare the Archer eVTOL’s 45 dBA output when flying overhead at around 2,000 feet to other modes of transportation.

The noise from standard city traffic measures from approximately 62 dBA (for a single car, 50 feet away, traveling 30 mph) to 85 dBA (for city traffic) at 30-50 feet away. A subway train that’s 200 feet away generates approximately 95 dBA, a helicopter at 1,000 feet generates approximately 78 dBA, and from 100 ft away a jet aircraft taking off generates approximately 140 dBA. With standard automobiles generating approximately 100 times more noise at 30-50 feet away than Archer eVTOLs do from 2,000 feet overhead, replacing even a portion of automobile traffic with eVTOLs may actually lower overall community noise levels. 

Lowering noise levels can affect more than just how people experience sound. Noise pollution impacts both public health and the economy, according to a study published in the American Journal of Preventative Medicine. Exposure to prolonged or excessive noise can exacerbate health problems ranging from stress and fatigue to hearing loss and cardiovascular disease. Homeowners who live near high traffic roadways see sharp decreases in their property values compared to lower traffic areas. 

Vehicular noise isn’t just a quality of life issue; it has quantitative consequences. Even a four-decibel reduction in vehicular noise can significantly improve health and economic outcomes.

Archer is on a mission to advance the benefits of sustainable air mobility by helping drive transportation towards a zero-emissions future. That includes streamlining transportation with beautiful, eco-friendly aircraft, but the bigger vision is about a better quality of life: shorter travel times, less pollution, accessible exploration, and less noise. Reducing noise pollution is one more reason to embrace the eVTOL revolution, and Archer is looking forward to playing its part.