Engineers are creating a silent future for drones and flying cars. One of the obstacles to the dream of flying cars is noise—imagine 1,000 leaf blowers crashing your barbecue.
This issue extends beyond flying cars to drones. Complaints about the high noise levels of propellers could lead to restrictions or regulations that might hinder the development of a new commercial drone industry.
Aerospace engineering students at the University of Cincinnati are developing noise suppression methods in the lab of Associate Professor Daniel Kuppaletti at the College of Engineering and Applied Science. According to Kuppaletti, flying cars can only be a successful venture if they operate quietly.
UC aerospace engineering students Natalie Reed, Matthew Walker, and Peter Sorensen presented their reports alongside Kuppaletti at the Science and Technology Forum and Exhibition this month in San Diego, California. This is the world’s largest aerospace engineering conference hosted by the American Institute of Aeronautics and Astronautics.
“I look at noise in terms of its impact on society,” said Kuppaletti. “These vehicles need to be unobtrusive in the environment; otherwise, noise impact will be maximized on certain population segments.”
He noted that, in most cases, the noise impact is felt more acutely in low-income areas.
Airports across the country receive tens of thousands of noise complaints annually from local residents. In a study by the FAA published last year, two-thirds of respondents reported that they are “greatly annoyed” by aircraft noise. The survey showed that noise from planes and helicopters is far more disruptive than that from cars, trucks, or neighbors.
Engine noise is also a significant issue for military and commercial aviation. Hearing loss and tinnitus are leading causes of disability claims filed with the U.S. Department of Veterans Affairs.
Drones do not pose the same risk of hearing loss as airplanes because they are only slightly louder than kitchen appliances. However, the distinct buzzing of the rotors does not blend with the natural sounds of the environment, making them seem annoying and distracting.
“One helicopter flying over your house won’t let you sleep,” said Kuppaletti. “If you want 1,000 drones flying over cities and urban centers, noise will be a huge problem.”
In the long term, the widespread deployment of drones could become an issue. While around 5,700 commercial airplane flights occur daily in the United States, drones, with their diverse applications, could perform thousands of flights in major metropolitan areas each day.
Kuppaletti mentioned that many factors influence how people perceive sound. Noise from airplanes is less noticeable in densely populated cities than in suburbs or rural areas. The time of day also matters; evenings are generally quieter, making aircraft noise more pronounced. “Studies have shown that simply observing an airplane can lead people to think it’s loud,” Kuppaletti said. “There are subjective human reflexes that you can’t control.”
Kuppaletti is exploring sound management for drones through engineering design. He tests sound in a room lined with sound-absorbing materials that eliminate echoes. Using an anechoic chamber—a room covered on all sides with sound-absorbing material and equipped with an array of eight microphones—Kuppaletti measures sound frequency, wavelength, amplitude, and other factors affecting our perception of noise. He and his students are developing a guideline that drone and flying car manufacturers can use to estimate the noise level of new designs based on UC’s engineering experiments.
Each rotor has its own noise signature. A simple change in the configuration of two rotors can increase or decrease sound by 10 decibels or more, according to UC student Reed.
Natalie Reed tested 16 rotor configurations for a paper she presented at the SciTech conference. “Changing the vertical gap affects the noise. So, I looked at what would happen if we altered the vertical or horizontal spacing,” she said.
UC student Sorensen studied the differences in sound between rotors rotating in the same direction versus opposite directions: either rotating together or counter-rotating. However, clear results have yet to be obtained.
“It’s an exciting time for the aerospace industry,” Kuppaletti said. “New aircraft designs are in the preliminary conceptual design phase. Based on the designers’ solutions, we will be able to influence aircraft noise levels in the future.”
He added that sound experiments at UC will help manufacturers develop more streamlined designs.