When we talk about pollution we typically are referring to unhealthy substances in the air or water. But an additional, pervasive source of pollution—environmental noise—can also pose significant risks. According to the journal Environmental Health Perspectives, noise pollution results in adverse health effects—from hearing loss to heart disease—in tens of millions of Americans each year.
Those of us working from home during COVID-19 developed a heightened awareness of environmental noise whenever leaf blowers, lawnmowers, garbage trucks, or motorcycles interrupted our Zoom calls. On the whole, however, ambient sound exposure diminished early in the pandemic because of reduced air and road travel. According to Environmental Research Letters, participants in a study in California, Florida, New York, and Texas on average experienced a three dBA decrease in environmental noise during the COVID-19 lockdown, which “likely represents a meaningful reduction in overall risk of sound-related health effects” such as hypertension, ischemic heart disease, and cognitive performance.
The benefits have been short-lived. Now that pandemic restrictions have been scaled back, air and road travel are currently surging, in some cases topping pre-pandemic levels. Environmental noise will likely be worse than ever as we return to normal.
That need not be the case. A series of new material technologies can help reduce mechanical noise at the source of emission—and offer us the promise of a quieter future.
Foamed or layered composites that absorb and reflect sound are one promising avenue of research. Scientists anticipate that such materials can help reduce aircraft noise, a significant source of unwanted environmental sound: Jet engines are so loud that they can cause an eardrum rupture when heard from a distance of 25 meters during take-off. Researchers at the University of Bath have recently developed an aerogel-based insulation for aircraft with impressive noise-reduction properties. The foam is made from graphene oxide-polyvinyl alcohol, an airy substance weighing only 2.1 kilograms per meter³—reportedly the lightest acoustic insulation product ever made. When applied within an airplane engine, the material would diminish sound levels by up to 80%, reducing take-off noise to a level similar to that of a hairdryer.
Scientists at the University of Alabama have also developed a foam material to quiet engine combustion noise. The composite foam consists of hafnium carbide and silicon carbide and is sufficiently heat-tolerant to resist direct exposure to fire. The high permeability of the material makes it suitable to envelop a combustion flame, acting as a noise reduction filter as the flame passes through it.
Another invention, developed by researchers at North Carolina State University and MIT, filters specific sound frequencies. The composite material is a honeycomb structure surrounded by a lightweight, reflective rubber skin. When the low-frequency noise of an airplane engine meets the panel, much of the sound energy is deflected off the membrane. The material could be used on the ceilings and floors of airplane cabins, contributing to sound dampening without increasing fuel costs by adding too much extra weight.
Researchers at Stanford University and Eindhoven University of Technology have been studying hummingbird wings in the hopes of designing quieter vacuum cleaners, drones, and other products.
Another area of environmental noise research concerns surface structures and aerodynamics of moving components. Scientists have been analyzing biological models for methods of mitigating noise: For example, researchers at Lehigh University have studied the sound profile of owl wings during flight. The number and configuration of downy feathers in their wings enable owls to hunt in near-silence—the noise of their wings is diminished above frequencies of 1.6 kHz. The scientists have mimicked the surface profile of these feathers in 3D-printed polymer textiles. By attaching these materials to aerodynamic components such as wind turbine blades, noise is reduced by 10 decibels without affecting speed.
Similarly, researchers at Stanford University and Eindhoven University of Technology (TU/e) have been studying hummingbird wings. The hummingbird’s wing speed of 40 flaps per second, or 40 Hz, would result in a grating noise in most human-made objects, but the hummingbird instead generates a subtle and pleasant-sounding buzz. To study the wing flap, the researchers used advanced “sound cameras” that combine optical cameras with an array of over 2,000 microphones, resulting in the development of a 3D acoustic field algorithm that can interpret and predict the sounds made by particular wing shapes and speeds. The team anticipates that the research will improve the sound quality of products like drones, fan blades, vacuum cleaners, and other rapidly moving human-made components.
These advances in noise-insulating research are changing the way we respond to environmental noise. “Noise pollution is becoming an ever-greater problem,” says TU/e researcher Rick Scholte. “And a decibel meter alone is not going to solve that. You need to know where the sound comes from and how it is produced, in order to be able to eliminate it.”