Acoustic black holes for less noise living07/01/2020
Shaped like a multi-directional hollow cylinder, acoustic black holes control structural vibration and sound radiation. Credit: ISVR University of Southampton.
ISABEL RUBIO ARROYO | Tungsteno
The roar of a plane taking off, the toot of a car horn, or the bustle of a busy bar. These are just some of the noises that reverberate in practically every metropolis in the world hundreds of times a day. The growth of cities and their overpopulation is propelling the search for solutions to make urban areas more liveable and reduce polluting emissions, not only of chemical substances but also of noise. Creating less noisy cities and acoustically healthy buildings is one of the challenges of the 21st century.
By 2019, Western Europeans were losing at least one million healthy life years from traffic-related noise, according to the World Health Organization. Noise pollution can also lead to cognitive impairment, sleep disorders, hearing loss or even cardiovascular disease, the WHO says. However, it is difficult to establish whether this is a causal relationship, because most people exposed to noise are also exposed to many other environmental factors.
The cities suffer from noise pollution derived from traffic. Stopping the negative impact of this noise requires reducing vehicle movement and the use of green transport. Credit: Wikimedia Commons.
Traffic, the main source of noise pollution
The impact of noise pollution is greater in cities. The Chinese city of Guangzhou, as well as Delhi, Mumbai, Cairo, Istanbul, Barcelona and Mexico City are among the world's noisiest cities, according to the Worldwide Hearing Index. "Traffic noise is an important component, but machinery, planes if you are near an airport or entertainment also contribute," explains Matthew Wright, professor of acoustics at the Institute of Sound and Vibration Research at the University of Southampton in the United Kingdom.
In fact, noise pollution from cities has evolved in recent decades as attempts have been made to create less noisy cities. In the 1980s, car engine noise "was the most important component." But they have become increasingly quiet, so now "most of the noise you hear if you live near a highway comes from the tires," says Wright. Even so, in a city where cars stop and start all the time, engine noise is still significant.
Technology and metamaterials to absorb noise and vibrations
There are also different technologies and new types of building materials that could help to eliminate the annoying sounds in cities. Active noise control, for example, involves creating a deliberate "anti-noise" to cancel out unwanted noise. There are also initiatives to transform noise pollution, such as that carried out in 2016 by Jordan Lacey, a researcher at the Royal Melbourne Institute of Technology (RMIT) in Australia. Microphones were used to capture both ambient and traffic sounds near a park. The signal was passed through an algorithm and mixed with other processed sounds to create a musical composition that was played through speakers. Another alternative for controlling noise and vibrations are acoustic black holes, where "sound energy is trapped in a structure." At the moment, most applications are in the aerospace sector and other forms of transportation. But Wright suggests that in the future acoustic black holes could be applied to other areas to curb noise pollution in cities.
In the case of buildings, the materials used in their construction will be decisive in blocking noise. "Some, like rock wool and fiberglass, are well known for their ability to absorb sound, but acoustic engineers now design metamaterials to have the exact properties required for a particular situation," he says. Metamaterials are man-made materials that have properties that have never existed in nature until now. For example, tests have been conducted with viscoelastic materials or phonic crystals that can block the transmission of sound at certain frequencies.
Curbing urban noise pollution also requires changing the way buildings are constructed, for example using metamaterials designed to absorb noise and vibrations. Credit: FOM Institute AMOLF.
There are also technologies for trying to hear a priori how spaces that are not yet built may eventually sound. The process is called auralization and consists of modelling and simulating the acoustic experience of being in a particular space. In addition, in buildings, it must be taken into account that "often sound does not just travel through walls and floors. Vibrations can travel through beams and pillars and transmit noise, and sound can travel through open windows", says Wright. Therefore, he advises architects to consider good acoustics along with heating, ventilation, fire protection and all the other aspects of building design.
Will it ever be possible to live next to an airport or above a bar and not hear its noises day after day? The development of electric aircraft could have significant effects on reducing noise pollution. There are experts at the University of Southampton's Institute of Sound and Vibration Research working on aircraft noise reduction, and the concept of the silent aircraft has also been explored by the University of Cambridge and the Massachusetts Institute of Technology (MIT).
In the case of bars, trying to make people quieter would probably not work. "Customers like loud bars, but the neighbours may not," says Wright. Here, soundproofing is already possible in some cases. For example, luxury hotels have noisy bars and quiet rooms. But "laws or financial incentives are needed" before most establishments will adopt such measures. Whether noise in cities will disappear completely one day is still an open question. "I don't know if it will happen, but I do know it won't be soon. Acoustic engineers still have many problems to understand and solve," Wright concludes.
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