Health : using speakers to combat street noise

Cars whizzing by, mopeds rattling – the noise of the big city can rob you of sleep. Researchers want to ensure silence in the apartment with the method "Active noise control.

Pssst. Noise pollutes. Researchers want to bring back silence

Bam! The baseball bat lands crashing on the windshield. The alarm system, which was still emitting a deafening howl, falls silent. David Owen smiles. Silence at last. The New York lawyer from the film satire "Noise" has a mission. He wants to rid the metropolis of noise and will stop at nothing to do so. The omnipresence of noise drives Owen so crazy that he fights back and becomes the town’s resident noise nuisance, for whom no remedy is too extreme. His longing for silence is shared by many people in real life. One person is disturbed by noises that leave the other person cold. While music fans voluntarily flock to the heavy metal festival Wacken and luxuriate in the decibel rush, others are annoyed by the buzzing of a mosquito. Our everyday lives are dominated by car horns, the drone of airplanes, the tinkling of cell phones, and the clacking of our colleagues’ keyboards.

To avoid noise, scientists sometimes resort to unusual methods. Researchers from Darmstadt, Germany, have found a way to combat noise with noise. To protect interiors from the noise of road traffic, they place loudspeakers in double-glazed windows. Acoustician Joachim Bos laughs when he talks about the work of his colleagues: "Of course, it sounds absurd at first: loudspeakers against noise. But the method is quite effective."

Scientists use that sound occurs in waves – in wave crests and wave troughs. "We knew that if we managed to use the loudspeaker to send out waves that slosh toward the incoming noise wave in such a way that wave trough meets wave crest, we would succeed in leveling the waves," explains Bos. Experts call this phenomenon destructive interference, because the sound mountain and sound valley cancel each other out.

Noise countering does not always work

When a car drives past the front door, the sound causes the air to vibrate. This vibration hits the outer window pane and slightly depresses it. The negative pressure created in the space between the windows causes the inner pane to move as well. The result: sound ends up in the home. With the loudspeakers, noise researchers want to keep the inner pane from vibrating. A sensor measures the surface vibrations of the outer window pane and adjusts the speaker to compensate for the incoming sound pressure. In this way you get double-glazed windows quiet, but not many other rooms. Because sound spreads out in a spherical pattern. Countering with counterwaves only works if the incoming wave is predictable and straightforward.

Inside a double-glazed window, sound propagates almost linearly because of the short distance, so scientists can calculate the sound pressure well. "Leveling spherical sound waves is as futile as trying to push onion slices into each other," says acoustician Bos. In the worst case, they achieve exactly the opposite effect: a wave crest meets a wave crest and amplifies the sound.

Bos thinks a different approach is more efficient. In addition to "active noise control" – loudspeakers are used to generate targeted counter-noise – there is also "active vibration control". The idea here is to influence potential noise sources so that they don’t make any sound at all. For example, there are devices that ensure that certain parts of wind turbines do not start vibrating. Car manufacturers also have an interest in preventing sound emissions in cars. Then it gets quieter in the cars.

30 km/h zones can help

Not only traffic noise burdens our everyday life. Open-plan offices can also influence our concentration, for example. Markus Meis from the Horzentrum Oldenburg (Oldenburg Hearing Center) has looked at the acoustic situation in workplaces and classrooms. His conclusion: you can hardly block out speech. "The clearer the language of colleagues, the more I am restricted in my performance." Moreover, unlike noise, you can’t simply ignore information that comes through speech, he said. According to the expert, open-plan offices or newsrooms are often set up in such a way that a flow of information can take place, i.e. sound is transmitted well. Good room acoustics are important here. Suspended ceilings, movable walls and carpets contribute to the fact that the area does not reverberate.

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If it were up to Michael Jacker-Cuppers of the German Acoustics Society in Berlin, the most effective method against noise would be quite simple: Don’t let it occur in the first place, or keep it to a minimum. Above all in the traffic there would be possibilities there. "Asia is taking on a pioneering role. In Tokyo, just 18 percent of people use private transport – there’s no better way to get traffic noise under control," says Cuppers. A 30-mph speed limit in city centers could also help. Construction noise bothers him the most. "That’s when neighbors remodel their apartments, leave their windows open, and the noise echoes across the street."So far, according to Jacker-Cuppers, there are too few incentives to ensure more silence. Nevertheless, he would not mutate into David Owen from the film "Noise". After all, baseball bats that crash into car windows to silence the alarm system also ultimately cause noise again.

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