Novel windows block out noise but let in fresh air

A team of South Korean engineers has invented a remarkable window that lets air in while keeping a great deal of noise out.

There are few things better than lazing around the house on a warm summer day, whose fragrant zephyrs speak of spicy isles and heaven-breathing groves.^* At least, until the neighbors start their leaf-blowers and the city needs to tear up the sidewalks. Noise pollution is one of the scourges of urban and suburban life, which can drown out nature’s melodies to cause annoyance, stress, and hearing loss. Now, however, a team of South Korean engineers has invented a remarkable window that lets air in while keeping a great deal of noise out.

Simply put, noise is any sound one does not want to hear. Measured in decibels (a 10 dB increase in sound corresponds to a tenfold increase in sound pressure and energy, with 0 dB the human auditory threshold), a beautiful countryside with only natural sounds has a sound level in the range of 10 to 20 dB. In a bedroom having sound levels above 45 dB, most people will experience considerable difficulty in getting to sleep and staying asleep.

At a distance of 50 ft (15 m), those leaf-blowers mentioned earlier have sound levels in the 70 to 75 dB range, and the jackhammers tearing up the sidewalks outside check in at about 90 dB inside your house. Most of us respond by closing the windows and doors, feeling that putting up with a stuffy, closed-in feeling is better than enduring the noises of an urban or suburban setting.

Now an alternative is appearing. A clever use of acoustic metamaterials by Professor Sang-Hoon Kim of the Mokpo National Maritime University and Professor Seong-Hyun Lee of the Korea Institute of Machinery and Materials in South Korea will enable the design and construction of windows that allow air to pass through, but reduce environmental noise by about 35 dB, depending on the frequency of the external sound. This level of sound reduction is about five dB better than that of a standard vinyl double-pane window.

Metamaterials are composed of patterns of locally resonant structures. Kim and Lee have discovered how to build a sound-blocking metamaterial that has continuous paths to allow the passage of air by using Helmholtz resonators. If you have ever heard a jug band, or simply blown air across the top of a soda bottle to make a whistle, you know about Helmholtz resonators. More generally, they consist of a constrained volume of air that can be accessed from the outside by a neck or a small hole.

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