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Prototype Window Blocks Out Sound But Not Air

By Wesley Fenlon

Airflow? Check. Cars honking and dogs barking? Not so fast!

When your window's open, a cool breeze wafts through the room, but it brings with it the sounds of passing cars and leafblowers. When the window's closed, a blissful silence descends, but it's accompanied by an uncomfortable stuffiness. But you can't have the air without the noise, right? Well, according to Korean scientists Sang-Hoon Kim and Seong-Hyun Lee, we might be able to get one without the other.

Photo credit: Flickr user wakingphotolife via Creative Commons.

The two scientists recently published a paper (PDF) about their research into an air-transparent soundproof window. Technology Review summarizes how the window works:

"A material’s bulk modulus is essentially its resistance to compression and this is an important factor in determining the speed at which sound moves through it. A material with a negative bulk modulus exponentially attenuates any sound passing through it. However, it’s hard to imagine a solid material having a negative bulk modulus, which is where a bit of clever design comes in handy.

Kima and Lee’s idea is to design a sound resonance chamber in which the resonant forces oppose any compression. With careful design, this leads to a negative bulk modulus for a certain range of frequencies. Their resonance chamber is actually very simple—it consists of two parallel plates of transparent acrylic plastic about 150 millimetres square and separated by 40 millimetres, rather like a section of double-glazing about the size of a paperback book. This chamber is designed to ensure that any sound resonating inside it acts against the way the same sound compresses the chamber. When this happens the bulk modulus of the entire chamber is negative."

Two prototype windows designed with 20mm and 50mm air holes that serve as diffractive elements for certain wavelengths. As the paper explains:

"The window is composed of many artificial atoms or diffraction resonators connected in series and parallel. The diameter of the hole in the artificial atoms should be much less than the wavelength of the sound wave for a strong diffraction...We created the resonator to intercept sound from the sound waves at some frequency ranges which led to a separation of the medium and sound. Afterwards, we applied sound waves in the range of 400−5,000Hz to the two windows. We then observed a serious transmission loss of sound within specific frequency ranges. The loss was 20−35dB with the 50mm window in the range of 700 − 2,200Hz."

20-35 decibels isn't a huge noise reduction and isn't exactly the same thing as soundproofing, but it is enough to lower the sound of a loud jackhammer to the volume of a normal conversation. And the size of the air holes can also affect the range of sound being targeted, meaning specially designed windows--near highways, or train tracks, or concert venues, for example--could prove to be even more effective.