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Novel windows block out noise but let in fresh air

By

July 11, 2013

A prototype window design that lets air pass through, but attenuates outside sounds by 30-...

A prototype window design that lets air pass through, but attenuates outside sounds by 30-35 decibels (Photo: Mokpo National Maritime University)

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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.

A variety of Helmholtz resonators were used to fabricate the test windows (Image: Mokpo Na...

The particular resonators used for the silent window are called diffraction resonators. These are hollow boxes measuring 6 x 6 x 1.6 in (15 x 15 x 4 cm) that are made from acrylic that is 0.2 in (5 mm) thick. There are two sets of three diffraction resonator designs, one set having a central air hole 0.8 inches (2 cm) in diameter, and the other set with a 2-inch (5-cm) air hole. Each set has different internal dividers that break the inner air space into one, two, or four equal-sized volumes. The dividers serve to change the resonant frequencies so that a wider frequency band can be silenced. In all cases a cylindrical air filter is inserted through the air hole to prevent the usual Helmholtz resonator whistle.

Why do the diffraction resonators pass air but not sound? One part of the answer is that when sound passes through the air holes, the sound waves are strongly diffracted into the entire volume of the diffraction resonator, so that very little of the sound can pass directly through the air holes.

The other part of the answer is that the diffraction resonators cause the air to have negative compressibility over a fairly wide frequency band. Normal sound waves in air are made up of a series of compressed and expanded regions. The energy of the compressed air drives that material to expand, and vice versa. However, when compressibility is negative, compressed air is less dense than air at lower pressure. As a result, compressed air does not expand, which is the mechanism that normally allows sound waves to travel through materials. Instead, sound waves are strongly attenuated as they travel through a material with negative compressibility.

The combination of these two effects defines a frequency band within which sound passing through the window is strongly attenuated.

Prototype window designs for testing sound attenuation properties. These have three layers...

Prototype windows were made by combining the diffraction resonators in a triple-layer thickness, four resonators wide and three resonators high, measuring 24 x 18 x 5 in (61 x 46 x 13 cm) thick. One window was made of resonators with the 0.8-in air holes, and the other with 2-in air holes. As shown in the figure above, the layers had one, two or four resonance chambers per air hole to try to achieve sound attenuation over a broad range of frequencies.

The test results showed that the window with the 0.8-in air holes reduced sound transmission by over 30 dB at frequencies from 200 to nearly 5,000 Hz, with over 20 dB attenuation even at very low frequencies. The window with 2-in air holes was intended as a compromise between sound attenuation and air passage. This window produced similar attenuation to that of the 0.8-in window for frequencies between about 700 and 2,000 Hz, with attenuation in excess of 15 dB for frequencies from 600 and 5,000 Hz.

The test windows are intended to represent proof of principle, as ultimate performance can be improved in many directions. Many of the overall properties can be improved by introducing more complex internal structures. The thickness of the window can be reduced by causing the air passing through the window to follow a curved path, which can also be used to obtain increased sound attenuation along with increased airflow. Additionally, the shape and size of the diffraction resonators are currently not optimal, and can be improved to give more favorable window size and performance.

The researchers have demonstrated the potential for their windows to change the urban/suburban living environment for the better. The silent windows have principles and structure simple enough that it need not be long before they hit the market – and my window frames.

* Apologies to Percy Shelley

Source: arXiv (PDF)

About the Author
Brian Dodson From an early age Brian wanted to become a scientist. He did, earning a Ph.D. in physics and embarking on an R&D career which has recently broken the 40th anniversary. What he didn't expect was that along the way he would become a patent agent, a rocket scientist, a gourmet cook, a biotech entrepreneur, an opera tenor and a science writer.   All articles by Brian Dodson
12 Comments

"One part of the answer is that when sound passes through the air holes, the sound waves are strongly diffracted into the entire volume of the diffraction resonator, so that very little of the sound can pass directly through the air holes."

Does that mean that the sound energy is transferred into the window structure?

Would this imply that the window cavity needs to itself be suspended or mounted on foam seals so that if the noise outside is loud enough it does not become a source of vibration? Energy needs to go somewhere.

Also what is the relationship between the depth of cavity and frequencies it can absorb? Can thinner cavities with various configs and diameters be sandwiched to widen the band stop, or is there a law of diminishing returns?

Nairda
11th July, 2013 @ 08:01 pm PDT

So fill your windows with transparent mufflers. Who knew?

Slowburn
11th July, 2013 @ 08:29 pm PDT

What about bugs? I need a window that keeps out bugs.

Facebook User
11th July, 2013 @ 08:40 pm PDT

I may have been a little too snarky. 30 odd years ago when I was still a preteen I suggested a multi-pane window with holes like the baffles in a muffler to reduce noise while letting air flow and everybody laughed at me.

Slowburn
11th July, 2013 @ 09:15 pm PDT

" (a 10 dB increase in sound corresponds to a tenfold increase in sound pressure and energy, with 0 dB the human auditory threshold)"

dB is a relative scale, and a 10db increase indeed corresponds to a tenfold increase as stated, but the second part of the sentence seem to imply that 0 db is a change so small that it cannot be perceived by the human ear. Being a RELATIVE scale, 0 db means NO CHANGE and by definition no increase whether measured by human ear or any other method.

In contrast, dBA (though perhaps flawed) is a commonly used ABSOLUTE scale, set so that 0 dbA is approximately at the human auditory threshold.

So, the window may attenuate sound by 30 db (i.e. relative to the un-attenuated sound), but a bedroom is noisy at 45 dbA (i.e. relative to human threshold)

R B
11th July, 2013 @ 11:49 pm PDT

re; Jennifer Linsky

You buy a fine mesh and stretch it over the window to screen out the bugs. Get the mesh with the largest openings that will not let the local bugs through for maximum airflow.

Slowburn
12th July, 2013 @ 01:52 am PDT

Hey, this is a great idea for those with babies in the house, so you can keep an eye on sleeping baby, while you watch the TV? I can think of a few other applications.

Nantha Kumar Nithiahnanthan
12th July, 2013 @ 05:31 am PDT

To "R B". The dB is, indeed, a relative measure. It has been converted to an absolute measure by arbitrarily assigning 0dB to a certain energy level. As the article notes, this 0dB value corresponds to the threshold of hearing for a "normal" person. So dB is both: a relative measure by definition and an absolute measure by convention.

piperTom
12th July, 2013 @ 07:54 am PDT

Good idea NKN

fireflies
12th July, 2013 @ 05:53 pm PDT

A small point. The statement is that a 10 dB increase is a ten fold increase of sound pressure and energy. Sound energy is related to the square of the sound pressure. Therefore, a 10 dB increase in energy is a 100 fold increase in sound pressure.

wmarsh
14th July, 2013 @ 10:46 am PDT

Windows are highly overrated. Most residences have way too many. I don't want air or sound coming through. I want my air filtered to the max (my wife and I suffer from allergies). That leaves lighting and viewing. Viewing is not worth the cost. I have lived with windows for many decades that were not used for viewing. Meanwhile, they increased the cost of heating and cooling. Last, the use for space lighting is preferable to artificial light. However, that can be done two ways without compromising the "R" value of the walls. Regular or tubular skylights work fine.

So the need for windows is greatly over estimated. Considering the cost of extra climate control, coverings, and the security problems, I would eliminate them with few exceptions.

Should I build a custom house, I might try designing high, narrow, room length windows for light only and appearance if I can get glass that stops infrared. And because my wife would go ballistic with no windows. Her objection would be: "Nobody does that."

Don Duncan
14th July, 2013 @ 11:12 pm PDT

Impressive though the attenuation levels are, I can see many downsides:-

1. Even after considerable development, it would still be very thick window that would be difficult to see clearly out of. (due to the many internal baffles)

2. The internal structure would be impossible to keep clean and bug-free.

3. It would be extremely difficult to make in glass. We all know the problems of plastic windows.

My first thought was "why do they need to be windows at all?"

If these structures were made into injection moulded plastic panels, they could be fitted into walls with no need for any form of transparency, leaving windows to do what they do best...

Paulg
15th July, 2013 @ 01:44 am PDT
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