What's the latest science has to say about room acoustics and noise control? The UK weekly New Scientist ran an article "Sonic doom: Making a sound barrier" this past Feb. that may help audiophiles and cheesy Michael-Green-hippie-type product developers ...
You have to be a subscriber to access the full article (maybe the video, too). Well, I am so I'll re-print much of it below (for science and education purposes only ;) ...
Driven to distraction by the urban din? A sonic crystal or a bubble sandwich could cut out the racket
IT IS 3 am. Thud, thud, thud go the footsteps up the stairs. Uh-oh. Sure enough, laughter and music are soon pouring through the ceiling. Even with my head buried under the pillow, bass notes pound through my skull. Eventually I drop off, only to be woken a few hours later by revving engines as the rush hour begins.
Being kept awake by noisy neighbours and traffic is maddening. It is not just the mounting resentment that your personal space is being compromised and the stress of trying to deal with it: chronic noise pollution can seriously damage your health too, and it is a growing problem.
See gallery: "Sonic doom: Noise in pictures"
So what can I do about it? Short of leaving the city, the only option is to find some sort of soundproofing. There are various kinds on the market, but they seem fairly crude, and anyway they are not that effective against the din of 21st-century urban living. As I settle down for another long night with lumps of wax jammed into my ears, I think surely there must be something more high-tech than nailing thick panels of plasterboard to my walls and ceiling. And what about windows - must I settle for nothing more amazing than double glazing?
The next morning I set to work, and find that engineers and physicists are by no means deaf to the problem. They have devised insulation that controls acoustics like a conductor does an orchestra; smart windows whose subtle vibrations cancel the most aggravating sounds (see "Good vibrations") as well as novel kinds of materials made from rubber sheets, metal poles or even bubbles, all designed to stop sound dead in its tracks. So can they really bring peace to our noisy neighbourhoods?
Noise pollution is a serious problem, affecting more and more people as cities expand, population density increases and machinery invades our lives. In recent years, researchers have started to appreciate the detrimental effect this has, physically and mentally, on the human body. Stress and loss of sleep increase your metabolic rate, your blood pressure can rise, and over long periods noise can increase your risk of developing a number of psychological disorders, including depression. Long term exposure to stress from traffic noise has been linked to an increased risk of heart disease. The World Health Organization estimates that each year, chronic noise contributes to the death from heart disease of more than 200,000 people worldwide (New Scientist, 24 August 2007, p 6).
Despite this, there is little information on just how many people are affected. Some cities in the US, such as San Francisco, are beginning to map urban noise levels, and the European Environmental Agency estimates that for roughly 20 per cent of the population of the European Union - around 80 million people - such levels are "unacceptable". In 2006 the European Union ruled that every European city with more than 250,000 citizens must monitor its noise environment. There are numerous laws regulating major sources of urban noise, such as building construction, aircraft, and so on, but for many people holed up in the city, these just aren't enough.
Stressed to kill
Some researchers, including urban design specialist Anne Vernez Moudon from the University of Washington in Seattle, advocate more serious action. "The time has come for the health sector to acknowledge ambient noise as an eminent public health burden and to embrace its abatement as one of its responsibilities," she wrote in the American Journal of Preventive Medicine(vol 37, p 161).
One way to tackle ambient noise is to pile on conventional sound insulation, as I discover at the UK's National Physical Laboratory (NPL) in Teddington, home to some of the world's most sophisticated facilities for the study of sound. Here, acoustic labs need complete quiet, so the walls are made of solid concrete, 30 centimetres thick.
This brute force approach works well. Thin, lightweight walls will wobble in synch with incoming sound, transmitting it through to next door. It is far more difficult to make dense concrete structures vibrate, so incoming sound waves tend to bounce off instead. In Teddington this method has proved effective, as NPL acoustic scientist Richard Barham tells me. And often it is the only option, unless you can suspend your home in a vacuum.
Yet even concrete isn't perfect. It is very good at transmitting "impact" noise - like the clacking of heels running up stairs. And while it blocks higher frequency sounds such as singing, those thumping bass lines still make it through. Such sounds can travel long distances through solid walls and they can even reflect around corners. "You have to do more elaborate things if you want to be effective against low-frequency sound transmission," says Keith Attenborough, an acoustics specialist at the Open University in Milton Keynes, UK - things like adding cavities stuffed full of foam. Such a cavity can absorb sound waves by resonating and the foam then dissipates the waves' energy as heat. But this kind of thing is best added during construction so it won't help me.
One light on the horizon is a recent invention called a sonic crystal. Clear your mind of glistening jewels - this resembles a crystal only in the sense that it has a regular, symmetrical structure. Sonic crystals are really not much more than arrangements of objects that scatter sound waves, but they have one very alluring feature: with careful design they can stop sound dead.
Arguably the first sonic crystal was a minimalist artwork made by the Spanish artist Eusebio Sempere in 1977, which is now outside the Juan March Foundation in Madrid (New Scientist, 23 March 2002, p 32). Órgano is a circular array of vertical pipes that is 4 metres wide. It is likely that Sempere did not purposefully build it to have acoustic properties, but when Francisco Meseguer of Madrid's Institute of Materials Science and his colleagues tested its acoustic properties in 1995 they discovered it allowed some frequencies through, but blocked others (Nature, vol 378, p 241).
Plumbing in the key of C
The key to this effect lies in the spacing between pipes. If incoming sound waves have a wavelength roughly equal to the distance between any two pipes, the sounds are reflected and interfere destructively. Órgano blocks sound transmission between 1500 and 4000 hertz (see diagram). It may not stop traffic noise, but this sculpture could be just the thing if your neighbour is up at all hours playing the piccolo.
The artwork has inspired researchers to devise sonic crystals that cut sounds at lower frequencies. Attenborough, with colleagues at the Polytechnic University of Valencia (UPV) in Spain and the University of Salford, UK, has tested a crystal that both reflects and absorbs sound. It is made up of long plastic pipes, or resonators, with narrow slots cut lengthways along them and a soft latex tube within. The idea is that the slots let sound waves in, so the pipes themselves can resonate, and the latex tubes then absorb this energy. At the same time, pairs of resonators vibrate together, creating larger resonators that absorb sound at lower frequencies. Attenborough has found that these measures extend the range of frequencies blocked to below 1000 Hz (arxiv.org/abs/1101.2332). Physicist Juan Sánchez-Pérez at UPV is aiming for a similar effect by using pipes of different diameters arranged in a pattern based on a fractal arrangement called a Sierpinski triangle (Europhysics Letters, Vol 92, p 24007). Fractal-like devices seem better than current designs, he says.
Still, I cannot see how this kind of sonic crystal will help me sleep more soundly. The pipes are best arranged in a pattern a metre or so deep, and that's too large to line my bedroom. Attenborough thinks it would be ideal for cutting noise along roads and railways, though. He is now trialling the design with the Transport Research Lab, a research company in Wokingham, UK, and Sánchez-Pérez, too, plans to test his fractal pipework outdoors.
So what hope for a sonic crystal that I can hang on my wall? Last year, Zhiyu Yang and colleagues at the Hong Kong University of Science and Technology in China announced a design that uses a latex membrane with a small weight at its centre. The membrane is stretched over a frame, and when sound waves hit the membrane, the inner part of the membrane resonates in the opposite phase to the outer part so the sound waves cancel out (Applied Physics Letters, vol 96, p 041906).
The device seems to work particularly well at low frequencies. By adjusting the mass of the small weight, the membrane can be tuned to block a particular range of frequencies, and when Yang and his colleagues arranged their membranes in panels like tiles on a wall and then stacked five of these panels on top of one another, they achieved a noise reduction of more than 40 decibels between 70 and 550 Hz. That won't completely block the sound of an electric guitar, but it's enough to muzzle next door's booming subwoofer so it sounds more like the hum of a distant power station. Since each panel is just 15 millimetres thick, adding a stack to your wall should be no more difficult than attaching plasterboard panels.
So my hopes are rising, yet my best chance of a good night's sleep may lie with bubbles. Valentin Leroy, a physicist at Paris Diderot University in France has developed a kind of bubble sandwich: a regular array of air bubbles inside a flexible layer of latex (Applied Physics Letters, vol 95, p 17). Like the latex-lined pipes, the design both scatters and absorbs sound. It is the regular arrangement of the bubbles that does the scattering, making sound waves interfere destructively. The bubbles themselves also undergo something called Minnaert resonance: they ring like tiny bells when sound waves strike them and the latex absorbs the energy of these oscillations.
Because they are encased in latex, the bubbles have a very low resonant frequency, meaning they absorb frequencies that are hard to block using other designs. Minnaert resonance means you can have bubbles that are much smaller than the wavelength of the sound, says Leroy, so the structure can be very compact: "This is a fascinating property of bubbles."
So far Leroy has only tested his bubble sandwich with sound waves at ultrasonic frequencies, but he plans to extend the work to the audible range. Bubbles that resonate at 1 kHz need to be 6 millimetres across, he says, while resonance at 100 Hz needs bubbles around 6 centimetres wide. It should be possible to stop a wide range of frequencies by having bubbles of different sizes within the structure, and in principle a 10-centimetre-thick latex bubble layer could help block everything from thumping bass to piercing soprano, Leroy says. Best of all, Leroy reckons such a sandwich should be cheap, robust and simple to manufacture.
So I may have to wait a while, but it looks like a good night's rest is within my grasp. It will probably take a coating of bubbles in my bedroom, and the bland concrete walls on our busiest streets will have to be covered with mosaics of rubber membranes or replaced with sound barriers that look like church organs. And though urban living will almost certainly continue its assault on our ears, at least it should be a little easier on the eye.
Whatever your home is made from, the easiest way for unwelcome noise to slip inside is through the windows. Double-glazing will reduce the noise that makes it through, but a team led by Rajesh Rajamani at the University of Minnesota in Minneapolis has now developed a single-glazed window that actively eradicates any noise that reaches it. It does this by detecting an incoming sound wave and generating a perfect out-of-phase duplicate, so that the waves cancel out.
Rajamani's team had to overcome some serious challenges to develop their window. The main problem is that the window itself needs to become both a microphone and a loudspeaker. Conventionally, neither of these devices are transparent.
The team found an answer in 2007 in the form of polyvinylidene fluoride (PVDF), a transparent piezoelectric material. This responds to the pressure changes in a sound wave by generating an electric current, and can also be made to resonate to produce sound waves by applying an alternating voltage across it. Rajamani deposited small squares of PVDF onto windows and connected them to a computer using transparent wires. When activated, these PVDF squares behaved as microphones to monitor the environment and when they detected unwelcome noise, the computer switched on other squares to vibrate at the same frequency but out of phase.
Tests suggest Rajamani's active windows reduce outside noise by 12 decibels - equivalent to converting the sound level of a truck to that of a leaf-blower (IEEE Transactions on Control Systems Technology,vol 15, p 704). Rajamani hopes that his windows will be marketable within a couple of years.
Jon Cartwright is a science writer based in Bristol, UK