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Introduction

"You will soon be able to silence the constant noise of a road drill or the beat from a nightclub without blocking the sounds you want to hear, according to Selwyn Wright, an engineer at the University of Huddersfield in Yorkshire, UK."(1) He is working on a so-called “Silence machine”.

Basically, it works by "analysing a stream of incoming sound waves from a noise source, and generating sound that is exactly out of phase"(1), neutralizing the incoming sound waves.

As with much of modern science, this idea originally start off as science fiction. The idea was first proposed by Arthur C. Clarke in his series Tales from the White Hart. The story goes that a professor in England develops a silencer device. A scorned young man whose ex-girlfriend is in the universities’ big play borrows the device for the big evening and sneaks it into the auditorium, with explosive results - the device overloads and explodes.

Practical applications

Minus the explosive consequences, the machine is both scientifically possible, and, on a smaller scale, already commercially available. Amongst others, Sony sell the NC10 headphones, which they claim “reduces noise by up to 70% (10dB)”. A Noise Cancelling Circuit in the main unit senses outside noise with built-in microphones and sends an equal-but-opposite cancelling signal to the headphones. Sony are marketing the headphones as ideal for travel on planes, trains, buses and cars, use near noisy air conditioners or on noisy streets.

There are many situations in which the ability to ‘create’ silence is advantageous, and although today's technology is not yet able to focus the waves accurately or quickly enough to blanket say, a room full of people talking (as in the Clarke story mentioned earlier), it is able to significantly reduce background noise and other, simple repetitive noises.
  • The IT industry
    Even a standard desktop PC makes an audiable humming noise due to its fan and the whirring of its hard drive, so imagine the noise created by several hundred high-end servers. The ability to dampen the noise from the machines in a server farm is a great benefit to the IT engineers who work with them.
     
  • Vehicles
    By dampening the noise generated while a car is in motion, a much more peaceful and enjoyable driving experience is given - a great bonus for both the passengers and driver, and an even better selling point for the car salesman. Similarly, with headphones, the noise from plane jets or trains can be dampened.
     
  • The Military
    Many modern US fighter jets have flat-panel speakers installed that produce the necessary anti-waves to dampen the noise within a fighter cockpit. By giving the pilot a quieter environment, the air force found that the pilots where able to better concentrate and act under pressure.
     
  • Industry / Manufacture
    Large industrial or manufacturing machines that produce a large amount of noise can be ‘quietened down’ by installing these “silence machines”, enabling the workforce around them to better concentrate.

The effects of background noise on people

Hearing loss from long term exposure to noise has been recognized as a hazard for a long time. However, the non-auditory effects of noise are still not fully known. The suspected effects include cardiovascular function (hypertension, changes to blood pressure and heart rate), and changes in breathing, annoyance, sleep, physical and mental health. This wide range of effects has led researchers to believe that noise has the ability to act as a general, non-specific stressor.

In addition, noise is annoying. If exposed to noisy environments, people generally prefer to reduce the noise loudness, avoid it, or leave the noisy area if possible. The same noise could be annoying to some people but acceptable to others. There is no definite relationship between the degree of annoyance or unpleasantness of noise and the risk of adverse health effects. For example, very loud music may be pleasant to one group of people and annoying to another group. However, both groups will be equally at risk of hearing loss.

Globally, some 120 million people are estimated to have disabling hearing difficulties. More than half citizens of Europe live in noisy surroundings; a third experience levels of noise at night that disturb sleep.

In the USA in 1990 about 30 million people were daily exposed to a daily occupational noise level above 85 dB, compared with more than nine million people in 1981; these people mostly in the production and manufacturing industries. In Germany and other developed countries as many as 4 to 5 million, that is 12-15% of all employed people, are exposed to noise levels of 85 dB or more. In Germany, an acquired noise-related hearing impairment that results in 20% or more reduction in earning ability is compensatable.

Noise can adversely affect performance, for example in reading, attentiveness, problem solving and memory. Deficits in performance can lead to accidents. In addition. noise above 80 dB may increase aggressive behaviour. A link between community noise and mental health problems is suggested by the demand for tranquillizers and sleeping pills, the incidence of psychiatric symptoms and the number of admissions to mental institutions.

Clearly, a quieter working and living environment is much better, and it is this that Wright is hoping to create. When the machine is perfected, "it probably will have some applications," says Tim Williamson of Britain's National Society for Clean Air and Environmental Protection.

Sound as a wave

In order to understand both the problem and it’s solution, a look at how sound works and can be manipulated is required. Sound waves are waves of pressure travelling through any given medium - for the purpose of this node, only air is being considered - which creates regions of higher pressure (compressions) and areas of lower pressure (rarefactions).

In air, the air molecules perform standard harmonic motion along the line of travel of the wave. The fact that sound is a wave is essential to creating anti-phase silence. With any set of waves, “beats” can be created when two or more waveforms of slightly differing frequencies meet at one point in space.

By taking a wave and adings its inverted mirror-image, near silence can be produced by superposition. The effect of beats is the superposition of waves that are not quite identical because of the slight difference required in their frequencies. Therefore, any given observer receives a signal that has either been added to or subtracted from as time passes. By maximising the amount of subtraction, a ‘dampening’ effect on background noise can be achieved.

By utilising a process like this, an anti phase wave can be produced and emitted. It is a process like this that is used within headphone or flat-speaker based systems today.

Selwyn Wrights work

His patented Silence Machine has several microphones for sound sampling, a powerful processor for creating the necessary anti-noise, and many loudspeakers for outputting the anti-sound at the incoming noise.

What is special about Wright’s machine over currently available technology, is that it is a lot more directional. He claims that his machine is able to create a “shadow” of silence, in which only the unwanted sound will be cancelled, and other noise is still audible. The size of the shadow areas where the sound and anti-sound waves cancel each other can be varied by changing the number of loudspeakers or their positions. The more directional the microphones and loudspeakers are in terms of sensitivity the better the end result.

Signal processors in the computer measure the frequency of every component in the noise signal, and use this information to create the anti-noise-sound with an identical frequency but the opposite phase. This means that a peak in the noise wave meets a trough in the anti-noise, cancelling out the sound, as shown earlier by superposition. Although the actual physics behind the system is actually fairly simple, there are a number of other problems to consider:

  • Generating the anti-phase wave
    This needs to be done as quickly as possible, and ideally output as soon as the input wave is received. This is largely a computing problem, and as the power of processors increases, this process can be tackled a lot quicker.
     
  • Public awareness of the technology
    Currently, this is not a well known system. By generating publicity and public knowledge of the system and its benefits, the system could come to more widespread use.
     
  • Abuse of the technology
    The ability to create definable areas of blanket silence would be a great advantage for the user. Some sort of legislation may have to be passed to stop events like that depicted in the Clarke story becoming a real-life occurrence of a new type of terrorism.
     
  • Military use would be a separate issue
    If the military was able to blanket enemy troops with targeted silence - effectively creating radio silence on the battlefield, they would have a great advantage over their opponents. Applications like this are becoming more popular with the military, as they stride towards developing more non-fatal weaponry.
     

Conclusion and Evaluation

Clearly, the technology is available to make Wrights’ machine work on a much larger scale, and the end result would be both useful and potentially very effective in the right situation. If an employer was able to increase the productivity of his staff, and safeguard their mental and physical health, then the machine would be a great advantage to the business: a happy, healthy worker, is, after all, a much more productive worker.

Resources and bibliography

  1. "Silence machine" zaps unwanted noise, Marina Murphy
    http://www.newscientist.com/hottopics/tech/article.jsp@id=99992094&sub=Gadgets%20and%20Inventions
     
  2. Whatis dot com: silence machine
    http://whatis.techtarget.com/definition/0,,sid9_gci906859,00.html
     
  3. Sony NC10 Reviews
    http://www.audioreview.com/PRD_124759_2750crx.aspx
     
  4. How Noise Affects Human Hearing, Communication and Job Performance
    http://buy.sennheiserusa.com/ASP/Sennheiser/pdf/Noiseef.pdf
     
  5. Noise pollution
    www.transport2000.org.uk/campaigns/NoisePollution.htm
     

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