Urban dwellers have undoubtedly experienced disruption by loud vehicles and traffic noise. While it may be part and parcel of urban life for the time being, researchers at School of Electrical and Electronic Engineering (EEE), Nanyang Technological University (NTU) have come up with two new methods to tackle these as well as other problems with the hopes of making cities quieter in the future.
A noise-cancellation system was developed that can be installed in home windows as well as an automated monitoring system that could help government agencies nab excessive noise offenders.
LIMITS TO URBAN NOISE CONTROL
For urban residents, a typical solution to noise is to close their windows to mute outside dins, and then switch on the air-conditioning but this is not an environmentally-friendly solution. While governments could erect noise barriers along roads and train tracks, these are costly, visually unappealing, increase the risk of graffiti, and are not suitable for mass deployment in many land-scarce cities.
With considerations like these in mind, NTU researchers have developed a noise-cancellation system, which relies on a technique called active noise control to help complement the use of noise barriers. This uses inverted sound waves to mute sounds from sources like construction sites or traffic intersections. The technology has been used in noise-cancellation headphones, industrial plant air-ducts and car exhaust pipes, but so far not in homes.
To help defeat noise, the NTU system places an array of small microphones and loudspeakers near homes' windows. The microphones detect incoming noise, and the loudspeakers generate sounds that are the same in amplitude but opposite in phase - anti-noises, in effect - to cancel the incoming noise.
IMPORTANT DESIGN FACTORS
To successfully achieve noise control, researchers ideally need to have some information about the types of traffic noise that residents in a particular block face, and how they perceive noises, as some noises are deemed more annoying than others.
This information could be partly obtained by using microphones on a moving vehicle to capture the typical noises in a housing estate. The information can then be programmed into the memory of the noise controllers to generate the best anti-noise wave-fronts.
Noise cancellation is never perfect in implementation because there are different reflections and diffractions of sounds through walls and windows, and this leads to different noise levels within a room.
TESTING NOISE CANCELLATION TECHNOLOGY
To help get an optimal result, researchers tested two brands of compact flat panel and cone speakers that could be installed with minimal obstruction to the windows. The flat panels are extremely thin but take up more surface area, while the cones are more voluminous but less obstructive in area.
Both types of speakers were pitted against typical traffic sounds of about 85 A-weighted decibels, or dBA — a form of measurement that takes into account how the human ear perceives loudness. They achieved noise reduction of more than 10 dBA.
The researchers found, however, that the cone speakers were better at reducing the noises, and also provided quiet within a larger part of the room in some cases.
The flat panel speakers responded more quickly to incoming noise, but also emitted a larger diffuse sound from their backs, which can interfere with the noise-cancellation process. This problem could be solved with effective acoustic absorbent material. Using cone speakers, which are cheaper, could also work. In addition to noise cancelling, adding soothing soundtracks to mask residual noises in the room could also be effective. A hybrid approach of noise control is more suited to cover a broader bandwidth of noise with changing noise patterns.
TACKLING VEHICLE NOISE POLLUTION
Mitigating ambient noise pollution through noise cancellation is also only part of the battle. Researchers designed an automatic noisy vehicle detection and surveillance camera system — dubbed NoivelCam — that can track moving vehicles on roads and capture footage of those that exceed stipulated noise level thresholds.
To monitor vehicle noise pollution, the Singapore National Environment Agency, Land Transport Authority and Traffic Police currently set up road blocks and check vehicles' tailpipe noise emission levels manually. The manpower needed however means this is carried out less than 10 times a year.
The checks are also conducted by getting the driver to accelerate the vehicle while it is in a static position, which may not reflect the actual driving conditions.
The NTU automated system consists of two microphones, a high-speed camera to take photographs, a wide-angled video camera and a computer. The prototype was designed to monitor a single lane in expressways, but the researchers plan to scale it up for multi-lane use in the project's next phase.
The directional microphones can track vehicular noise on only the monitored lane as they weaken sounds from other directions. When the noise exceeds the stipulated threshold, the high-speed camera is activated to capture the offending vehicle's number plate. The video camera continuously records footage which can be used as proof, or to evaluate possible false triggers due to loud vehicles in neighbouring lanes.
Tests in NTU's premises and on a busy expressway showed that the system is generally able to differentiate between noises from the monitored lane and its neighbouring lanes. The researchers are currently improving the system so that it can even classify the acquired noise signals into different vehicle types for greater confidence in the results.
The team has also added 3G connectivity to the system to stream the recorded data to a server, and plan to incorporate other features such as data analytic tools to generate useful information including vehicle speed, traffic density and sound level patterns over time.
NOT JUST PEACE OF MIND
Beyond providing peace of mind at home, these inventions could even help protect health: The World Health Organization (WHO) calls noise pollution "an underestimated threat that causes a lot of short and long-term health problems, such as sleep disturbance, cardiovascular effects, poorer work and school performance, and hearing impairment." Researchers hope that implementing technologies like these can help improve the quality of life for urban residents going forward.