New technologies have helped engineers visualise noise in 3-D in Hong Kong, where some main roads are just outside bedrooms and more than a million people are exposed to excessive traffic noise.
Showing noise in three dimensions has become an important method in planning for the city, as government officials try to improve on poor planning in the past.
Large-scale urbanisation, carried out quickly in the early 1980s, is partly to blame for the 5,000 noise complaints that the government receives every year - more than 13 a day.
Now government engineers are using advanced technology to help show the public the environmental effect in projects whose noise may annoy residents.
To show people how they would be affected by particular works, government departments, through the Environmental Protection Department's website, have been putting 3-D noise maps online for assessment of big public projects.
Local residents can see the difference with or without particular structures, such as noise-absorbing barriers, or semi-enclosures.
The maps also help decision makers ascertain what to do to manage noise and reduce noise levels.
For instance, 3-D noise maps of the Wan Chai Bypass - a proposed four-kilometre trunk road running between Central and Causeway Bay - and the Hong Kong section of the Guangzhou-Shenzhen-Hong Kong Express Rail Link are used in their environmental impact assessments and are accessible through the department's website.
Such maps were used in the consultation for retrofitting noise barriers in Tai Po in 2008, which 'effectively assisted councillors in making the decision' in the end, according to a paper by the department.
A department officer gave talks on what Hong Kong had been doing on this front last week at the annual Acoustics conference.
The noise maps are made using a geographic information system, computer modelling techniques, and 3-D computer graphics.
No actual noise measurements are usually done in making these maps. They are based on information such as traffic flow data, road/rail type, and vehicle type data.
The modelling takes account of features that affect the spread of noise such as buildings and the shape of the ground and whether it is acoustically absorbent (such as fields) or reflective (concrete or water).
Gone are the days when noise levels were measured on the spot and presented in reports full of maps and tables that few but engineers read.
Two-dimensional noise maps, which the government adopted about 10 years ago, do not suit the hilly terrain. And it is hard to tell who in high-rises are most disturbed, and where the noises are from.
Hong Kong's topography is characterised by steep slopes, with little flat land. Only about 25 per cent of the 1,104 square kilometres has been developed as urban, and 40 per cent is country parks.
The city's vehicle density is one of the highest among developed economies: there are 283 vehicles per kilometre of road, according to a 2009 study.
Almost 40 per cent of homes in Yau Tsim Mong district are exposed to noise levels above 70 decibels (dB), though Kwun Tong is where the highest number of homes reach that level, according to official figures done in 2003 that are still in use.
Kowloon City, Sham Shui Po, Tsuen Wan and Wan Chai are other districts that top the list.
At the other end, Southern, Sai Kung and the Islands districts are the quietest.
More than a million people in Hong Kong are exposed to excessive traffic noise, defined as greater than 70dB, according to official statistics.
That may explain why almost one in four of the 21,000 complaints the department receives every year are against noise pollution, according to the spokesman.
'Given Hong Kong's population and development density, noise pollution is a frequent concern in the community,' he said.
Just by turning the maps from 2-D to 3-D has greatly helped the public engage in consultations, the paper said.
The spokesman said the government would keep track of the latest development in the fields of noise modelling, geographic information systems and computational graphics, and would try to make use of them more so that noise effects were more transparent.
Noise mapping is not new. Noise maps have been made in many European countries since the 1970s, but it was in 2007, when European Union law required cities with more than 250,000 people to be noise mapped, that noise concerns were more loudly heard.
Now the London map, for instance, covers noise from traffic, aircraft, railways and industry.
Planners are encouraged to use the maps to reduce noise pollution by re-routing traffic, say, or rethinking city design.
In Hong Kong, the department developed a citywide 2-D traffic noise map in 2006, and in recent years 3-D maps for specific projects.
The noise mapping technology has been used to help assess how much selected households are exposed to traffic noise in a large-scale study by Chinese University on the health effects of the noise in the city.
Our sense of hearing is able to cope with a huge range of sounds.
A car running at 60 km/h seven metres away measures 70dB; a train racing past a station measures 94 dB; at 120 dB, we start to experience pain.
The findings of the university's study, which was commissioned by the department, were released this year. They were presented in the conference last week by Professor Lam Kin-che, of the university's geography and resource management department.
A poll of more than 10,000 households found 36 per cent of Hong Kong adults had been disturbed by noise while trying to take a break at some time in the preceding 12 months.
Interviewers asked members of 10,000 households whether loud noise emanating from outside their homes, such as that from traffic, construction and renovation, had prevented them conducting household tasks. Some 24 per cent said there were times when they could not concentrate on their work because of noise, while 15 per cent said their homes were sometimes too noisy for them to talk in a relaxed fashion.
However, a majority - 64 per cent - reported that noise had no impact on their daily life.
Approximately how many times more powerful the sound of a jet engine is than the smallest audible sound