Noise pollution in cities is causing out health to suffer, according to the World Health Organization. Recently, researchers at the Free University of Brussels in Beigium have developed an applocation: NoiseTube. It allows everyone who downloads it the ability to measure the amount of noise they are exposed to by transforming their smart phone into a sound level meter. NoiseTube works by recording sound levels and GPS locations. Once launched the app shows decibel  levels of green or red depending on the level of noise. That information is then sent to the NoiseTube server via the Internet where a "noise map" is produced and then made available to the user. Since its launch, the app has been popular with community groups who want to monitor their own noise levels. According to D'hondt of the Noise Tube project, these groups don't think that city produced noise maps give an adequate interpretation of the level of noise they're experiencing. City produced noise maps are based on statistical models involving very few actual measurements.  NoiseTube on the other hand gives a detailed account of the level of noise experienced at a specific time and location. "Ten people walking the same area from 9 to 10 am every day for one week can make a valid  and detailed map for an area of a couple of blocks," D'Hondt said. "It's reasonable effort by the community groups to produce these maps, and with it they can go to the authorities and have real evidence about what they're experiencing." So far these sorts of noise maps have been produced for a number of European cities. The app has been downloaded by 10,000 people with 2,700 people registered on the NoiseTube website. The reliability of city produced noise maps is questionable because   _  .
the research is based on only a few actual measurements they can only be used at a specific time and location their operating principles are not clear the collected information isn't enough to prove it