杏吧原创

AMBULANCES racing through city streets put pedestrians and other drivers in danger because their sirens make the wrong sort of noise. Deborah Withington of Sound Alert, a company in Leeds, told the BA that a siren鈥檚 sound does not provide enough information for the human brain to decide where it is coming from. The company is about to road test a new siren that it says does the job better.

The problems with existing sirens make road junctions dangerous places when an ambulance is approaching at speed. In one year alone in the US, accidents involving ambulances killed 67 people and injured 537.

Researchers at Sound Alert asked 52 people with normal hearing to locate the source of a siren sound coming from one of eight hidden speakers during a simulated driving trial. These tests showed that today鈥檚 sirens, including the 鈥渉i-lo鈥�, pulsar, wail and 鈥測elp鈥� varieties, are good at alerting people but are difficult to locate. 鈥淓ven the best of the sirens in use at the moment has an error of localisation of over 45 degrees,鈥� said Withington. 鈥淩oad users always tell me that they know there is an ambulance coming but they don鈥檛 know where it is.鈥�

For people to locate a sound without seeing its source, it must include a broad range of frequencies. The human ear can hear sounds at frequencies between 20 hertz and 20 kilohertz, but existing sirens only use the range from 500 hertz to 1.5 kilohertz. A mixture of all audible frequencies 鈥� known as white noise 鈥� would be the easiest to locate, but this would not make an effective warning signal.

Sound Alert has developed and patented new patterns of sound which can be pinpointed more easily, said Withington. The pattern its researchers have chosen for road trials is a series of 鈥渨haa鈥� sounds of rising frequency followed by a blast of white noise. The end result sounds something like 鈥渨haa, whaa, whaa, shhhhhhh鈥�.

Derek Smith of the West Yorkshire Metropolitan Ambulance Service said: 鈥淲e endorse the project and we will be doing road tests on the new sirens.鈥�