Most of us have had the experience of driving in traffic, and hearing the approaching ambulance siren, but being unable to discern where it's coming from. US research found that only 26 per cent of occupants of a closed car could determine the direction of a siren.
The first problem is that modern cars have excellent soundproofing. This is good for a comfortable ride, but not good for external sounds such as sirens. In addition, internal noises such as air-conditioners, stereos, and talking passengers mask those sounds.
Research shows that road noise at 60km/h effectively cancels siren sound inside a vehicle until it is within 100 metres. With air-conditioning on and radio on low, the audible distance is reduced to 30 metres. Turn the radio up, and the distance can drop to 15 metres or less.
It's important for a siren to be omni directional, beaming out in all directions from the emergency vehicle. Then the sound bounces around the environment, off hard surfaces such as buildings and other cars. All this makes for a confusing signal to interpret.
A reported study by the American College of Orthopaedic Surgeons observes "the siren sounds of an ambulance proceeding at 100km/h barely precede the ambulance, so that vehicles ahead of it cannot respond to its warning''. The study showed that the distance for getting the attention of a motorist travelling at 100km/h to be ''within 2 metres of the ambulance's front bumper".
Then, there are human factors. Sound perception is impeded by hearing loss, alcohol, and other drugs. The sound of a siren alters a person's sense of speed and distance. Disoriented pedestrians have been known to step off a kerb into the path of an oncoming emergency vehicle.
Sirens work best if they contain a range of frequencies. There's the low-pitch WAH WAH, and the high-pitch WE WE. This helps people with impaired hearing, who might not hear one range as well.
Lower-pitch sounds travel further, but are more difficult to pick direction. This is why car horns are high pitched.
- Information provided by Janette Thorburn, principal audiologist, Hearing Australia.
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