The following is a modified extract from the report, The theoretical and practical aspects of visual warning methods in use on emergency vehicles, John Killeen, 1999.
This summary will indicate some of the shortcomings in the effectiveness of siren warning systems, demonstrating an even greater emphasis and reliance on visual warning methods. There has been a great deal of research and development work done on sirens during the last twenty years. This has led to the almost universal use of the multi-tone electronic siren module driving 100-150 watt loudspeakers.
Sound is perceived and heard by sensing direct waves of energy and multiple waves of reflected energy. The sound waves generated by a siren bounce easily between buildings, off surrounding cars and from other hard surfaces.
Solomon1 (P57) and De Lorenzo/ Eilers2 (P1333) discuss the difficulty experienced by subjects in locating the source of a siren tone, due to the interference of reflected sound producing a non-directional noise stimuli.
They go on to describe the extraordinary lengths that modern vehicle manufacturers undertake to soundproof the interiors of their cars. In addition, the presence of noise, generated internally by air conditioners and stereos within the passenger compartment can overwhelm the very low levels of sound that may succeed in penetrating into the car from the exterior environment.
The effects of soundproofing in modern vehicles
Solomon1 (pp 58-9) describes the outcomes of audiometric testing between an emergency vehicle mounted siren (115 decibels) and passenger vehicle compartments at different distances and angles.
The road noise at 60 kph (with
no air conditioner or radio noise), effectively cancels out the siren sound inside the vehicle until the ambulance is within 100 metres. If the radio volume in the car was low and the air conditioner switched on, the siren's audible distance decreased to 30 metres. When the radio volume was raised further (not to maximum), the internal sound level was 90 decibels against the
lower penetrating siren intensity of 82 decibels.
No siren sound was audible inside the car at a distance of 15 meters. The siren could be heard inside only at a distance of one or two meters separating the vehicles.
See diagrams of this research in the PowerPoint presentation under Downloads
There was a marked reduction in the audible intensity of the siren between the two vehicles when the approach angle was altered by an intersection (90 degrees), this angled approach resulting in no siren sounds being audible within the passenger vehicle.
The effective range of sirens
A US National Safety Council newsletter3 (April 1997, p1) reports on the results of a study by the AmericanCollegeof Orthopaedic Surgeons. The study demonstrated that the siren sounds of an ambulance proceeding at 100 kph barely precedes 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 100kph to be within 2 meters of the ambulance's front bumper.
De Lorenzo & Eilers2 report that US Department of Transport (USDoT) studies indicate thatover a sirens effective frequency range, the average signal attenuation (through closed windows...) resulted in a maximal siren effective distance of siren penetration of only 8 to 12 meters at urban intersections. In corroboration with other studies the US DoT report concluded that at intersections the recommended
maximum safe entry speed of 15 kph.
These studies above take no account of the presence of people on the road that may have
hearing difficulties, alcohol and/or drug impairment.
Also, the sound of the siren is known to alter an individual's sensory perception of speed and distance. Similarly, disoriented pedestrians who are overwhelmed by loud siren noise nearby may become confused, step off the kerb and directly into the path of an oncoming emergency vehicle.
The Krovkov Effect
The influence of high intensity sound is known to cause the Krovkov effect to occur as reported by Green4 (p7). Krovkov discovered that high intensity sound decreases the eye’s sensitivity to red, attenuating the perception of orange. The blue response in the eye is increased and the green-yellow sensitivity remains unchanged.
The directionality of sirens
The warning sound generated by a siren causes individuals to turn and search for the source. Life experience tells the person that an emergency vehicle is close by and they may need to take evasive action. The siren tones are usually non-directional: - the US National Safety Council newsletter3 (April 1997, p1) reporting that the USDoT study found that only 26% of the occupants of a closed car could determine from which direction the siren noise came from. The time taken to search for and locate the emergency vehicle after hearing the siren is a crucial factor in decreasing the reaction time.
The visual conspicuity of the emergency vehicle is the key to a fast and reactive response by nearby drivers, pedestrians and onlookers. These people may hear the advance warning of a siren and look around for more visual information or for those that hear nothing at all, relying on visual cues to finally see the emergency vehicle as it moves closer.