Research indicates that electronic yelp sirens can cause hearing problems in personnel. If we look at the graphs of the sirens we can see that in the initial period of each cycle of sound there is a very steep rise time followed by rapid fall times. The wail siren takes around 1.92 seconds to emit 75% of its peak volume (amplitude) in 30% of its total cycle time. Although its initial rise time is steep, it is spread over a long time period and is unlikely to cause hearing damage.
However, a yelp siren reaches 75% of its peak frequency in around 0.08 seconds and repeats it up to 250 times a minute.
When our ears hear loud noises the ocular muscles tighten to avoid damage to the eardrum. When sounds are increasing and decreasing in intensity very quickly these muscles become overstretched and in time can lose their elasticity, leading to permanent hearing problems. The fine hairs in the cochlea of the inner ear, known as stereocilia, bend in response to sounds and generate electrical signals which the brain interprets as sound. Overstimulation by high frequencies can damage the stereocilia causing permanent hearing loss.
Unfortunately, damage to hearing is cumulative and adds up over time. Hearing loss is usually permanent.
In contrast to electronic sirens the Quadrasonic™ sirens emit sinusoidal soundwaves which have gentler rise and fall times allowing the eardrum muscles have more time to adapt to the increase in volume levels. The sinusoidal sound waves are less likely to cause hearing damage. The stereocilia are activated at a slower rate and are less stressed by the type of sound wave used in Quadrasonic Siren Systems.