Industrial Fire Journal - Fire & Rescue - Hemming Group Ltd

A siren’s call

innovation corner

Published:  01 April, 2009

He’s invented a locatable siren that allows members of the public to pinpoint the location of an approaching emergency vehicle and – what’s more – it results in lower cab noise levels. F&R speaks to Jeff Berman.

Accidents involving emergency response vehicles en route to an incident, or indeed taking part in a chase, are not uncommon. In Europe, there are around 2,834 serious injuries per year in accidents involving emergency vehicles, and 308 fatalities. That equates to a daily average of 14 per day serious injuries, or 1.5 fatalities, and it is estimated the daily cost is over £4m.The question is, could some of these accidents have been avoided if bystanders and victims had been able to pinpoint the exact location of an emergency vehicle?

Jeff Berman came up with the idea for a locatable siren after gaining a Master of Science degree in electronic product design.

In 2002, Berman’s company PDI Products developed a special sound wave that was used in remote controlled, locatable audible guidance beacons for blind people, following a two-year feasibility study. Today, Berman is finalising a special technology for use on emergency vehicles.

Berman’s new technology, he explains, is formed of a number of specific frequencies (or sounds). “When the human brain hears a single frequency, it automatically tries to block it out and you can end up not hearing it. The human senses work on the principle of change, and we notice change. When there is only one tone the brain loses interest. If there are two tones, the brain is stimulated because it notices the differences between the two.”

That is why, he adds, two-tone sounds (like those used on door bells) are considered to be better alerting signals than single tones.
A new dimension to the two-tone siren involves a second tone that is sent from a different physical location. The effect, he says, adds to the spatial awareness ingredient; “So the brain senses that it is receiving even more information. It is like the difference between listening to mono or stereo.”

At the heart of the technology is a new sound wave profile. Sound waves, he explains, come in different shapes – some resemble triangles, others squares. Each wave has different characteristics both in how it is perceived by the brain (psychoacoustic reaction) and how it interacts with outside surfaces. “Many current sounders and sirens bounce off surfaces without perceptively changing pitch or volume. This means that people cannot tell if the source of the sound is coming from the sounder, or from the wall it is bouncing off.” The sounds that Berman has designed change in volume and pitch as they reflect off surfaces, making it easier to perceive the location of the source.

PDI Products is now at the stage of building computer simulations of the system using loudspeakers to demonstrate effectively the differences between a non-locatable and easily locatable sound signals. “One of the demonstrations we will set up will show sound waves coming straight at you, and then disappearing behind you.”

Fitting locatable sirens, he adds, would lead to cost benefits. He points at the cost of vehicles being off the road, repair costs, and additional vehicles covering a shortfall. And then there is the cost of staff sickness due to accidents, stress, and early retirement.

Current sirens produce high levels of noise inside the vehicle cabs due to the piercing nature, explains Berman. “The high frequencies used and the sharp waveforms have little attenuation as they go right through the cab structure. The rigid fixed position of roof-mounted sirens causes the roof structure to amplify the noise – acting like a drum.” The noise distracts the driver, and secondly, it can heighten his already activated physiological condition, which could lead to errors of judgement. And of course, the sirens could also cause long-term hearing problems.
Radio communications are also made difficult by the siren noise inside the cab, and it can be hard for the crew to hear communications from the control centre, and vice versa.

Modern radio comms equipment use AGC – automatic gain control – which adjusts the sound signal to a prescribed level. This means that if someone is talking softly, their voice is amplified and the recipient can hear them. Conversely, if the noise level is high, the sound is reduced to an acceptable level. However, points out Berman, most AGC does not distinguish very well between the background (siren) sounds and the foreground sounds of the person speaking. “This results in the overall volume being heard by the comms controller being reduced to allow for the loudness of the sirens. The crew’s speech level is thus reduced making it difficult to hear them.”

This effect is exacerbated by modern digital sound processing technology. Digital processing handles simple digital signals such as electronically produced sirens, better than complex analogue signals such as speech, and can prioritise the digital signal.

Microphones and headphones that reduce or cancel background noise are available, admits Berman, but he says that they can be expensive and work best when the background noise is fairly constant, such as in an aeroplane. The changing siren sounds coupled greatly reduce their efficiency. “The siren system I have devised addresses all of the above issues using acoustically isolating mountings set in specific locations on the emergency vehicles, and by the specific frequencies and waveforms used.

“The specific frequencies and volumes emitted by my system can also be linked to noise-cancelling microphones if further improvement in cab noise is specified – so the crew can be heard by the controller – and also to noise cancelling headphones, which will greatly reduce and potentially virtually eliminate the noise of the sirens for the crew member receiving the comms from the control centre, and also for the driver.”

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