Information versus imperatives

I asked Professor Doctor John Drury, University of Sussex, about the usefulness of live voice-over alarms in an evacuation.

Dr Drury pointed out that alarms are often ambiguous. People often don’t react to them immediately because they think it is just a test of the alarm or fire drill. Yet the time taken to respond to a signal of danger is one of the best predictors of survival in an emergency. If people are given information about the nature and location of a danger then they are more likely to react promptly and escape.

An evacuation study in Newcastle, explained Dr Drury, tried four grades of information given to travellers on the underground metro, ranging from a simple alarm or siren to a public address (PA) system announcement informing about the location of the danger and instructing people of where to exit.

The fastest most effective evacuation occurred with the informative PA system. Moreover, such explicit information did not result in panic behaviour.

Quick response is crucial to survival

Contrary to common belief, mass panic is rare in emergencies and disasters. Proffessor Ed Galea, founding director of the Fire Safety Engineering Group at the University of Greenwich, has conducted a three-and-a-half year study (High Rise Evaluation Database) into the evacuation of the Twin Towers, Sept 11, 2001. The study found that “classic panic action” – or people behaving irrationally – was limited to 1/124.
The normal panic model, however, suggests that a group of people is not as intelligent as the individual, and that a crowd may respond irrationally.

Crowds are often demonised by the media, and the traditional disaster film nearly always has an uncontrollable mass of people exhibiting the “everyman for himself” syndrome.

Interviewees from disasters such as the Jupiter sinking in 1988 and Hillsborough Stadium disaster in 1989, commented on the sensible reaction of the crowd, which exhibited helping behaviours and a sense of orderliness and togetherness. This is reflected in the way that people react when they first hear the emergency alarms and this, crucially, goes on to affect their chance of survival.

People trying to gather information for themselves and others instead of evacuating are more likely to be killed. World Trade Centre occupants seeking information about what was happening, took between 1.5 and 2.6 times longer to respond, according to Professor Galea’s report. More than half the occupants stayed to carry out a task before evacuating.  “Providing people with good information can significantly reduce occupant response times.” (Galea)

Professor Galea’s team summarised in five rules what could make the difference between life and death in an emergency evacuation. The first one was response.
• Don’t do anything to delay your departure
• Know your way out
• Don’t stop on the way to reassure family and friends
• Don’t discard your shoes on the stairs
• Know how long it will take to get out.

Keeping people informed

There are physical and logistical problems of getting people to respond and keeping them informed.  How do we keep informing people on-the-move, where equipment has stopped functioning or background noise drowns out the installed public address system?

People use their cognitive maps to exit – they will go to an exit that they know – so it is important that they know which exits to avoid and why.
This information could be changing constantly especially if the danger is a progressive or mobile one. For emergencies taking place over a large area, is it necessary to have the capability of broadcasting over your radio? The most effective way of providing the necessary information is via a loud, and most importantly, clear public address system that has been thoughtfully and knowledgably designed.

What’s in a Watt?  Here comes the science

There are many factors to consider when choosing a megaphone, mobile public address system or tactical evacuation system. A decibel is a measurement on a logarithmic scale. Well used examples of decibels say a normal conversation is around 60dB, a chainsaw at 1m away 110dB, a jet aircraft at 50m 140dB.

The sensitivity of electro-acoustic equipment is normally measured as 1 watt at 1 meter. This measurement gives you an idea of the quality of the loudspeaker in the megaphone. A standard loudspeaker in a megaphone has a sensitivity of circa 104dB at 1 Watt. High quality precision compression speakers have a sensitivity of between circa 112dB and 115dB at 1 Watt. It should be clear that there is no direct correlation between the loudness of a megaphone and the power output of the loudspeaker amp (Watts).

The HiFi world of the Watt has nothing to do with the power of a megaphone, and holding a 30 Watt megaphone in your hand will tell you very little about how loud it is. You must check the quality of the product. Also, if a high quality megaphone is able to reproduce lower frequencies in the speech band of 300Hz – 4kHz, then all the better as this reproduces a warm speech sound with high intelligibility that sounds a lot better than the electro speech tones heard through a cheap megaphone.

How loud does my system need to be?

Once you have found out what the range of equipment is capable of – and at what distance there is still speech intelligibility – you can now factor in background noise.
These numbers will also affect the type of system you need.  Crowds are noisy and fire insulates sound, as does humidity. Are there likely to be other sirens wailing? People in vehicles? Windows opened or closed?

These factors will all affect the effectiveness of your megaphone and you need to be at least 10 to 15dBs louder than the background noise for people to understand you.  Of course an important number factor is distance. Double the distance is usually calculated by adding 6dB. Other practical things you might want to take into account:

• How far away from the incident must the speaker stand?
• Is there a danger of gas? Gunfire? Explosion?
• Does the system have remote broadcast with UHF wireless or with an existing integrated radio system?
• Do you have recorded messages (MP3) for multilingual broadcasting? 
• How long should the batteries last – do you need more power from a vehicle or changeable batteries?
• Can the operator/s hold the system that long - do they need a stand? Shoulder strap? Will they be moving around a lot?
• Is it weather proof?
• Where and how will you store the system?

Importantly, a system also need to be “full” when you need it, so you may need a system that manages it’s own charging. It’s no use buying a nice compact system that says it has 30 Watts, if it has no speech intelligibility and isn’t loud enough for your emergency. Then it’s useless. Get the right kit, give enough information, and you might just save a life.

For more information about Dr John Drury, visit www.sussex.ac.uk/psychology/profile92858.html. Suzanne Coop can be contacted through the Sound-to-Go website, www.sound-to-go.co.uk.

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