MNS & ENS Explination
Designing and Specifying the Right Loudspeaker System for Emergency Mass Notification
in
Public Venues
A properly designed and specified loudspeaker sound system is essential for an effective emergency mass notification in any public venue. In an emergency, everyone in the affected area must be able to get the same information – and instructions – “loud and clear” for the best possible outcome.
Whether it’s a public-safety matter, such as an accident, fire or terrorist incident, or a weather-related incident, such as a tornado or wildfire, sound volume and clarity are equally important. When commotion and confusion are compounded by language problems, only the highest levels of sound volume and voice clarity will shorten the time to process information and follow potentially life-saving instructions.
If you are considering fixed-position high-power speaker arrays (HSPAs) for your venue’s emergency mass notification system, you should follow these steps throughout the design-and-bid process. Paying attention to the details of your needs and sound technology will help you get the right system for your needs.
Start with an on-site acoustic sound survey.
- The sound survey will help you determine the best locations for fixed HSPAs and the sound levels you’ll need at each targeted point. Some points to keep in mind:
Measure sound levels – typically expressed as decibels (dB) where you want the sound to be heard. You’ll need to overcome the loudest-possible ambient noise, such as traffic, crowds, wind noise, or mechanical equipment, depending on the type of venue you have. Log the Leq (Equivalent Average Level) background sound levels and chart the noises heard during the survey. Each test at each location should last at least five minutes using a tripod-mounted sound analyzer system set at 5’4” in height.
- You’ll want your loudspeaker to deliver sound at the targeted area 6 to 10 dB above the ambient level. Six dB is the minimum; 10 is double the ambient noise level. Make sure every vendor who bids on the project has your sound measurements so they know your requirements at each point.
- Correlate your target sound levels with venue’s physical characteristics to determine the placement of speaker arrays on towers, poles, or rooftops. Buildings and trees for example can block sound waves. Sound waves can pass over spaces between buildings or over ravines and other steeply lower places. Manufacturers design speakers to meet specific needs. You can get the flexibility to focus sound in a narrowly defined area or a wider area. Just know the sound level you need to reach at the target area and make sure you have taken all of your site’s physical characteristics into account.
Specify The Right Equipment
Not all loudspeakers are created equal, and not all sound pressures are equally loud. This difference is because the human ear does not respond equally to all frequencies. Power and intelligibility are both necessary. Humans are much more sensitive to sounds in the frequency range of 400 to 6500 Hz, and harmonic distortion should be less than 1 percent at full RMS power.
Don’t “cheap out” with a system designed for sirens or a lesser-quality system. For voice, several manufacturers, such as IMLCORP®, offer systems such as our SoundCommander® line, designed specifically for outdoor mass notification. Your system should meet and exceed all standards for range and clarity, as well as integrate with other mass-notification platforms, such as indoor speakers, alarm systems, email and telephone dialers.
Here are some technical factors to consider when specifying your system:
- Speaker arrays for fixed systems should have a full 3600 range to ensure coverage in all directions. However, placement conditions may necessitate using arrays that can concentrate more sound in a 1800 range.
- Each fixed-system speaker array should have a minimum effective range of up to 800m in diameter.
- Wireless UHF transmission systems have severe limitations. High-quality voice transmission cannot be technically possible because of legal restrictions regarding the very narrow frequency band spread allocated for UHF transmitters for emergency notification systems. The baud (transmission) rate is too low, and the audio band width of the channel is limited to a total of about 2000 Hertz (Hz) audio frequency response at best. Systems that depend on licensed UHF transmission can work well as digital controllers for setting off sirens, reporting unit status, etc. because of the narrow frequency limitations of the transmitters, the loudspeaker drivers included with those systems are also not designed for the wider fidelity required for quality intelligible voice audio.
- Systems should have battery backup capable of providing at least four hours of continuous operation.
The system should be able to handle multiple broadcast nodes – groupings of HPSAs – throughout the coverage area. Each node can have up to four HPSAs. - Systems that meet military specifications will be able to withstand severe outdoor weather conditions, including hot-cold swings, precipitation and airborne particulates, such as salt from seaside locations and effluents from industrial facilities
Integrate with other mass notification technologies.
"There is no single solution when it comes to notifying people in an emergency. Getting the same information at the same time to everyone affected is necessary to help you protect and save lives".
When it comes to notifying people, you have many “targets” to consider – and all are important. A lot of moving parts must work together.
- People in buildings need to get the same message at the same time through an integrated system of indoor alarms and speakers, graphic displays, electronic signage, and messages delivered via the Internet and mobile devices. The system should tell them to stay where they are or where to find a safer location.
- People not yet in the affected area need to hear the loudspeaker announcement. Therefore, you should consider speakers that you can aim outside your venue.
- Emergency responders from various agencies converging at the scene can serve people more effectively by hearing the instructions given to the public and other responders. This is why voice must be clear and intelligible. The minimum standard is that 50 percent of words be understood – a 0.5 level – but that’s inadequate. Being 70 percent understandable – a 0.7 level – is much better, and a higher level is better still. It is possible to achieve levels above 0.8.
Use pre-recorded messages in multiple languages to save time and avoid confusion.
You can fill a huge void if your emergency mass notification system has a component that can play pre-recorded warnings and messages in several languages.
IMLCORP®’s AlertCommander warning and voice message delivery system, which works with our SoundCommander® loudspeakers, is an example of how this technology should work. You should be able to choose from a large library of warning tones and pre-recorded voice messages that cover a variety of emergencies. It should be a matter of policy and procedure to determine the warning tones, messages and languages you will need. Then a system of computerized folders, all accessible within your message-delivery-device software, to group messages and to sequence when to play them back.
Three important requirements for your message-delivery system are:
-The ability to work with a computer at the command center and from a wireless handheld device so that you can change messages to provide more up-to-date information.
-The ability to switch from pre-recorded messages to live messages as your emergency-response needs. You should be able to make live announcements easily from your command center or the field.
The capability to easily combine messages from its library and record and play new messages as conditions require.
-The ability to access your system from an off-site location.
The time and conditions of any type of emergency at a public venue may require the activation of the emergency mass notification system from a cell phone, landline or off-site computer. Further, you may need to change messages for your loudspeakers and other notification platforms as events unfold. Make sure that the system you specify has this capability and that your policies and procedures definitively state who has access to the system and how it is to be used.
One final point to keep in mind is that integrating all the components of a comprehensive emergency mass notification system is highly feasible. Specifying and installing a system does require attention to many details, and it does require clear policies and procedures to activate the system and use it effectively to protect lives and property. But it can — and should — be done.
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