Achieving Maximum Intelligibility in Mass Notification Systems

ATI Systems
A mass notification system (MNS) is designed to deliver accurate and timely voice warnings to all affected people during a catastrophic disaster or threat, protecting people from danger and guiding them towards safety.

During major emergencies and disasters, accurate and timely voice warnings to all affected people are among the most important factors to successfully protect lives, contain property damage and minimize community impact.

A mass notification system (MNS) is designed to deliver such warnings during a catastrophic disaster or threat, protecting people from danger and guiding them towards safety. 

The MNS can cover indoor and outdoor areas, and use a variety of methods including visible signals and social networks, but the single best way of reaching the maximum number of people in the shortest amount of time is by way of intelligible voice messages.


Intelligibility is a measure of how easy it is to understand a voice message such as live public address (PA) or a pre-recorded message delivered from an MNS.  


It is affected by the quality of the original message transmitted, the level of background noise, reverberation and, for messages transmitted over an MNS, the properties of the communication system.

The original message must be spoken clearly and more slowly than the typical person to person speech.

Background noise can mask an emergency message in what is known as the signal to noise ratio, and reverberation can blur the speech over time by lengthening vowels and shortening pauses.

All of these factors must be considered in MNS design to create the most effective Giant Voice or PA system.

Selecting Your Mass Notification System

The size and shape of the outdoor space or indoor area are also very important considerations in MNS design. 

When designing a system for maximum intelligibility in an outdoor area, conditions to consider include topography, atmospheric conditions, and ground cover.

For indoor systems, reverberation is a more important factor.  In both cases, background noise levels and the other factors listed above must also be considered.

Acoustic Technology Inc. (ATI Systems), a competitor in the 2017 ‘ASTORS’ Homeland Security Awards program, has been considering the questions of MNS effectiveness and background noise for over 30 years.

ATI Systems started out as an acoustic consulting firm, advising industry and government groups about emergency alerting, and helped in developing some of the early standards in this field.

As part of this work, ATI Systems measured numerous siren signals and background noise levels at various sites.  With this large database of empirical data and in-house expertise in the field of acoustics, ATI Systems developed an in-depth acoustic model for optimal MNS design.

ATI banner

ATI Systems’ acoustic computer model utilizes state-of-the-art methodology to predict MNS sound coverage.

The computer model uses a 15-decibel differential above ambient noise for the entire acoustic coverage area, and takes into consideration factors such as:

  • Hemispherical wave divergence
  • Atmospheric absorption of sound, and the
  • Local situation of the MNS, be it outdoor or indoor

ATI Systems conducts a comprehensive acoustic analysis to evaluate the voice coverage of the facility or outdoor area, develops the recommended layout of speakers or other alerting devices, and provides the resulting acoustic coverage mapping.

The acoustic model utilizes the specific characteristics of the loudspeakers, including power and directivity, together with typical voice frequency characteristics.

An example of an ATI outdoor acoustic coverage map
An example of an ATI outdoor acoustic coverage map

Outdoor Acoustic Coverage

Designing outdoor systems requires taking into consideration all factors that affect outdoor acoustics including: wind effects, ground cover absorption, and barrier attenuation effects (such as those posed by buildings and above-ground structures).

GIS mapping of various layers is used to determine elevation heights of sound obstructions such as hills, mountains, high buildings, and other obstacles.

HPSS16 with bidirectional speakers
HPSS16 with bidirectional speakers

For communities, sound obstructions can be mainly obtained from GIS data.

For special facilities such as a military base or a campus, ATI utilizes various techniques to obtain structural heights.

A three-dimensional model of the actual facility and speaker locations is used to predict sound propagation, accounting for topographical features, buildings structures, ground and terrain effects, atmospheric effects, and geometrical spreading, and includes diffracted, direct and reflected sound paths.

This model is used to identify the optimum location of each outdoor speaker station to ensure clear, intelligible voice messages that could be heard throughout the area to be covered by the MNS.

ATI’s High-Powered Speaker Stations (HPSS) are used outdoors to provide the adequate acoustic coverage.  Class D amplifiers for maximum intelligibility are being used.

Directional speakers are used to reduce echo and multiple arrivals of acoustic waves.

Indoor Acoustic Coverage

Indoor areas have different acoustic considerations.

Factors to be considered include the square footage of the building and layout, the shape of the space and the existing background noise level.

ATI Systems has worked with a large number of different types of indoor acoustic spaces including tunnels, industrial facilities and colleges, which include smaller rooms and hallways.

The Control Station (CS) provides a centralized location for activating, controlling and monitoring all system equipment. The CS also has the ability to broadcast live public address, pre-recorded messages and siren tones.

Speaker locations are determined to ensure 100 percent emergency alert coverage with maximum intelligibility.

For the hearing impaired, visual lights and message boards are used.

The acoustics of an indoor space is also affected by the acoustical absorption of its boundaries, and if the walls, floor and ceiling are covered by absorptive or non-absorptive finishes.

In many cases, one will find non-absorptive finishes which will reflect sound off their surface, creating additional considerations for echo and reverberation.

By examining the individual contributions and reflections from each speaker location, multiple arrival effects and shadow regions can be identified to assist with speaker orientation and placement in order to achieve optimal intelligibility and coverage.

For indoor applications, ATI developed the Indoor Speaker Unit (ISU) that is well designed for excellent acoustic characteristics and can reach the design goals. 

Indoor MNS Using the ISU to Maximize Intelligibility

An important consideration for the intelligibility of messages delivered by any system that uses multiple speakers is the perfect coordination of those devices.

If the speakers delivering the message are not in complete synchronization, then different parts of the message may arrive at listener’s locations at different times, making it much more difficult to understand.

This is particularly important for indoor systems, which often include greater numbers of speakers, installed closer together.

Warning and public address systems designed by ATI Systems use a central Communications Control Unit (CCU), which has been designed to ensure that all speakers are in perfect synchronization.

In the case of indoor systems, the CCU is connected to an ISU which controls and powers the actual speakers.

ISU with cabinet door open to show electronics inside
ISU with cabinet door open to show electronics inside

The ISU provides up to 400 Watts of continuous audio output power for reliable alert tone notification, voice instructions and public address, and can also be configured to provide activation of visual alerting devices such as strobes and message signs.

The ISU is monitored, controlled and activated by the ATI Central Control Station, and can also be optionally configured as a standalone voice evacuation system to operate independently using a local control panel.

It utilizes a high efficiency Class-D amplifier, the unit puts out 400 watts of power with 4 zoned, Class-B supervised, 70V Speaker outputs to deliver the highest level of voice intelligibility.

The ISU uses a 24VDC NAC power supply to drive the 4 Zoned, Class-B supervised outputs up to 10 Amps  It features the computing power of an advanced 32 bit integrated micro controller and the unit is able to receive activations and relay vital monitoring information back to the operator almost instantaneously.

The ISU can support a multitude of devices not limited to, strobes, push buttons, pull station, local operating consoles, and capability of interfacing with existing equipment such as public address systems and fire panels.

It has the ability to receive and decode signals from various monitoring and sensor devices.  When featuring the ATI ICG, the unit is IPV6 capable and has an advanced IP Stack supporting the latest IP security standards and protocol.

The ISU comes standard with RS485 interface, allowing for a robust and flexible communication peripheral pathway and supports multiples form of communication including but not limited to IP, hardwired, both digital and analog radio, and DTMF.

The ATI team with experts in acoustics and GIS can design the best systems available in the market today.

(See ATI Systems MNS in Action. Courtesy of ATI Systems and YouTube)

This combined with ATI’s high quality acoustic products and optimal communication choices make ATI an industry leader for MNS.

ATI Systems ‘MassAlert’ in 2017 ‘ASTORS’ Homeland Security Awards Program

The 2017 ‘ASTORS’ Homeland Security Awards Program, is organized to recognize the most distinguished vendors of Physical, IT, Port Security, Law Enforcement, First Responders, (Fire, EMT, Military, Support Services Vets, SBA, Medical Tech) as well as the Federal, State, County and Municipal Government Agencies – to acknowledge their outstanding efforts to ‘Keep our Nation Secure, One City at a Time.’

As an ‘ASTORS’ competitor, MassAlert will be competing against the industries leading providers of emergency mass notification systems.

ASTORS HSA 2017Good luck to ATI Systems and MassAlert on becoming a Winner of the 2017 American Security Today’s Homeland Security Awards Program!

For more information about MassAlert and the entire line of Acoustic Technology Inc. (ATI Systems) offerings, please visit