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  • Writer's pictureAsad Lalani

Enhancing Speech Intelligibility: The Power of Acoustic Design for Life Safety in Complex Spaces

Updated: Jun 5, 2023

Acoustic Design for enhanced Speech Intelligibility by LC ACOUSTICS. Picture showing a busy concourse.

How The Proper Acoustic & Electroacoustic Design Enhances Speech Intelligibility for Fire and Life Safety in Complex Spaces

In the realm of fire and life safety, ensuring clear and intelligible communication is paramount. In complex spaces such as large buildings, auditoriums, or public venues, acoustic design plays a crucial role in optimizing speech intelligibility. By carefully considering the acoustic characteristics of these spaces, properly executed acoustic and electroacoustic design can enhance emergency communication systems and improve overall safety. In this technical blog, we will explore the significance of acoustic and electroacoustic design and its impact on speech intelligibility in complex spaces.

What’s Acoustics Got to Do with It?

So, what does acoustic design have to do with the development of a life safety design?...

When it comes to fire and life safety in complex spaces, one crucial yet often overlooked aspect is speech intelligibility. In emergencies, clear and effective communication is paramount for timely evacuation and efficient response.

Fire and life safety design is a crucial aspect of any building project, especially in complex spaces such as airports, bus and train stations, atriums, malls, museums, laboratories, offices, workspace, and residential establishments, as it aims to protect the occupants and the property from the hazards of fire and smoke. However, fire and life safety design are not only about installing fire detection and suppression systems but also about ensuring that the environment of the building is conducive to effective communication and evacuation in case of an emergency. This is where the unsung hero, “ACOUSTIC DESIGN”, comes to save the day!

Image of 3D acoustic model of a concourse showing STI distribution map and average STI score. Image by LC ACOUSTICS Design Services Group

Acoustic design is the process of creating spaces that have optimal sound quality for speech intelligibility and an overall pleasant and healthy acoustic ambiance. Acoustic design involves controlling the sound sources, sound paths, and sound receivers in a space, as well as the reverberation, noise, and background sound levels. Acoustic design can enhance fire and life safety by improving the audibility and clarity of emergency voice communication systems (EVCS), fire alarm systems (FAS), and public address systems (PA).

What Is Speech Intelligibility and Why Is It Important?

One of the key challenges in fire and life safety design is ensuring adequate speech intelligibility for emergency communication systems, such as public address, fire alarm, and voice evacuation systems.

Chart showing STI scores and corresponding English language descriptors.

Speech intelligibility refers to the ability of listeners to understand spoken messages clearly and accurately. When speech intelligibility is compromised, the efficiency of emergency communication systems is hindered, potentially jeopardizing lives.

The importance of speech intelligibility in fire and life safety design cannot be overstated. It directly impacts how well occupants can receive and comprehend instructions from mass notification and voice communication systems. Poor speech intelligibility can lead to confusion, panic, delayed response, or even failure to evacuate, all of which have severe consequences for life safety.

Speech Intelligibility depends on several factors, such as the quality and output gain level of the sound source, the distance and direction of the sound propagation, the ambient noise level, the reverberation time, and the acoustic properties of the space.

How To Achieve Speech Intelligibility in Fire and Life Safety Design?

Acoustic study and proper design are essential for achieving high speech intelligibility in spaces where fire and life safety systems are implemented. The acoustic study involves measuring and analyzing the acoustic characteristics of a space, such as reverberation time, and background noise level. Creating a balanced acoustic ambiance involves the study of absorption, reflection, diffusion, and transmission of sound.

A Public Address System (PAS) designed for life safety and mass notification purpose requires careful planning and specific sound engineering techniques that adhere to applicable codes and standards. An electroacoustic design would carefully consider parameters such as required sound pressure level, signal-to-noise ratio, dynamic range, uniformity of coverage, and system headroom.

Above mentioned acoustic and electroacoustic design factors affect how well speech sounds are propagated and perceived by listeners in a variety of locations within a complex venue.

Proper design involves applying principles of physics in the domain of acoustic & sound engineering along with proven and accredited techniques to optimize the acoustic performance of a space and its fire and life safety systems. Some of the design strategies include:

  • Application of acoustic treatment to reduce reverberation time or control sound reflections. (Acoustic design must be conducted by experienced engineers who have extensive experience in the file of internal room acoustic design.)

  • Creating acoustically distinguishable spaces (ADS) that have different acoustic characteristics from adjacent spaces, such as corridors or lobbies, to improve speech intelligibility and localization. (This requires full coordination with the fire and life safety design disciplines of a project.)

  • Complying with relevant codes and standards that specify minimum requirements for speech intelligibility in fire and life safety design. (Local codes such as civil defense and AHJ-specific requirements must be studied and considered during the acoustic design for life safety.)

  • Selecting appropriate sound sources and signaling devices, such as message servers, alarm playback, fireman’s microphone, announcer microphone, and other certified input sources; loudspeakers, horns, and sounders, which can produce adequate sound pressure levels (SPL) and frequency ranges for the intended message and audience. (Fire and life safety compliant devices must be used.)

  • Strategic placement of sound sources and signaling devices to ensure even coverage and directivity of the sound field, avoiding dead spots or hot spots. (Minimum design and performance requirements of applicable codes and AHJ requirements must be achieved by electroacoustic design.)

  • Adjusting the sound levels and equalization of the sound sources to match the ambient noise levels and frequency spectra of the space. (This is subjective to the acoustic ambiance and should be coordinated with acoustic design.)

  • Applying sound masking techniques to reduce unwanted background noise and enhance speech privacy. (To be effective this requires careful needs assessment and proper sound masking design.)

Don’t Let Any Stone Unturn!

Throughout this blog, we have been reiterating that the acoustic environment of the building plays a vital role in determining the audibility and intelligibility of the fire alarm system. The acoustic environment is influenced by numerous factors such as,

THE BACKGROUND NOISE level is the sound pressure level of all sounds in each space, excluding the sound source of interest (for instance output of the Public Address System through its loudspeakers). The background noise level can vary depending on the type and function of the space, such as offices, classrooms, auditoriums, corridors, stairwells, concourses, departure gates, lounges, and so on.

LC ACOUSTICS chart exhibiting L90% spectral data of noise recorded at a project site.

Some of the leading sources contributing to the background noise of space are HVAC systems, electrical & mechanical devices (such as computers, photocopiers, printers, pantry equipment, lifts, and escalators), human-produced noise (such as people talking in a public space, walking, using instruments), transmitted noise (such as sound from outside of building bleeding into the space, or sound flanking from adjacencies, structural borne noise).

The background noise level can affect the audibility of the fire alarm system by masking or reducing its sound level. Therefore, it is important to measure and control the background noise level in different spaces and ensure that it does not exceed the recommended values for different occupancies. Most importantly the smoke extraction systems must be studied for their overall noise levels and specifically during 100% operation.

REVERBERATION TIME is the measure of how long it takes for a sound to decay by 60 decibels after it stops in a given space. When the reverberation time in space is high, it means that sound takes longer to diminish after it is produced. This prolonged sound decay can have a detrimental effect on speech intelligibility.

LC ACOUSTICS: Reverberate decay curve result from the acoustic simulation (Ray-Tracing Method)

Overall, high reverberation time hampers speech intelligibility by introducing acoustic distortions, reducing clarity and audibility, increasing background noise, and impeding the understanding of linguistic cues. By addressing and controlling the reverberation characteristics of space through proper acoustic design, speech intelligibility can be improved, enhancing communication, and ensuring effective emergency response.

THE SOUND TRANSMISSION LOSS is the measure of how much sound is attenuated or reduced when passing through a barrier, such as a wall, door, or fire-stop barrier. The sound transmission loss can affect the audibility of the fire alarm system by preventing or allowing its sound to propagate from one space to another. Therefore, it is important to design and construct the walls and doors with adequate sound insulation properties and ensure that they do not have any gaps or openings that can compromise their performance. Design coordination and ADS development are crucial for the successful implementation of fire and life safety systems.

Space Aesthetics and Layout

It is of paramount importance to study the architectural, geometrical, and interior design details of spaces that are part of fire and life safety design.

ACOUSTICALLY DISTINGUISHABLE SPACES (ADS) refer to areas within a building that have intentionally different acoustic characteristics from adjacent spaces. In fire and life safety design, ADS are created to enhance speech intelligibility and localization during emergency communication. ADS helps to minimize sound interference and improve the clarity of emergency messages.

Acoustically Distinguishable Spaces stack-view of a project.

They allow occupants to easily identify the origin and direction of the sound, facilitating efficient evacuation and response to emergencies.

ADS can be achieved by using physical barriers, such as walls, doors, or partitions; or by using electronic means, such as zoning, paging, or sound masking. For example, in a large building with multiple corridors and lobbies, creating ADS can involve adjusting the acoustic treatment, sound absorption, or Public Address system (PAS) design to differentiate these areas from the adjacent spaces. This helps to prevent sound propagation between different zones and improves the overall audibility and intelligibility of emergency announcements.

ADS are an important consideration in fire and life safety design as they contribute to effective communication and facilitate a swift and orderly response during emergencies. They help ensure that critical information reaches occupants clearly and enables them to make informed decisions for their safety.

Devices and Components

SOUNDING DEVICES, SYSTEM COMPONENTS, AND ELECTROACOUSTIC CHARACTERISTICS or voice communication systems performance parameters are related to their frequency response, directivity, distortion, power output, placement, orientation, spacing, and so on. The characteristics listed above can negatively impact both the audibility and intelligibility of the fire alarm system.

It is important to select and install loudspeakers, sounding devices, or voice communication systems that are suitable for different spaces and applications and ensure that they are properly calibrated and tested.

Beam-steering column array type loudspeaker system with its DSP components

Besides the loudspeaker and sounding devices, the other PA system and mass notification system equipment must be carefully selected to achieve specific performance criteria. These may include selecting appropriate audio sources (such as message servers), amplifiers, microphones, and controllers. The fire alarm system must comply with the relevant codes and standards, such as NFPA 72: National Fire Alarm and Signaling Code, EN 54: Fire Detection and Fire Alarm Systems, or other local regulations.

Live By The Codes!

Acoustic study and proper electroacoustic design can help achieve the speech intelligibility requirements that are mandated by various codes and standards for fire and life safety systems. Some of the relevant codes and standards include:


- NFPA 72: National Fire Alarm and Signaling Code

- EN 54: Fire detection and fire alarm systems

- ISO 7240-16: Fire detection and alarm systems - Part 16: Sound system control and indicating equipment. (ISO 7240-16 Supersedes IEC 60849:1998)

- ISO 7240-19:2007: Fire detection and alarm systems — Part 19: Design, installation, commissioning, and service of sound systems for emergency purposes.


- ANSI/ASA S3.5-1997 (R2020): Methods for Calculation of the Speech Intelligibility Index.

- ANSI/ASA S12.60-2010/Part 1 (R2015): Acoustical Performance Criteria, Design Requirements, And Guidelines For Schools, Part 1: Permanent Schools.

- ANSI/ASA S12.60-2009/Part 2 (R2014): Acoustical Performance Criteria, Design Requirements, And Guidelines For Schools - Part 2: Relocatable Classroom Factors.

- The Acoustical Society of America (ASA) Standard on Acoustics in Educational Settings (provides recommendations and criteria for acoustic design and performance in educational facilities, including fire alarm systems and voice communication systems).

- The Smithsonian Institution Fire Protection and Life Safety Design Manual (provides guidance and requirements for fire alarm systems, including public address, in various types of buildings and occupancies).

Dubai International Airport Strums the Right Notes!

There are many examples of how acoustic study and proper electroacoustic design have improved fire and life safety design by achieving better speech intelligibility in complex spaces. Airports come at the very top of that list.

Airports are challenging environments for fire and life safety systems due to their large size, high ceiling heights, multiple zones, diverse occupancy types, high background noise levels, and multiple sources of information.

For instance, Dubai International Airport (DXB) is one of the busiest airports in the world, serving over 90 million passengers per year. It has four terminals, three concourses, two runways, and a total area of over 15 square kilometers.

Dubai International Airport Terminal-3 Baggage Reclaim Hall.

The airport has implemented a state-of-the-art voice alarm system that covers all areas of the airport with over 1000 zones and 40,000 loudspeakers. LC Acoustics has rendered services in the Acoustic design and verification of the Public Address system (PAS) of Dubai international airports (by 3D acoustic modeling, simulation, and site verification methods).

The airport has implemented a comprehensive acoustic design strategy that covers both the PAS and the acoustic designs. The airport systems include voice alarm systems, public address systems, flight information display systems (FIDS), gate information display systems (GIDS), baggage information display systems (BIDS), check-in information display systems (CIDS), security information display systems (SIDS), wayfinding information display systems (WIDS), emergency information display systems (EIDS), and passenger information display systems (PIDS).

The sound systems use digital signal processing (DSP) to adjust the sound levels and equalization of each loudspeaker according to the ambient noise levels and acoustic characteristics of each zone. The sound systems also use ADS technology to create acoustically distinguishable spaces that have different sound levels and frequency spectra from adjacent spaces, improving speech intelligibility and localization (such as utilizing beam steering linear array speakers).

of Acoustic study and proper design has ensured that emergency messages are delivered clearly and effectively to all passengers and staff in different areas of the airport, such as terminals, concourses, gates, lounges, baggage claim areas, security checkpoints, parking lots, so on.

Tie-in the Knot

In conclusion, acoustic study and proper design of electroacoustic systems play a crucial role in fire and life safety design. They are essential in meeting the requirements set by authorities having Jurisdiction (AHJ) and local civil defense agencies. These design considerations directly impact the performance and functionality of emergency communication systems, ensuring their effectiveness during critical situations.

The importance of these parameters cannot be overstated, as they directly contribute to the safety and well-being of building occupants in the event of a fire or other emergencies. Through the application of sound engineering principles, mass notification design techniques, and thorough acoustic study, the proper design enables the achievement of high speech intelligibility, adherence to relevant codes and standards, creation of acoustically distinguishable spaces, and ultimately, improved fire and life safety outcomes in complex spaces.

By prioritizing acoustic considerations, architects, engineers, and designers can create environments that support clear and effective communication, minimize confusion, and panic, and enable swift and efficient evacuation. It is through these comprehensive design practices that we can enhance the protection and well-being of individuals within buildings, ensuring their safety when it matters most.

For reader’s interest, some of the acoustic design strategies that can be applied in an airport design include,

- Using directional speakers to focus sound on specific areas (such as those spaces that have higher reverberation time and high background noise levels.)

- Using distributed speakers to provide uniform sound coverage.

- Using digital signal processing to enhance speech clarity.

- Using ambient noise sensors to adjust sound levels automatically.

- Using compliant components to override non-emergency announcements.

- Using visual aids to supplement auditory information.

Sources of information:

- Fire Protection & Life Safety Design Manual - Smithsonian Institution

- Guidance for institutions on fire and life safety design - Architects Registration Board

- NFPA 72: National Fire Alarm and Signaling Code - National Fire Protection Association

- EN 54: Fire detection and fire alarm systems - European Committee for Standardization

- ISO 7240-16: Fire detection and alarm systems - Part 16: Sound system control and indicating equipment - International Organization for Standardization

- ISO 7240-19: Fire detection and alarm systems - Part 19: Design, installation, commissioning, and service of sound systems for emergency purposes - International Organization for Standardization

- ANSI/ASA S3.5: Methods for Calculation of the Speech Intelligibility Index - American National Standards Institute / Acoustical Society of America

- ANSI/ASA S12.60: Acoustical Performance Criteria, Design Requirements, and Guidelines for Schools, PART -1 & PART -2 - American National Standards Institute / Acoustical Society of America


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