Speech Intelligi­bility

The Importance of Speech Intelligibility in Emergency Situations:

Optimal Speaker Configurations for Voice Alarm Systems

In emergency situations, such as a fire or other hazardous scenarios, precise and understandable instructions are crucial to safely evacuate people from danger zones. Voice alarm systems (VAS) play a key role in these situations as they can deliver acoustic warnings and specific instructions. A central challenge in this context is ensuring speech intelligibility, as it significantly affects the success of the evacuation. The following blog discusses why optimal speech intelligibility is essential and the technical considerations that should be taken into account when configuring speakers in such systems.

Why is speech intelligibility so important?

In emergencies, especially in large buildings or open areas such as shopping malls or airports, it is vital that people clearly and unambiguously understand the instructions delivered through speakers. Inaccurate or poorly understood announcements can lead to panic, misinterpretation, and dangerous behaviour. Therefore, it is essential that the speech transmission is designed to provide high intelligibility even in acoustically challenging environments. Speech intelligibility is influenced by various factors, including room acoustics, ambient noise, the quality and placement of speakers, and the technical characteristics of the sound system. To ensure that the transmitted messages are clear and understandable, the Speech Transmission Index (STI) is often used to assess speech quality.

The Speech Transmission Index (STI) – A Detailed Explanation

The Speech Transmission Index (STI) is an internationally recognized metric for objectively evaluating speech intelligibility in rooms. The STI is based on measuring how well speech information is transmitted from a sender (e.g., a speaker) to the listener. The STI takes into account both the acoustic properties of the room and possible distortions, noise, and echoes that may impair speech intelligibility.

How does the STI work?

The STI measures the ability of an audio system to transmit the fine modulations of the speech signal, which are crucial for understanding spoken language. Essentially, the STI analyzes how well speech modulations at different frequencies can be reproduced by a speaker and received by a listener. Speech consists of several frequency bands, each conveying different aspects of intelligibility, from low frequencies representing vowels to higher frequencies responsible for consonants. An STI value ranges from 0 to 1, with a higher value indicating better speech intelligibility:

• STI 0.00 to 0.30: Very poor intelligibility. The spoken message is hardly or not at all understood.
• STI 0.30 to 0.45: Poor intelligibility. Some words are understandable, but the overall message is difficult to grasp.
• STI 0.45 to 0.60: Moderate intelligibility. Communication is possible but not always clear and unambiguous.
• STI 0.60 to 0.75: Good intelligibility. Most words are easily understood, with only minor intelligibility issues.
• STI 0.75 to 1.00: Very good intelligibility. All information is transmitted clearly and precisely.

To be effective in emergency situations, voice alarm systems should achieve an STI value of at least 0.50 to 0.60, as this ensures sufficient intelligibility even in environments with background noise or more challenging acoustic conditions.

Factors Influencing the STI

Several factors can influence the STI of a voice alarm system:

1. Room Acoustics: Rooms with long reverberation times, such as large halls or areas with many reflective surfaces, can significantly degrade the STI. Reverberation causes speech modulations to “blur,” preventing important frequencies from clearly reaching the listener.
2. Ambient Noise: Background noise, such as machinery or crowds, can lower the STI by masking or distorting the speech signal. A high signal-to-noise ratio (SNR) is necessary to ensure that speech is loud and clear over the background noise.
3. Distortions and Signal Loss: Distortions in the speaker signal, caused by poor cable connections or substandard amplifiers, can also affect intelligibility. Technical components must be designed and installed to provide consistently high audio quality.
4. Speaker Placement and Number: Uneven speaker distribution can result in significant differences in sound pressure levels, causing the STI to vary in different areas of the room. A well-planned speaker installation is essential to ensure uniform intelligibility throughout the space.

STI Measurement Methods

STI measurement is typically conducted using specialized measuring devices or software that feed the audio system with a test signal and analyze the sound waves generated by the speakers. These test signals are artificially modulated noises that simulate the human speech spectrum. The software or device then evaluates how well the modulations in various frequency bands are transmitted by the system and calculates the STI value. Measurements are often taken in different areas of a room to ensure uniform intelligibility. In large or complex environments such as train stations, airports, or stadiums, where voice alarm systems play a central role, achieving consistent intelligibility is a major challenge. Acoustic simulations and STI measurements during the planning phase of the VAS are therefore crucial for optimal results.

Factors Affecting Speech Intelligibility

In addition to the STI, there are other technical and physical factors that influence speech intelligibility and should be considered when designing speaker systems for voice alarm systems.

1. Speaker Placement and Selection: Proper placement and selection of speakers are crucial to ensure even and clear speech transmission. Excessive distance between speakers can result in low sound pressure or excessive reverberation in certain areas, reducing intelligibility. Ideally, speakers should be positioned to achieve consistent sound levels throughout a building. Directional speakers or speakers with a defined dispersion angle, such as horns or sound columns, are particularly useful in large, open spaces or long corridors to control sound distribution and minimize reflections that could impair intelligibility.
2. Sound Pressure Level and Signal-to-Noise Ratio (SNR): The sound pressure level (SPL) must be sufficiently high to drown out ambient noise, but not so high as to affect intelligibility or cause discomfort to listeners. An optimal SPL ensures that announcements remain audible in noisy environments without compromising clarity due to excessive volume. The signal-to-noise ratio (SNR) describes the ratio between the desired signal (the spoken message) and the background noise. A high SNR is necessary to ensure that speech is clearly heard above the ambient noise.
3. Room Acoustics: The acoustics of a room significantly affect speech intelligibility. Rooms with hard, reflective surfaces like concrete, glass, or metal can interfere with sound propagation and lead to long reverberation times that blur the speech signal. Careful acoustic planning, such as using sound absorbers or diffusers, can reduce reverberation times and significantly improve speech intelligibility.

Final thoughts

Speech intelligibility is a critical success factor for voice alarm systems, as it can save lives in emergency situations. The Speech Transmission Index (STI) is the key tool for assessing and ensuring high speech quality. When planning and installing speaker systems for voice alarm systems, careful consideration of speaker placement, sound pressure levels, room acoustics, and the signal-to-noise ratio is essential to ensure the intelligibility of emergency announcements.