Overview
EN ISO 9921:2003 - Ergonomics: Assessment of speech communication specifies requirements and methods for evaluating the performance of spoken messages used as verbal alert and danger signals, information messages, and general speech communication. The standard describes practical ways to predict and assess both subjective and objective speech performance for direct person-to-person exchange, public address (PA) systems, and personal communication systems (telephones, intercoms, mobile devices).
Key concepts covered include speech intelligibility, speech quality, vocal effort, and environmental effects such as the Lombard effect. The document supports the three design stages of a communication solution: (a) define application and performance criteria, (b) predict performance during system design, and (c) assess in-situ performance after installation.
Key Topics and Requirements
- Scope and purpose: Defines when speech should be used (warnings, information) and when non-voice signals may be preferable.
- Three communication modes: Direct communication, public address, and personal communication systems - each with distinct assessment needs.
- Performance metrics and objective methods:
- Speech Transmission Index (STI) - objective prediction/measurement of intelligibility.
- Speech Intelligibility Index (SII) - objective method related to the Articulation Index.
- Speech Interference Level (SIL) - simple metric using A-weighted speech level and octave-band ambient noise.
- Subjective assessment: Guidance on listening tests and intelligibility scoring; Annexes provide test methods and rating scales.
- Speaker and listener factors: Vocal effort, gender, accents, non-native speakers, distance, and hearing protection effects are described.
- Design and prediction: Practical predictive methods and examples (informative annexes) to estimate intelligibility before installation.
- Normative/informative annexes: Speaker/listener characteristics, STI details, SIL definition, test examples, and application case studies.
Applications
EN ISO 9921 is practical for:
- Designing and commissioning public address systems in transport hubs, auditoria, factories.
- Specifying and testing alarm and emergency voice messages to ensure warnings are intelligible.
- Evaluating workplace communication where verbal instructions are critical (control rooms, shop floors).
- Assessing personal communication devices (intercoms, telephones, mobile handsets) for intelligibility and user effort.
- Supporting occupational safety decisions: when to use speech vs. non-speech signals.
Who uses this standard
- Ergonomists and human factors engineers
- Acoustic and audio-system consultants
- Safety and facility managers
- System integrators for PA and intercom systems
- Telecom and product designers concerned with speech intelligibility
Related Standards
EN ISO 9921:2003 is a practical, standards-based reference for ensuring clear, effective speech communication in safety-critical and everyday environments. Keywords: ergonomics, speech communication, speech intelligibility, STI, SIL, SII, vocal effort, public address, alarm messages.
