ISO 9921:2003

Overview

ISO 9921:2003 - Ergonomics - Assessment of speech communication provides standardized methods to predict, measure and specify the performance of spoken communication. The standard covers verbal alert and danger signals, information messages and general speech communication in person-to-person, public-address and personal communication-system contexts. It defines key terms (speech intelligibility, speech communicability, vocal effort, etc.), describes measurement approaches, and gives practical guidance for specification, design/prediction and in‑situ assessment of communication systems.

Key topics and requirements

• Scope and approach
  • Addresses requirements for intelligibility and vocal effort across different application types (alerts, meetings, public areas).
  • Recommends a three‑stage process: specification of performance criteria, design and prediction, and in‑situ assessment.
• Means of communication
  • Defines three modes: direct communication, public address (PA) systems, and personal communication systems (telephones, intercoms, mobile radios).
• Objective and subjective assessment
  • Describes subjective speech‑intelligibility tests and objective prediction/measurement methods (see STI, SII).
  • Includes normative/infomative annexes on test construction and predictive indices.
• Key metrics defined
  • Speech Transmission Index (STI) - objective method for speech intelligibility (referenced IEC 60268‑16).
  • Speech Intelligibility Index (SII) - objective prediction method based on the Articulation Index.
  • Speech Interference Level (SIL) - difference between A‑weighted speech level and average ambient noise in octave bands centered at 500 Hz, 1 kHz, 2 kHz and 4 kHz.
  • Vocal effort - quantified by A‑weighted speech level at 1 m and described subjectively.
• Human factors
  • Accounts for speaker/listener variables: Lombard effect, non‑native speakers, hearing loss and listener coverage requirements (e.g., 50 % vs 96 % population coverage).

Practical applications and users

ISO 9921 is intended for professionals involved in designing, evaluating or regulating speech communication systems:

• Acoustical and ergonomics consultants
• Public‑address and audio system designers and suppliers
• Safety and emergency planners (evacuation warning systems)
• Facility and venue managers (auditoria, transit hubs, workplaces)
• Telecommunication and intercom system engineers
• Standards bodies and compliance auditors

Practical uses include setting performance criteria for PA installations, predicting intelligibility during design, selecting equipment and verifying in‑situ intelligibility for safety‑critical messages.

Related standards

• IEC 60268‑16 - Objective rating of speech intelligibility by STI (normative reference)
• ISO/TR 4870 - Construction and calibration of speech intelligibility tests
• ISO 7731, IEC 60849, ISO 11429 - complementary standards on non‑speech warnings and visual signals (referenced in ISO 9921).

Keywords: ISO 9921:2003, ergonomics, speech communication, speech intelligibility, STI, SII, SIL, vocal effort, public address systems, speech assessment, acoustic prediction.