ISO 26101-1:2021

Overview

ISO 26101-1:2021 - Acoustics: Qualification of free-field environments defines a standardized methodology to qualify acoustic spaces (anechoic and hemi-anechoic chambers) as providing a free sound field. The standard specifies test methods (discrete-frequency and broadband), instrumentation and source requirements, procedures for microphone traverses, result presentation and measurement uncertainty. It adopts the divergence loss method to compare measured sound-pressure decay with the ideal inverse-square behaviour and includes normative annexes with qualification criteria and source directionality tests.

Key technical topics and requirements

• Scope: Qualification of anechoic and hemi-anechoic spaces for general and referenced acoustical measurements; intended as the primary reference for other standards and test codes.
• Divergence loss method: Main measurement principle - evaluate direct and reflected sound contributions and compare spatial sound-pressure decrease to the inverse-square law.
• Instrumentation:
  • Sound level measurement system meeting Class 1 linearity and IEC 61672-1.
  • One‑third-octave filters meeting IEC 61260-1 for banded measurements.
  • Microphone nominally omni-directional; for >5 kHz, small-diameter microphones (WS2F-equivalent) are normally required.
• Test sound source:
  • Approximates a point source; omni-directional per Annex B directionality criteria.
  • Generates SPL ≥ 6 dB (preferably ≥ 15 dB) above background at all traverse points.
  • Stability requirement: monitor microphone correction applied if source varies by more than ±0.2 dB.
• Microphone traverses & source placement: Traverses originate within the source volume; referencing documents or Annex A specify traverse paths and source locations for qualification.
• Reporting & uncertainty: Results presentation formats and uncertainty evaluation aligned with ISO/IEC Guide 98‑3 (GUM); Annex C discusses measurement uncertainty.

Practical applications and users

ISO 26101-1 is used to:

• Qualify and certify anechoic/hemi-anechoic chambers for research, product testing and regulatory compliance.
• Provide a traceable qualification method referenced by standards for loudspeaker, microphone, automotive, aerospace, consumer electronics and noise-testing laboratories.
• Support chamber designers, test-lab managers, acoustical engineers, conformity assessment bodies and standards writers who require validated free-field environments.

Related standards

• ISO 26101 series (Part 2 announced: determination of the environmental correction)
• ISO/IEC Guide 98-3 (GUM) - uncertainty of measurement
• IEC 61260-1 - octave/fractional-octave filters
• IEC 61672-1 - sound level meter specifications

Keywords: ISO 26101-1:2021, acoustics, free-field qualification, anechoic chamber, hemi-anechoic, divergence loss method, omni-directional sound source, microphone traverse, measurement uncertainty.