Overview
ISO 11820:1996 - "Acoustics - Measurements on silencers in situ" specifies methods for measuring the acoustic performance of silencers installed in real plant conditions. It defines procedures for determining either insertion loss or transmission loss of silencers used in ducts, openings and enclosures under operating conditions (e.g., gas turbines, HVAC plant, exhaust stacks, compressors). The standard covers measurement ranges (octave and one‑third‑octave bands, typically from 63 Hz to 4 kHz and, where required, lower/higher bands), treatment of flow‑related noise (regenerated sound), and the limits of applicability (not for closed high‑pressure systems where structure‑borne sound dominates).
Key topics and requirements
- Measurement types: Procedures for measuring insertion loss (Dis) and transmission loss (Dts) depending on silencer type and installation.
- Acoustic quantities: Sound pressure levels and derived sound power levels in octave/one‑third‑octave bands; measurement at source and receiver positions.
- Flow & operating data: Requirements to record static/dynamic pressure, flow velocity, temperature and operating conditions that affect silencer performance.
- Corrections: Guidance on background noise corrections and field corrections for non‑ideal sound fields (Annex A).
- Instruments & calibration: Requirements for acoustic instruments and for pressure/flow/temperature sensors; calibration of directional microphones and microphones with turbulence windscreens is covered (Annex B).
- Installation conditions: Rules for positioning microphones, measurement surfaces and dealing with transitions when silencer and duct cross‑sections differ.
- Evaluation & reporting: Methods for evaluating measured sound and flow data and mandatory information to record and report for acceptance tests and acoustic analysis.
- Practical notes: Regenerated (flow) noise is treated as an operational property of the installed silencer and is not separated out in field tests; results are not directly comparable with laboratory tests (e.g., ISO 7235) because of differing field conditions.
Applications and users
ISO 11820 is intended for:
- Acoustic engineers and test laboratories performing on‑site silencer acceptance tests and performance verification.
- Plant operators and maintenance teams evaluating refurbishment or cleaning effects on silencers.
- Specifiers and purchasers assessing in‑situ attenuation for HVAC, power generation, industrial exhaust and intake systems.
- Regulators and consultants needing standardized, reproducible field measurements of silencer performance.
Keywords: ISO 11820:1996, silencers in situ, acoustic measurements, insertion loss, transmission loss, flow noise, HVAC, exhaust stack, silencer acceptance tests.
Related standards
- ISO 7235 (laboratory measurements on ducted silencers) - results not directly comparable.
- ISO 3744 (sound power determination - engineering method)
- ISO 5221 (air flow measurement in ducts)
- IEC 651, IEC 804 (sound level meters / integrating-averaging meters)
