Acoustics - Measurement of insertion loss of ducted silencers without flow - Laboratory survey method (ISO 11691:2020); German version EN ISO 11691:2020
1Key Takeaways
This document specifies a substitution method for determining the insertion loss under laboratory conditions of round or rectangular, primarily sound-absorbing duct silencers without flow, as well as other duct elements for use in air handling systems. The document applies to silencers with a design velocity of no more…
2Expert Interpretation
An in-depth analysis of the laboratory measurement method for insertion loss of duct silencers under no-airflow conditions in DIN EN ISO 11691:2021, covering test setup requirements, implementation specifications for Accuracy Class 3, key technical changes, and a comparative analysis with ISO 7235.
Analysis of the Standard's Core Content
DIN EN ISO 11691:2021, the latest acoustic measurement standard released by the German Institute for Standardization, specifies the laboratory method for determining the insertion loss of duct silencers under no-flow conditions. This standard applies to circular (80-2000mm diameter) or rectangular cross-section silencers designed for flow velocities ≤15m/s, focusing on the performance evaluation of absorption-type silencers.
Key Technical Changes
| Changes | 2010 Edition | 2021 Edition |
|---|---|---|
| Modal filter | Not mandatory | Must be installed after the sound source, consistent with ISO 7235:2003 |
| Test channel cross section | Differences allowed | It is recommended to have the same cross section as the test piece |
| Appendix structure | Includes ZA/ZB appendix | Only retain the documentary appendix |
Measurement system requirements
The standard requires that the test system must include the following components:
- Acoustic measurement equipment: Meets the requirements of IEC 61672-1 Level 1 and requires regular calibration
- Sound source system: Consists of a 12-inch loudspeaker, a 1m long φ400mm circular pipe and a modal filter
- Transition section: A cross-sectional conversion structure with a 180° opening angle
- Test channel: At least 3.5m of straight pipe section on each side
Typical case: In the test of a rectangular cross-section silencer, a modal filter filled with 30mm thick mineral wool was used to achieve a modal suppression effect of 8dB in the 125Hz frequency band.
Measurement Accuracy Analysis
This standard belongs to Accuracy Grade 3. The comparative standard deviation (σR) of each frequency band is as follows:
- 50-100Hz: 1.5dB
- 125-500Hz: 1dB
- 630-1250Hz: 2dB
- 1600-10000Hz: 3dB
The expanded uncertainty (95% confidence level) should be twice the σR value.
Implementation Recommendations
1. Modal Filter Selection: It is recommended to use a short silencer similar to the test specimen, which must meet the following requirements:
- ≥3dB insertion loss in the 50/63Hz frequency band
- ≥5dB insertion loss in the 80-10000Hz frequency band
2. Channel Coupling: When the measured value is close to the upper limit of ISO 7235, the following should be applied:
- Use elastic seals
- Use a high-damping composite wall structure
3. Data Report: Key information such as the test specimen structure parameters, test environment volume, instrument model and serial number, etc. must be included.