Acoustics - Measurement of insertion loss of ducted silencers without flow - Laboratory survey method (ISO 11691:2020); German version EN ISO 11691:2020
1Key Takeaways
The document specifies a method for the simplified determination of the insertion loss of duct silencers intended for use with flows with velocities of 15 m/s or less. It is applicable for silencers with circular cross sections and diameters between 80 mm and 2000 mm and for sile
2Expert Interpretation
This in-depth analysis of the DIN EN ISO 11691:2021 standard for laboratory measurement of insertion loss of duct silencers under no-airflow conditions covers the test requirements, equipment configuration, and data processing specifications for accuracy level 3, suitable for the performance evaluation of silencer components in HVAC systems.
Interpretation of the Standard's Core Content
DIN EN ISO 11691:2021 specifies an alternative laboratory measurement method for the insertion loss of circular or rectangular duct silencers operating primarily on the sound absorption principle under no-airflow conditions. This standard applies to HVAC system silencer components with a design flow velocity of ≤15 m/s and a measurement frequency range of 50 Hz to 10 kHz.
Key Technical Changes
| Version Comparison Dimensions | 2010 Edition | 2021 Edition |
|---|---|---|
| Modal Filter | Not Mandatory | Must Be Installed, Consistent with ISO 7235:2003 |
| Test Channel Cross Section | Differences Allowed | Same Cross Section Recommended |
| Appendix Structure | Includes ZA/ZB Appendix | Simplified as Literature Reference |
Test system requirements
The standard requires that the test system must include:
- Class 1 sound level meter (compliant with IEC 61672-1)
- Broadband noise source (including modal filter)
- Transition connector (180° opening angle)
- 3.5m long test channel
- Reverberation chamber (preferred) or other test environment compliant with ISO 3741
Modal filter must provide: insertion loss ≥3dB in the 50-63Hz frequency band and ≥5dB in the 80-10kHz frequency band.
Measurement Uncertainty Analysis
The standard clearly gives the comparative standard deviation σR for each frequency band:
- 50-100Hz: 1.5dB
- 125-500Hz: 1dB
- 630-1250Hz: 2dB
- 1600-10kHz: 3dB
The expanded uncertainty (95% confidence level) is recommended to be twice the σR value.
Implementation Recommendations
1. Equipment Selection: Prioritize test channel systems with consistent cross-sections, with circular muffler diameters ranging from 80-2000mm.
2. Vibration Control: When measured values exceed the ISO 7235 limits, measures such as elastic sealing or heavier channel walls must be adopted.
3. Data Reporting: Must include muffler structural parameters, test environment description, and frequency band labeling.
4. Standard Linkage: Must simultaneously reference ISO 7235 (including airflow measurement) and ISO 6926 (reference sound source requirements).