Technical knowledge
Low-frequency noise: what it is and how it can be investigated
Low-frequency noise is more than simply “bass sound”. Investigation should consider frequency, time variation, background conditions and the relationship between the reported phenomenon and the actual operation of potential sources.
Low-frequency noise is more than simply “bass sound”. Investigation should consider frequency, time variation, background conditions and the relationship between the reported phenomenon and the actual operation of potential sources.
Why it can be difficult to identify
- An occurrence may last only seconds or minutes and may not coincide with a short visit.
- Overall weighted indicators may reduce the visibility of narrow components at low frequencies.
- The building, room and receiver position may reinforce or attenuate particular frequencies.
- Electrical interference, wind, traffic or local equipment may create similar signatures.
Possible sources and mechanisms
- Fans, pumps, compressors, motors, transformers and generators.
- Rail, metro, road traffic or mobile equipment.
- Airborne transmission through façades, ducts and openings.
- Structural transmission through building elements or the ground.
- Modulation or operating regimes that produce intermittent events.
What should be preserved in measurement
- Time history with resolution compatible with event duration.
- Band spectra and, where appropriate, FFT analysis.
- Operating conditions, observations and reported times.
- Comparison of periods with and without occurrence and reference conditions.
- Enough information to exclude unrelated events.
Simultaneous noise and vibration
- Synchronous measurement can show whether an event occurs at the same time in acoustic and vibration signals.
- Time correlation alone does not prove origin, but it can strengthen or weaken transmission hypotheses.
- Comparison between several locations can help understand the path and guide further measurements.
How to interpret results
- Conclusions should distinguish observation, correlation and attribution of cause.
- The technical reference is selected according to purpose, site, frequency range and applicable framework.
- The absence of an event during the survey does not prove it never occurs; it limits the available evidence.
- The report should state valid events, criteria, limitations and next steps.
This content is informative. Methods, criteria and framework should be defined for each specific case.