Online Noise Monitoring: Components, Uses and Requirements
An online noise monitoring system is a permanent or semi-permanent station that logs sound levels around the clock, sends them to a web dashboard and raises an alert when a threshold is exceeded. It is increasingly used on construction sites next to communities, in factories with EIA conditions, at entertainment venues and on rail projects. This page explains what a system consists of, how it differs from an attended survey, when it is required, what makes its data defensible, and whether to rent, buy or take it as a service.
What an online monitoring system consists of
- An outdoor microphone with rain and wind protection, and a Class 1 sound level meter to IEC 61672-1 calibrated by an ISO/IEC 17025 accredited laboratory
- A logger storing Leq, Lmax, Lmin, the L10/L90 statistics and frequency spectra at intervals from 1 second to 15 minutes
- A 4G/5G or LAN modem uploading to a server, with battery or solar power where there is no mains
- A web dashboard with real-time charts, daily summaries and SMS, email or LINE alerts on exceedance
- Options: short audio clips on exceedance to identify the source, a weather station, and vibration sensors in the same enclosure
A station without a Class 1 instrument and a current calibration certificate produces data that looks good on a screen and cannot be used in front of an authority or a court. If the purpose is compliance or defending against a complaint, the instrument specification matters more than the dashboard.
How it differs from an attended survey
| Attended survey | Continuous online monitoring | |
|---|---|---|
| Answers | Do we pass right now? | When do we exceed, how often, and did the fix work? |
| Suits | Steady noise, periodic checks, licence reports | Fluctuating, unpredictable or night-time noise |
| Operator | An analyst on site noting conditions and sources | Unattended; needs rules to flag invalid data |
| Evidence | A single signed report | A continuous dataset with calibration and edit logs |
| Cost | Per visit | Per month, including equipment, platform and care |
The two are usually combined: an attended measurement by the Pollution Control Department method gives the number an authority accepts, and the online system gives the context of how often that number occurs. The basics of planning a programme are in noise monitoring.
When an online system is required
- EIA or EHIA conditions requiring continuous monitoring during construction or operation, especially for metro lines, expressways, ports and power plants
- Construction next to a hospital, school or housing, where the contractor has to prove the agreed hours and levels were kept
- A factory or venue with a complaint history that has to show the local authority the noise is now under control
- A dispute where the noise occurs only at certain times — nights or weekends — which an attended visit usually misses
- Projects that want a continuous before/after record of a mitigation measure
Data quality: what makes the data defensible
- Field calibration with an acoustic calibrator on a fixed cycle, with the deviation recorded every time
- A current ISO/IEC 17025 calibration certificate for the meter and the calibrator
- Explicit validity rules: rain, wind above 5 m/s, equipment faults and sounds unrelated to the source of interest are flagged with a reason, never silently deleted
- Microphone position to the standard method — 1.2–1.5 m high, at least 3.5 m from reflecting surfaces — or a clear note where that was not possible
- An edit log and backups, so the other party can audit the dataset
Reports Geonoise issues from an online system state all of these, because data whose validity can be questioned is worth the same as no measurement. See what a measurement report contains.
Alerts and thresholds
Alert thresholds should be tied to the limit that actually applies, not to a round number: for example, a one-hour Leq at the boundary back-calculated from the limit at the receiver, an Lmax for night-time events, or the level agreed with the community in a management plan. A threshold set too low fires all day and is ignored; one set too high misses the events that become complaints.
Rent, buy or take it as a service
- Monitoring as a service: Geonoise installs, maintains, quality-checks the data and issues monthly reports — for projects that need an authority-grade report without a team of their own. See noise monitoring service
- Buying a system: for factories and large contractors with several sites and someone to look after it. Class 1 instruments and monitoring stations are distributed by PLACID, which is run separately from Geonoise's consulting
- Short-term rental: for phased construction or a dispute needing two to four weeks of data; ask PLACID about instruments and calibration
- Smart systems: automatic source classification with AI, to separate traffic from machinery — see acoustic AI monitoring
Whichever route you take, the number used in a legal context should come from a measurement by the Pollution Control Department method made by someone experienced. The online system adds context to that number; it does not replace it.
Frequently asked questions
Can online monitoring data be used as legal evidence?
Yes, when the instrument is Class 1 with a current ISO/IEC 17025 calibration certificate, the microphone is positioned to the standard method, and the field calibration and data-validity logs exist. Authorities usually also want an attended measurement by the Pollution Control Department method.
How many stations do I need?
At least one at the boundary facing the most sensitive receiver, and usually one more at the receiver or at a background reference point. Linear projects such as metro lines use several along the alignment.
What does online monitoring cost?
It depends on the number of stations, the duration and the level of care. It is normally priced per station per month including equipment, platform and reporting; a quote for your actual scope is available from the monitoring service page.
Does it work in rain or strong wind?
The station keeps logging, but the periods of rain and wind above the limit have to be flagged invalid and excluded from the calculation. A good station carries a weather sensor to do this automatically.
Can a phone app or a cheap sensor replace it?
For a rough trend, perhaps; as evidence, no. The error can reach 10 dB and there is no calibration certificate. Thai authorities and courts accept only calibrated instruments used by the prescribed method.