Noise Barriers: How They Work, Types and Real Results
A noise barrier is the most-requested mitigation for factories, expressways, construction sites and car parks in Thailand, and the one most often built wrong. This page explains how a barrier reduces noise, how many decibels it realistically delivers, how high and long it has to be, which material suits which job, when a barrier will not help at all, and why the design should be calculated before any money is spent. Geonoise designs and verifies barriers; it does not sell or install them.
How a barrier reduces noise
A barrier works by breaking the line of sight between the source and the receiver. Sound that used to travel straight to the receiver now has to bend over the top edge — diffraction — and the more it has to bend, the less arrives. The reduction depends on the path difference between the direct route and the route over the edge, not primarily on how thick or heavy the barrier is.
- A barrier that just breaks the line of sight gives about 5 dB
- A barrier well above the line of sight and close to either the source or the receiver gives 8–12 dB at mid and high frequencies
- The practical ceiling in the field is around 15 dB: sound around the ends, through joints and reflected off neighbouring buildings sets the limit
- Low frequencies — plant hum, generators, bass-heavy music — diffract more easily, so a barrier helps less than people expect
These are physics-based rules of thumb. The figure for a specific site comes from a calculation model. The source–path–receiver logic is set out in noise control.
How high, how long and where
- Height: the barrier must rise above the line from the source to the receiver — the second-floor window of the house, not eye level of a person standing. A receiver on a tall building is usually above the top edge, and the barrier does nothing for them
- Length: extend each end beyond the source–receiver line by at least the distance from the barrier to the receiver; otherwise sound goes around the ends and the gain collapses to 2–3 dB
- Position: as close to the source or to the receiver as possible. A barrier half-way between performs worst
- Continuity: no gap at the base, no door left open, no unsealed panel joint. An open area of 1 % costs several decibels of the total
Barrier types and their pitfalls
| Type | Suits | Strengths | Watch out for |
|---|---|---|---|
| Concrete / masonry | Factories, expressways, permanent boundaries | High mass, durable, little transmission | Reflects sound to the other side; needs a foundation |
| Absorptive metal panels | Sites with receivers on both sides, plant yards | Reduces reflection, light, fast to erect | The absorption rating must come from a laboratory test, not a catalogue |
| Transparent (acrylic / polycarbonate) | Bridges, elevated roads, where light and view matter | No shadow, architecturally acceptable | Fully reflective; joints and scratching need care |
| Earth bund | Factories, quarries, car parks with space | Cheap per metre, non-reflective, looks natural | Takes a lot of land; height limited by the slope |
| Temporary (construction) | Piling, demolition, events | Movable, phased with the works | Usually gapped and too low; real result below the supplier's claim |
Absorptive facing on a barrier reduces the reflection back towards the other side; it does not make the barrier block more sound. The absorption coefficient (α or NRC) of a product should come from an ISO 354 laboratory test, such as sound absorption testing at ALT. Geonoise neither tests nor sells these materials.
When a barrier will not help
- The receiver is above the top edge — a condominium from the fifth floor up with a view of the plant
- The source is high — rooftop exhaust fans, stacks or cooling towers — which call for silencers or enclosures instead
- The problem is low frequency — generators, compressors, bass from a venue — which has to be solved at the source with mass, vibration isolation and sealing
- The sound is structure-borne — machine vibration travelling through the ground or the slab into the neighbouring building
In these cases a barrier is money spent for no result. The right step is to trace the sound path first; an acoustic camera shows where the sound comes from and which way it travels.
Design by calculation before building
A barrier is expensive to fix afterwards: adding a metre of height to a finished wall is usually impossible because the foundation was not designed for it. The process Geonoise uses is:
- Measure at the boundary and at the receivers by the Pollution Control Department method, with spectra, so the required reduction per frequency band is known
- Build a SoundPLAN model to ISO 9613-2 and test the options — 3 m against 4 m, close to the source against on the fence line, absorptive against reflective — see noise modelling and prediction
- Report the result as a noise map and a level at each receiver against the legal limit or the EIA condition
- Measure again after construction to verify, and to give the authority or the community a before/after record
If the goal is legal compliance, start with a compliance measurement: it tells you how many decibels you actually need before any decision about a barrier.
Barriers and Thai law
Thai law does not require a barrier; it requires a result at the receiver: a general limit of Leq 24 h 70 dBA and a maximum of 115 dBA under the National Environment Board notifications, and a disturbance level no more than 10 dBA above background. The EIA for many project types lists a barrier or bund as a mitigation measure, and failing to build what the EIA specifies is a breach of its conditions. The framework is summarised in Thai noise law.
Frequently asked questions
How many decibels does a noise barrier reduce?
Typically 5–12 dB at the receiver when the barrier breaks the line of sight, is continuous with no gaps and long enough. The practical maximum is around 15 dB. The figure for a real project has to come from a calculation model.
How high does a noise barrier need to be?
Higher than the straight line from the source to the highest receiver you need to protect, such as the second-floor window of the neighbouring house. Two to three metres covers a nearby single-storey house; no practical barrier covers a high-rise.
Can absorbent panels replace a barrier?
No. Absorbent panels reduce reflection inside a space or on the face of a barrier; they do not stop sound passing. Insulation comes from mass, continuity and breaking the line of sight. A product's absorption rating should come from an ISO 354 laboratory test.
Do trees or hedges block noise?
Barely, in decibel terms. A dense belt 30 m deep may give 3–5 dB, and hiding the source has a psychological effect, but it is not a measure you can rely on in front of an authority.
Does Geonoise sell or install barriers?
No. Geonoise designs the barrier in a calculation model — height, length, position and the material properties required — and measures the result after construction. The contractor and the materials come from the open market, so the recommendation is tied to no supplier.