How high does an acoustic wall need to be to screen generator and plant noise?

A generator that runs all day, or all night, is one of the most wearing noises a site can make. It does not stop for breaks, and a low drone carries a long way. If you live next to it, you may have been told “there is a barrier up” and still hear it clearly. If you run the site, you may have put up 2 m fence panels and still be getting complaints.

In both cases the likely cause is the same: the barrier is too low. This guide gives you a simple way to check the height, explains why it works, and is honest about where it does not.

How high should an acoustic wall be?

As a rule of thumb, an acoustic wall should be about 1.5 m higher than the top of the noise source.

This is a working rule used at Hushtec for first estimates. It is not a substitute for a noise assessment, but it tells you quickly whether a proposed barrier is in the right range.

Why does the wall need to be taller than the generator?

A barrier works by blocking the straight path between the noise and your ears. Sound then has to bend over the top, and it loses energy doing so. The further it has to bend, the quieter it gets.

Two published references show the pattern:

• Under the BS 5228 method used to predict UK construction noise, a barrier is commonly credited with about 5 dB when the top of the plant is just visible over it, and about 10 dB when the plant cannot be seen at all.
• US Federal Highway Administration guidance gives about 5 dB once a barrier breaks the line of sight, and roughly 1.5 dB more for each extra metre of height.

A 10 dB drop sounds roughly half as loud. A 5 dB drop is noticeable but modest. So a wall level with the top of the generator gives you the smaller figure, and the extra 1.5 m is what moves you towards the larger one.

Why does a 2 m barrier not work for a generator?

Most temporary acoustic barriers are panels hung on standard site fencing, about 2 m high. That suits low sources such as hand tools, small pumps and compact plant.

A 2.5 m generator stands above a 2 m barrier. From a ground-floor window you can often still see the top of it, and from a first-floor window you can see most of it. The panels may be good quality, but the noise is going over them, not through them.

Where does the 1.5 m rule stop working?

• Homes that overlook the site. If your bedroom window looks down on the generator, a 4 m wall may not hide it. The wall has to be high enough to block the view from the highest affected window.
• Barrier a long way from the source. A wall works best close to the plant or close to the listener. In the middle, it needs to be taller.
• Low-frequency hum. Deep engine tones bend over barriers more easily than higher-pitched noise, so the reduction you hear can be less than the headline figure.
• Gaps and short runs. Noise also goes round the ends and through gaps. A tall wall that is too short, or has openings, underperforms.
• Airflow. Generators need cooling air and a clear exhaust path. A wall should not box the set in without the supplier’s agreement.

What else reduces generator and plant noise?

A wall is not always the best first step. Ask the site whether it has looked at:

• Moving the generator further from homes, or behind a building or site cabin
• A quieter, acoustically enclosed set, or a smaller set matched to the real load
• Battery or hybrid power units, which can let the engine switch off at night
• A mains connection in place of a generator

What are the options for a temporary acoustic wall 4 m or higher?

Freestanding acoustic wall systems. Purpose-built frames carrying acoustic panels.

• Commonly used for: generators, plant compounds, demolition, data centre and battery storage (BESS) sites, and work near protected habitats
• Pros: reach 4 m to 8 m; some need no excavation or ground anchors; can be moved
• Cons: cost more than fence panels; need level ground, delivery access and a wind assessment

Scaffold-mounted barriers. Acoustic panels fixed to a scaffold structure.

• Pros: height is flexible; scaffolders are easy to find
• Cons: the scaffold must be designed for wind load on a solid face; slower to build and move

Stacked shipping containers.

• Pros: heavy and robust; can double as storage
• Cons: need a large footprint, cranage and firm ground; hard, reflective faces can bounce noise back unless lined

Where Hushtec fits. Hushtec, a New Zealand acoustic barrier manufacturer with a UK manufacturing site in Wakefield, West Yorkshire (where its Performance and Premium panels are made), makes a freestanding system called RapidWall. Its brochure lists standard heights of 2 m, 4 m, 6 m and 8 m, no ground anchors or footings, wind certification to 31 m/s and a sound insulation rating of 32 dB. Other heights are made to order. It is one option among those above, and it will not suit every site.

Disclosure: this site is funded by Hushtec Ltd and the author works for Hushtec.

What should you ask the site or the council?

1. How tall is the generator or plant, and how tall is the barrier around it?
2. Can the plant be seen over the barrier from the nearest homes, including upstairs windows?
3. Has a BS 5228 noise assessment been done, and what reduction does it assume for the barrier?
4. Can the generator be moved, swapped for a quieter set, or switched off at night?
5. Are there gaps in the barrier, and does it extend well past both sides of the plant?

This guide is general information, not acoustic or legal advice. Barrier heights for a specific site should be confirmed by a qualified acoustic consultant.