Acoustic fences: when a barrier actually reduces noise

An acoustic fence only works if it blocks the line of sight between the noise source and your ear. That is the whole principle. If you can see the wheels of the lorries from where you sit in the garden, the fence is doing almost nothing for the noise from those wheels, whatever it cost.

Break that line of sight with something solid and heavy enough and you can expect a useful reduction, often in the region of 5 decibels at the point where the sight line is just interrupted, rising towards 10 decibels or a little more with a taller, well detailed barrier. Beyond that the improvement gets expensive quickly, because sound bends over the top.

What a barrier does to the sound path

Sound travels in a straight line until something gets in the way. A barrier forces it to bend over the top edge, which takes energy out of it, and the further it has to bend the more it loses.

That bending distance is what acousticians call the path difference: the extra distance sound has to travel over the barrier compared with the direct route. Increase the path difference and the barrier performs better. You increase it by making the barrier taller, or by moving it closer to the source, or closer to the listener.

A barrier in the middle of a long garden is the worst position. The same fence tight against the road boundary, or tight around the plant it is screening, will do considerably more.

Two limits apply whatever you build. High frequencies bend less than low ones, so a barrier is much better at removing tyre hiss than the rumble of a lorry. And sound also arrives by reflection off other surfaces and around the ends of the barrier, which sets a practical ceiling on what any garden fence can achieve.

Height, mass and the line of sight rule

The two properties that matter are height and mass. Everything else is detail.

Height is judged from where the sound comes from to where you actually are. Sitting on a patio, standing at a first floor window and lying in an upstairs bedroom are three different geometries, and a 2 metre fence that shields the patio may do nothing at all for the bedroom.

Mass is the other half. A barrier needs to be heavy enough that sound does not pass straight through it, and the usual rule of thumb in practice is a surface density of at least 10 kilogrammes per square metre. A solid close boarded timber fence with overlapping boards and no gaps is usually at or above that. A lightweight lap panel fence from a garden centre is not, and neither is a hedge.

On hedges and planting, the honest answer is that vegetation of any realistic depth does very little for noise level. It does help how noise is perceived, because people react differently to a source they cannot see, and that is a real effect. But do not expect it to show up in a measurement.

Gaps, gates and the base detail

A barrier with gaps is not a barrier. This is the single most common reason a garden acoustic fence disappoints.

Sound leaks through small openings far more readily than intuition suggests, and a run of fencing that is acoustically excellent except for a 20 millimetre gap under every panel can lose most of its benefit. The details to insist on:

• No gap at the base. Either the panels run into a gravel board bedded into the ground, or a concrete kerb runs beneath the whole line.
• Boards that overlap or tongue together, rather than butt joints that open as timber dries.
• Posts detailed so the panel meets them without a vertical slot.
• Gates that are the same construction as the fence, close against a stop on all four edges, and are actually kept shut.
• No decorative trellis on top unless it is solid, because open trellis adds height without adding a barrier.

Gates are worth a separate thought. A gate is usually the widest opening in the run and sits exactly where the path to the house is, which is often exactly where people sit. If the gate cannot be detailed properly, it is better to move it than to accept it.

Barriers against road traffic noise

Road noise is the most common reason householders look at this, and it is where the geometry is least forgiving.

The source is low, at the tyre and road surface, so a boundary barrier close to the road can work well for a ground floor garden. Upper floors are a different matter. A first floor bedroom window looks over the top of any fence you are likely to be allowed, so the sight line is unbroken and the barrier delivers little there.

Which is why the usual advice for upper floor road noise is to work on the building rather than the boundary: better glazing, acoustic trickle ventilators, and where the ventilation strategy allows, a way of ventilating without opening the window onto the road.

Local topography matters too. A road in a cutting is already partly screened. A road on an embankment above the garden is almost impossible to screen with a fence, because the source is above the barrier line.

If you are buying a house on a busy road, our note on what an acoustic barrier actually achieves sets out the realistic range before you spend anything.

Barriers against a fixed plant source

A neighbour's air source heat pump, an extract fan from a commercial kitchen or an air conditioning condenser is a different problem, and often a more tractable one.

Fixed plant sits in one place, usually low, and often close to a boundary. A screen built tight around the unit, on three sides, with a lid where the manufacturer's clearances allow, can be far more effective than a boundary fence because the path difference is large relative to the distance involved.

Watch three things. The manufacturer's ventilation clearances, because a starved unit runs harder and louder and may fail. The reflection off your own wall, which can send noise back into your garden. And the tonal character of the source, since a hum or whine is far more annoying at the same level than broadband noise and may need treatment at the unit rather than a screen.

Where the plant belongs to a business rather than a household, the route may be environmental health rather than construction, and a planning permission does not authorise a nuisance, as our note on statutory nuisance after permission explains.

The height limit on fences

Height is where permitted development bites. Under Class A of Part 2 of Schedule 2 to the GPDO 2015, a gate, fence, wall or other means of enclosure may be erected without planning permission up to 2 metres in height, reduced to 1 metre where it is adjacent to a highway used by vehicular traffic or a footway of such a highway.

Several exclusions apply.

That last row catches people. Fixing a 600 millimetre acoustic panel on top of an existing 1.8 metre fence gives a 2.4 metre structure, which needs planning permission even though the new part is small.

Where a taller barrier is needed, a householder application is the route, and the case is made on appearance and on neighbour amenity as much as on acoustics. Set out the noise problem, the geometry, and why the height is the minimum that works, in the Planning Statement accompanying the application. This is general guidance rather than legal advice, and a Lawful Development Certificate is the only formal way to confirm that works are lawful without permission.

What to ask an acoustician to model

If the noise matters enough to spend money on, a short piece of specialist input pays for itself, because the answer is geometric and a measurement at the wrong height tells you nothing.

Ask for four things. Measured levels at the positions you actually use, at the relevant heights. The predicted insertion loss of the proposed barrier at those positions. A comparison of two or three heights and positions, so you can see what the extra half metre buys. And a plain statement of what the barrier will not fix.

For a planning application where noise is the issue, a Noise Impact Assessment is the document that carries this, and councils usually expect a suitably qualified acoustician, with professional review strongly advised.

And be ready for the answer to be that a fence is not the solution. That is worth knowing before the posts go in.

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