Noise Attenuation in the Garden: How to Reduce Noise Pollution in Landscape Design
Noise, or more accurately, unwanted sound, is becoming an ever-increasing problem which garden designers may be asked to address as part of their design commission. Many people may have lived with unwanted sounds filtering into their homes and gardens for many years, and only recently become more aware of the problem. Others moving into new build sites may be acutely aware as there is little in the way of established vegetation to absorb the sound waves invading their new environment.
Noise attenuation may be relieved by increasing the distance between the source and affected site, or by interrupting and absorbing the sound waves on their journey through the air. As it is rarely possible to increase the distance, barriers of some type need to be erected.
Noise Becoming Pollution
There are many sources of noise pollution. Motorways, even some distance away, can provide a constant drone, which becomes an irritant, especially at certain times of the day, or during weekends when you are trying to relax in the garden.
What is ‘noise’?
Noise is caused by vibrations in the air which cause variations in the air pressure, resonating outwards from the source. Some sites are affected by aircraft noise, and therefore the problem may be coming from more than one source. This is all basic knowledge, but when it comes to discussing noise issues, it is important to understand and obtain correct information in order to begin to address it.
Noise may become more problematic during tenancy, both in frequency of time of day or week, but also the sound frequency of the vibrations, or decibel levels which may become more uncomfortable to the hearer. Higher pitched sounds, or lower, more persistent noise may become the problem. As a designer, it is useful to understand the type of noise pollution the home owner is seeking to subdue or attenuate.
Bear in mind, that things such as noise and sound are subjective. There is no sensible way that as a designer you can be expected to offer a solution that may be measured (even though sound can be measured using a decibel recorder, the actual levels and pitch will be in the ears of the owner, and therefore one should avoid making any promises of level reduction) in a manner that could be considered contractual.
Assessing the Source and Intensity of Problematic Noise Levels
Recognition of the source of the noise should be noted in a survey, conducted over several days. Aircraft sounds will usually be arriving from overhead, whilst traffic noise may be from more than one direction and level of irritation. A motorway drone from the east, fast moving traffic from the west and a school playing ground from the north. Single point noises are generally more easily resolved, unless they are particularly loud and to an unacceptable pitch.
Sound waves are significantly reduced if they pass through solid objects or are reflected off them. However, they may be trapped or absorbed by other materials that are not solid. Solid objects may redirect sound waves, such as mounds or berms may deflect soundwaves away from the source, usually directed skywards as they ‘bounce’ off the rounded shape of a mound or curved surface.
Sound travels across surfaces, and may be deflected or channeled, an occurrence often seen in cities when sound bounces between walls and glass buildings, increasing in intensity by conflation. New build sites may act in a similar way until vegetation grows and helps to absorb those deflected sound waves.
Some larger buildings, or developments, make use of vertical green walls, either planted with living plant material of synthetic products, designed to absorb vibrations and sound waves.
These may be constructed on brick or concrete walls, or as panels, including along boundaries, erected between a development and highway or railway track.
Becoming Accustomed to Noise Levels and pitches.
Remember too, that noise levels may be reduced by perception in the ears of the owners simply over time, whereby occupants become accustomed to the sounds and unwittingly start to ignore the level or degree of irritation. Put simply, they learn to live with the problem!
Looking at ways to reduce noise levels, in real terms, start with overhead sounds from aircraft.
Whilst there is little that can be done to reduce the volume arriving on site from the clear air above, sound may be absorbed through vegetation, by reducing the deflected sound waves from bouncing off a smooth surface. Hence a lawn laid to wildflowers, with taller grasses with large open seed heads will effectively reduce the opportunity for sound waves to resonate off the ground.
Physical Changes in Ground Level.
Mounds or berms need not be too high. Even at 1200mm, aerially sourced sounds may be ‘lifted’ or steered upwards, and if the berm is planted with noise absorbing trees and shrubs, this will achieve a high level of reduction in decibels. Experiments along highways indicate that trees planted with gaps are more effective than densely planted sites, because people believe that they are hearing less noise. A solid planting of trees increases people’s perception of noise as their ears are trying to locate the source, i.e. actively listening for the cause.
Acoustic Panels
Acoustic fencing is available in various heights (up to around 2400mm) which provide a degree of relief. Some have slats that are angled to deflect sound waves downwards, into the ground, or skywards, much as the sound barriers in use at airfields. This recognition of the source of the sound waves becomes very important. If the source is from above, the last thing you need to do is return the sound waves from whence they came!
Acoustic barriers and fences may be visually hard, and require a lot of screening to disguise their harshness. However, they may be the only effective resolution if the nearby road is the cause of the problem, and a hard screen the only practical option. Therefore, sound absorbing panels will be preferable to sound deflecting ones. These absorbing panels have double sides, with a gap in the middle to effectively smother the sound waves. These gaps may be filled with additional material such as heather or cocoa matting which will greatly increase the capacity for capturing noise.
Vegetation as Attenuation
Trees and shrubs are invaluable in that they can reduce the amount of high frequency sound waves more readily than low frequency noise. The effect of planting trees is to disseminate the sound waves, through the branches, absorb them into the leaf surface, or interfere with the passage of the sound. Therefore, the more leaves and branches within the structure, the greater the reduction in noise. It necessarily follows therefore, that trees with larger leaves will be more effective than thin leaves needle-like leaves.
Some tree leaves, such as Sorbus aria lutescens, have soft, hairlike undersides which are very effective in capturing and holding sound waves. Experiments have proven that the trunk of a tree is the most effective in reducing sound, although I would aver that must depend on the type of tree and smoothness of the bark. Certain treelike shrubs such as Acer campestre, with its rugged bark on both trunk and branches, make excellent sound absorbing screens and barriers.
Others include Prunus lusitanica, Ilex in variety, Quercus ilex, Ligustrum, Griselinia and Pittosporum tobira. Many others may be useful as pollution filters, and mixed in with these basic foundation sound absorbers. I have nominated the plants above as they will ‘capture’ sound waves in an effective manner.
Bear in mind that as trees grow, their capacity to absorb sideways or lateral sound waves diminishes as their branches become higher. Underplanting will certainly help with these changes, although ground leaf fall has been proven to be an effective method of ‘collecting and retaining’ sound waves.
Independent Studies.
There are a number of studies available on noise attenuation produced by engineers and others, primarily addressing environmental issues such as those required for Planning and/or Governmental reasons, and others involving noise pollution regarding motorways and local nuisance. To the average homeowner, and garden designer, these are fairly irrelevant, unless you are working on a large-scale estate project.
The Institute of Acoustics have published various papers, including Applied Acoustics, which confirm that plants and planting in specific groups and density can be effective as sound barriers, acoustic barriers – especially directional blades – can deflect sound rather more effectively than attempting to suppress it.
Summary
To summarise; work to identify the source of the noise, level and frequency i.e. low-level humming or high pitch penetrating sounds, and their direction preferably over a period of time, noting any particularly noisy periods.
Look at the whole of the site and space available. Deflect as many sound waves as you can at the garden boundaries, absorb, deflect, suppress and smother others using whichever methods the site will allow.
If all else fails, try and provide at least one area – one noise free oasis – within the garden into which the owners may retire away from the majority of the offending sounds by using a combination of the methods outlined above.
