Plants and their acoustic benefits

Trees and shrubs have been used for many years to reduce traffic noise from busy roads. Research now shows that plants can also help to reduce background noise levels inside buildings. Some plant species are more effective than others and the benefits are most pronounced in buildings with hard, reflective surfaces.

Research

Tests carried out in the early 1990s by Rentokil Initial’s Research and Development Department suggested that interior plants can absorb, diffract or reflect background noise in buildings, thereby making the environment more comfortable for the occupants. The effect appears to be dependent on plant type, planting density, location and sound frequency.

To investigate the potential acoustic benefits of interior plants in more detail, further research was carried out by a post-graduate student, Peter Costa, at South Bank University, London. Rentokil supported this work by providing access to computer data, technical advice, plant specimens and test sites.

To quantify the acoustic effect, the sound absorption coefficients of a number of plant species were measured and compared with other building materials (see table).

The higher the absorption coefficient, the better the material is at absorbing sound – a coefficient of 0.25 means that a quarter of the sound is absorbed, 0.50 half the sound and so on.

The study indicates that plants are generally more efficient at absorbing high sound frequencies than low. Good examples of this are Spathiphyllum wallisii (Peace Lily), Philodendron scandens (Sweetheart Plant), Dracaena marginata (Madagascan Dragon Tree) and Ficus benjamina (Weeping Fig).

High frequencies cause the most irritation to building occupants so the benefit of having plants becomes clear.

How do plants help?

Plants absorb, diffract and reflect sound. The balance varies with the frequency at which the sound is generated and the room's physical properties. The type of plant, its size, shape, the container, top dressings and the compost all have an effect on the sound reduction capabilities of plant displays.

Absorption
Plants alter room acoustics by reducing the reverberation time. Plants work better in acoustically live spaces, such as those that have hard surfaces like marble walls, exposed concrete and stone floors. The impact of plants is less likely to be noticeable in an acoustically quiet space, containing soft furnishings, carpets, heavy curtains or well upholstered chairs, which have a much greater capacity to absorb sound.

Acoustically live space

Acoustically quiet space

Diffraction and Reflection
At lower frequencies plants may diffract and reflect sound.   This is because the leaf size is small by comparison to the noise wavelength.  Plants with lots of small leaves are useful as they scatter and diffuse sound.  At higher frequencies the leaves may reflect sound towards other surfaces that may then absorb the noise.

How to use plants to reduce noise

The principles that apply to the placement of noise reducing furnishings can also be applied to the use of plants.

Large plant containers
Bigger plant containers contain more compost and have a greater area of top dressing.  Both of these have a significant effect on noise reduction, so it follows that they make a larger impact on the room acoustics. Experiments have shown that arrangements of different plants in groups appear to work better than individual plants.

Several small arrangements are better than one big one
Positioning several arrangements around a space works better than concentrating the plants in one location. In this way the surface area of the plants exposed to noise may be maximized and individual work areas in an office space will all benefit from a localized effect.

Edges and corners are better than the centre
Plants placed near the edges and corners of a space are better than plants in the middle. This is because sound is reflected from the walls straight into the foliage.

Consider using screen plants instead of office partitions

Open-plan offices are often very noisy places.  The hum of computers, the ringing of telephones and the buzz of conversation all make for a surprisingly noisy environment.  Often these spaces are divided up with partitions or ranks of filing cabinets. Plant screens are an effective alternative.  The tops of filing cabinets can also be used to place plants.  Small bushy plants in narrow troughs take up little space, but they can still be effective.

Author: Kenneth Freeman

Additional material: Peter Costa, Mike Lothian

Copyright 2003 Rentokil Initial plc

Sources of further information: Costa, P. and James, R.W. (1995) Environmental engineering benefits of plants.  Proceedings of the Workplace Comfort Forum, London, UK.