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 in buildings. Some plant species are more effective than others and the benefits are most pronounced in buildings with hard, reflective surfaces.
Tests carried out by Rentokil Initial Research and Development suggested that interior plants can absorb 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 banks, 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.
Plant Species Sound Frequency
125Hz 250Hz 500Hz 1kHz 2kHz 4kHz
Ficus benjamina 0.06 0.06 0.10 0.19 0.22 0.57
Howea forsteriana 0.21 0.11 0.09 0.22 0.11 0.08
Dracaena fragrans 0.13 0.14 0.12 0.12 0.16 0.11
Spathiphyllum wallisii 0.09 0.07 0.08 0.13 0.22 0.44
Dracaena marginata 0.13 0.03 0.16 0.08 0.14 0.47
Schefflera arboricola - 0.13 0.06 0.22 0.23 0.47
Philodendron scandens - 0.23 0.22 0.29 0.34 0.72
Bark mulch 0.05 0.16 0.26 0.46 0.73 0.88
Thick pile carpet 0.15 0.25 0.50 0.60 0.70 0.70
Plasterboard 0.30 0.15 0.10 0.05 0.04 0.05
Fresh snow, 100mm 0.45 0.75 0.90 0.95 0.95 0.95
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.
Use in Building
The noise-reducing benefits of plants will be most pronounced in acoustically "live" buildings, i.e. those with hard reflective surfaces such as marble floors, plaster walls and large expanses of glass. Plants will have very little effect in acoustically "dead" areas, such as rooms with thick carpets, curtains and panelled walls.
Big planters have more effect than small planters
Bigger planters have more mulch and more plants. It follows that they make a larger impact on room acoustics. Arrangements comprising different plants in groups of three or five appear to work better than individual plants.
Several arrangements are better than a concentrated location
Positioning several arrangements around a room would work better than concentrating the planting in one location. This way the surface area of the plants exposed to the noise may be maximised.
Near edges and corners are better than in the centre
Near the edges and corners would be better than in the centre of a room. In these positions sound reflected from walls may be intercepted more easily by the plants.