Peter Costa at South Bank University in London, showed that plant displays are effective at absorbing, diffracting and reflecting sound.
They work best in hard surface spaces and when placed around a space rather than grouped together or in the centre – this is because sound is reflected from the walls onto the foliage.
Of course it depends on the room’s properties, size etc as well as the size and shape of the plant. Plants with lots of small leaves are useful to ‘scatter’ sound; a mix of plants works well too.
Using plants in troughs as barriers or screens in large open plan offices helps to ‘soften’ the noise of an office – computers, printers, telephones, conversation.
For a more scientific explanation here are some extracts from Peter Costa’s study ‘Constructive use of Vegetation in Office Buildings’ (1994)
“Apparent insulation by absorbents is achieved where the sound level in a room may be reduced by reducing the reflections by sound waves that ricochet around the room.
Four validation trials were undertaken to examine the use of plants in room acoustics. These show that, particularly at higher frequencies, plants reduce the reverberation time and, hence, make the room quieter. To achieve a useful difference at low frequencies increased planting densities are required. These trials also confirmed that better performance was achieved in live rooms than in quiet rooms.
How do Plants Work Acoustically?
Plants offer absorption, diffraction and reflection of sound. The balance between these mechanisms may vary with the frequency at which the sound is generated and the nature of the room itself. The species, specimen size, pot size, moisture content of the potting medium and the type of mulch all have an effect on the absorption offered by a plant. Care should be used in consideration of the contribution made by plants in the study of room acoustics. A key point in plants and acoustics is that the plants should be big, healthy and full bodied. To work at their best they should be happy plants that look good.
Plants alter room acoustics by reducing the reverberation time which is usually considered a benefit. The acoustic performance of plants is best in an acoustically live room that has hard surfaces such as marble walls, exposed concrete and stone floors. The effect of plants is unlikely to be noticeable in an acoustically quiet room, characterised by soft furnishings such as carpets, mineral fibre tiles and heavy pleated curtains. This is because the absorption of the soft furnishings is greater, and, therefore, masks the absorption by the plants.
In general, to make a noticeable difference, higher densities of indoor planting than currently used would be required.
The plants work more consistently at higher frequencies which is where many annoying high pitch noises may be encountered. At lower frequencies the performance of the plants has been found to be variable and there are other materials that function far better than plants at these frequencies.
Diffraction and Reflection
At lower frequencies where the wave lengths of sound may be about a metre or more, plants may offer diffraction because the leaf size is small by comparison to the wave length. At higher frequencies the leaf sizes may reflect the sound onto other surfaces that may then absorb the sound. Larger leaf sizes would offer increased reflection at lower frequencies.
Where Plants Work Best Acoustically
The principles that apply to many acoustic absorbents may be applied to determine where to locate plants to maximise their contribution.
Better at higher frequencies
The use of plants to “fine tune” room performance at higher frequencies should yield useful results. At lower frequencies other readily available materials are likely to provide more noticeable benefits.
Big plant pots offer more than small ones
Bigger planters contain more mulch and support larger plants. It follows from this that they make a larger impact on the room acoustics. Arrangements comprising different plants in groups of three or five appear to work better than individual plants.
Several arrangements work better than a concentrated location
Positioning several plant arrangements around a room would be more effective than concentrating the planting in one location. The motivation for this is to maximise the surface area of the plants exposed to noise, increasing the opportunity for reflections onto other surfaces.
Near edges and corners are better locations than in the centre
Near the room wall surfaces and corners would be more effective than in the centre of the space. In these positions sound reflected from walls may be more effectively intercepted by the plants.”