One after the other organization has introduced the ‘new working’ style. No more separate little offices, but flexible open-plan office spaces. These crowded and noisy space are called ‘office gardens’ in the Netherlands, but most could use a little extra green! Especially when you think of the ability of plants to absorb or muffle sound. Because noise polution is one of the most heard-of complaints of the ‘new’ office worker.

But how do you know for certain that plants can improve acoustics and, if this is the case, how does this improvement work? In order to measure the sound absorbing qualities of plants, Roby van Praag (2009) not only compared different types of plants and the way they were arranged, but also combined them with various wall covering materials and plant substrates. Key questions here were: how can the planting in or on a building be used innovatively to improve the acoustics of an area and what works best?

Van Praag worked together with interior landscaping firm Zuidkoop and with artist Zeger Reyers. Reyers developed a green wall, consisting of an aluminum framework with slanting aluminum planters filled with Seramis granules. They investigated the effects of single potted plants as compared with wooden or aluminum green interior walls, the influence of loose materials, like the substrate (such as potting soil, hydro culture and Seramis) on the acoustics, the plants themselves and the positioning of the plants in the space.

examples of office horizontal gardensWooden wall, Seramis in plain view in wooden wall and the aluminum wall

What is reverberation time?

Waves of sound ‘touching’ a material can go three ways: the wave can be reflected, it can pass through the material or it can be absorbed by the material. Plants, for instance, appear to be bad sound absorbers. A porous wall covering or carpet does a much better job. Plants, however, do let sound waves pass through which shortens the so-called reverberation time that is caused by reflection.

Plant, wall and substrate for the best result

The measurements demonstrated that for the reverberation time the type of plant or their position don’t really matter. Broadleaf plant species, however, do have the best results when put together. A green wall will have more effect on the reverberation time than a large collection of grouped plants and a thicker wooden wall has more effect than an aluminum wall, although the latter in fact does the job surprisingly well. Whether the placing of the walls makes any difference has not been determined from the study.

It’s interesting to know that the substrates that were used appear to be good sound absorbers for the frequencies between 500 to 2000 Herz, which happens to be the frequency range of the human voice. For higher frequencies another material is best used. Because of the fluctuation of the test results it can’t be concluded which substrate shows the best acoustic result. It’s clear, however, that Seramis granules are outstanding competitors. It would be a smart move to have the substrate ‘out in the open’ as much as possible, like is already done in many green walls.

Some guidelines

In van Praag’s report a study performed by P. Costa of the South Bank University in London (1995) is frequently referred to. This is one of the few other reseaches into the relationship of plants, rooms and acoustics. Costa tested several single potted plants and combinations of plants in various areas in a room. His conclusions generally confirm the ones made by van Praag:

  • Plants need to be large, healthy and well developed;
  • Plants need to have large leaves preferably;
  • Arrangements with three to five plants seem to work better than single plants;
  • Several planting arrangements in one room appear to work better than a concentrated set-up in one spot;
  • It’s more effective to place plants in corners or alongside walls than in the middle of a room

green wall
Green Wand Systeem Zeger Reyers, aluminum planters with Seramis granules

What else can plants do?

If the workers’ only concern is sound isolation, you would do better to use standard products with optimum sound absorbing qualities, but in open-plan offices there are more things that matter. People tend to feel unsettled in them, and plants appear to do a lot for our general sense of well-being. Plants clean the air  and keep the humidity at the right level. It’s this combination of characteristics that makes the green wall so very popular in offices and healthcare, as well as in the hospitality business.