Vertical sound absorbers for improved acoustics
In some premises the conditions are such that a wall-to-wall sound absorbing ceiling is not enough for creating good acoustics on its own. New findings show that in, for example, day-care centres and schools for lower grades it is not only a matter of the reverberation time, but to reduce the sound pressure level on the whole. This is best done by maximizing the amount of sound absorbing material and implies that also the walls have to be used for sound absorbers. To achieve good conditions it is also important to distribute the absorbers over more surfaces than the ceiling.
In other situations it might be difficult or is not proper to use the ceiling area for sound absorbers, or maybe only parts of it can be used: In older buildings the original ceilings might have nice plaster details, or paintings, which you want to be visual. It might even be against the law or regulations to hide them. Or, often in modern buildings, the expression of the interior is such that the architect or designer wants installations or the structure of the roof to be exposed. When the concrete structure is an active part of the thermal system, it must not be covered.
Reverberation time (RT)
is by far the most frequently used parameter for calculations and measurements within room acoustics. The formulas used are normally the Sabine formula, or some modified version of it. They are easy to use - you need the room volume and the amount of sound absorption, calculated by the statistical absorption coefficient αp
These formulas, however, are designed for ideal conditions with diffuse (i. e. non-grazing) sound fields. In reality the sound field is far from being diffuse. Most probably it will consist of two main parts: one grazing and one non-grazing.
Grazing sound field
Non-grazing sound field
Grazing sound fields, which mainly exist in the mid and high frequencies, consist of sound energy developed in a plane that is parallel to a sound absorbing surface (which normally is the ceiling). The reverberation time in a room is mainly determined by the grazing sound fields. This means that, in practice, the reverberation time is considerably higher than the value calculated for diffuse sound fields.
The best way to control the sound energy in the grazing sound fields is to absorb it by sound absorbers on the wall. It can also be redirected towards the (sound absorbing) ceiling by scattering or diffusion from furniture, interior fittings and linings. Wall panels are mainly efficient for scattering in higher frequencies; furniture can be efficient in both high and mid frequencies.
Sound absorbers installed one by one or in fields will scatter the sound due to the level differences created. Splitting the absorbing area into smaller units this way creates a mix of hard and soft surfaces. This increases the effect on the reverberation time even more.
More benefits with vertical absorbers
In many premises there is a need for really good room acoustics in order to reduce the noise level. The more sound absorption there is in a room, the lower the noise level will be. It has been shown that physical reduction of sound pressure levels (=less noise) in a room also leads to an additional sound reduction due to psychological reactions: people do not talk so loud.
For environments with a need for good speech intelligibility, the STI (or RASTI)
value might be a more appropriate parameter than reverberation time. Although STI is partly determined by the reverberation time, it is better correlated to the amount of sound absorption in the room. Adding sound absorption by placing absorbers on the walls will decrease the reverberation time, improve speech intelligibility
and also reduce the sound pressure level.
The amount of sound absorption can be used to calculate speech intelligibility and reduction of the sound pressure level. But it is not safe or reliable to calculate the reverberation time (RT) based on the amount of sound absorbers only.
Practical solutions with vertical acoustics
There are, at least, three things to take into consideration when introducing vertical acoustic treatment into a room:
The most straightforward application is to install wall panels
continuously wall-to-wall or to cover a wall partly. From the acoustic point of view it is ideal to cover at least two perpendicular walls, or parts of them, since, in this case, both of the horizontal sound fields are being treated, and flutter echo is avoided.
Another way of installing wall panels is to split them into smaller fields – or even use them one-by-one scattered over the wall. This can even be done in regular or irregular patterns and can be an outlet for all types of creativity.
A common way of arranging wall panels in, for example, classrooms or offices is to install a horizontal band of absorbers at a suitable height and use them as pin-boards. Even if the wall absorber is more or less covered by ordinary papers, the sound absorption will remain good. Also in this case it is preferable to use more than one wall and to combine with a sound absorbing ceiling.
The wall absorbers should be placed in the height of people's ears in both sitting and standing position.
2a) short wall
2b) long wall