Room acoustics

Room acoustics deals with creating optimum acoustic conditions in a room. While soundproofing of components (walls, floors, doors and windows) lies at the heart of architectural acoustics, room acoustics predominantly deals with the following three measures: sound absorption (also known as sound attenuation), sound shielding and sound scattering.

In doing so, the frequency ranges from 125 Hz to 4,000 Hz are considered. The aim is to bring the conditions of the acoustics of a room, in all frequency ranges, to a pleasant level. The most important parameters for assessing room acoustics are reverberation time and speech intelligibility.

The energy of a sound wave decreases with the distance between the listener and the source of the sound. When sound waves hit a smooth, reverberant surface, they are reflected back at the same angle. However, when a sound wave hits an absorbing surface, part of the sound energy is absorbed and only the remaining part is reflected, depending on the absorption coefficient. This reduces the reverberation. In room acoustics, we usually take the following types of sound into account:

• Direct sound (e.g. sound between a person and their counterpart)
• Early reflections (e.g. sound reflected off a desktop)
• Diffracted sound (sound between A and B passes an obstacle)
• Reverberation (sound that is reflected in the room multiple times)

Sound that is reflected back and forth several times between two parallel sound-reflecting surfaces is referred to as flutter echo. This occurs especially between floor and ceiling or between two walls. In order to prevent this, at least one of two opposing surfaces should be sound-absorbing or sound scattering, even in a small room.

Every frequency has a sound wave with a specific wavelength λ. The wavelength is given in metres and is calculated from the speed of sound (343 m/s at 20 °C), divided by the frequency: λ=c/f

Therefore, those sound waves that are in the frequency range relevant to room acoustics (125 Hz to 4,000 Hz) are of a size of 2.7 m to 8 cm. This shows that the wavelength of the sound is well within the range of room dimensions and the size of furniture. While high frequencies with short wavelengths can be absorbed simply by carpets and curtains, the absorption of low frequencies with long wavelengths requires very big absorbers.