Sound expands in space and time and creates various desirable but also undesirable effects in rooms. Those who develop loudspeakers are usually aware of this fact. The big challenge, however, is that every room in which a loudspeaker is used is different. The geometry is different and the surfaces are also different from room to room or from wall to wall. This manifests itself in room modes (also called standing waves) and reverberation times of different lengths and frequencies. Swiss radio SRF2 has an easy-to-understand podcast(german only) on this topic.
An important aspect for a good music reproduction is the following. The time until the sound of the first reflection reaches the listener is decisive for the spatial impression of music reproduction. It is important that this time is not too short. Otherwise, our brain cannot distinguish this reflection from the direct sound. The minimum value should not be less than 5 ms, i.e. a distance of at least approx. 1 m should be maintained from the lateral walls, ceiling and floor (depending on the listening position).
Another important value for good music reproduction is the reverberation time. This is the time until the sound has disappeared again after clapping hands, for example. For normal listening rooms the reverberation time should be between 0.3 and 0.7 s, for high frequencies it should be shorter, for lower frequencies it may be a bit longer. Both the first reflection and the reverberation time cannot be influenced electronically, but require physical measures. To achieve a shorter reverberation time in today’s rather underdamped living spaces, acoustic damping elements are needed. These can be part of the furnishings (carpets, acoustic curtains, fabric sofas, etc.) or in the form of special technical absorbers (porous absorbers, Helmholtz resonators, acoustic ceilings, etc.).
Standing waves, which inevitably form between parallel walls or between floor and ceiling and show up as specific booming frequencies, can be identified and electronically eliminated. This also pleases the neighbors, because standing waves have up to 20 dB more level and pass relatively well through ceilings and walls. Standing waves occur up to about 200 Hz (depending on room size, see also Schroeder frequency).
Every airplain music system can eliminate standing waves by room calibration. The loudspeakers are positioned as optimally as possible and the complete spectrum is measured at the listening position. Afterwards, the necessary filters are created.