Working Group
1.73
Room Acoustics
Room Acoustics
In free space sound
propagates straight forward similar to light and neither diffraction nor reflection
changes the direction of propagation. In closed rooms however the walls with their
acoustic reflection properties determine the character of the sound propagation and of the
resulting sound field. Due to diffuse reflections on rough surfaces the reflected sound is
even able to distribute its energy to all directions.
This is why the ear of a listener at different positions in a room receives
different sound signals influenced by reflections, absorption, scattering and diffraction.
The assessment of the acoustic quality of a room is depending on its purpose: for a
distinct transmission of spoken words a direct sound without influence of the room is
desired. For listening to a symphony orchestra a large hall with longer decaying sound
(reverberation) is preferred.
On the other hand matters
turn around completely when the same presentation takes place in a small and acoustically
heavy absorbed room.
In room acoustics it is tried to describe and measure physically the acoustic
properties that lead to a subjective assessment. Criteria can be derived from correlating
both the subjective and objective results that are appropriate to design rooms
corresponding to the acoustic purpose.
Besides the distribution of the sound level in a room mainly the reverberation time
T60 is of great importance. After the American acoustician
Sabine that is the time that passes until the sound pressure of a turned off sound event
has decayed to one thousandth of the start value. That means a decay of -60dB for the
sound level where the foot index in the specification T60
comes from.
The reverberation time in
optimum condition depends on room size, design and first of all its intended purpose. A
speaker studio in a broadcasting company has a reverberation time of less than 0.2 s for
example, the cathedral of Cologne around 16 s.
The reverberation time now is not the same for all frequency components of the
sound. We distinguish the form of the reverberation time curve plotted against the
frequency according to the type of the room and its assignment to certain music epochs and
styles. This way the reverberation time curve has a slight decrease to low frequencies in
music rooms of the baroque which conveys a clear and transparent sound impression. On the
other hand in large halls for example of the romanticism the reverberation time increases
slightly to low frequencies. A full warm and mighty sound character is the result. The
reasons for these different room acoustic properties get clear at the moment you compare
the delicate and often fast played baroque music (Vivaldi, Bach and others) with the
solemn music of the romanticism which is formed by calm and solemn brass passages
(Bruckner, Tschaikowski and others).
Besides the reverberation time as most known parameter are a whole lot of room
acoustic parameters to describe the acoustics of a room. They are listed on a special page
named "computersimulation". It gives a
detailed description of the calculation possibilities with the help of computer programs.
|
