Working Group
1.73
Room Acoustics
Soundfields
The physical description
of soundfields can be limited to the representation of the scaled sound pressure p in
dependence of the place (x,y,z-coordinate). The attached velocity v, a pointed
vectorial quantity, can be calculated with the sound pressure distribution p(x,y,z).
For the coverage by measurement techniques it is sufficient to measure the sound pressure
by amount and phase in a precisely defined point grid. Wide band measurements with gliding
sine sounds deliver the frequency dependence and reveal directly the room resonances as
maxima in the transfer functions. At low frequencies the room resonances (modes) are far
apart and can be viewed seperately. The determination of resonance frequencies, their
damping and spatial distribution correspond to the method of the modal analysis, a method
developed for resonance investigation on mechanical structures. In particular with single
modes being very closely side by side the special evaluation proceedings of the modal
analysis are necessary to identify the vibration forms.
The knowledge of accurate
soundfield structures dependent on the frequency is often necessary to analyse measurement
problems and optimize standardized measurement techniques to keep down measurement
uncertainties.
The modal approach is
mainly necessary at low frequencies and with small rooms, because in this case the
requirements are missing to use room simulation programs. Furthermore it is assumed in
building acoustics, that the soundfield in front of measurement objects (walls, windows,
absorbent etc.) is ideally diffuse, that is the sound incidence is probably identical from
all directions. That applies from a certain frequency limit. A clue to that limit between
low and high frequencies is the Schröder frequency fs . On a room
with the volume V (in m³) and reverberation time T (in s) applies:
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