Curved Walls and Home Cinema Acoustics - October 17th, 2008
Concavely curved surfaces in home cinema rooms are generally considered as critical or even dangerous in that they have the tendency to impede the uniform distribution of sound energy in the cinema room or to concentrate it to certain spots.
Formally, the law of specular reflections is valid for curved surfaces as well as for plane ones, since each curved surface can be approximated by many small plane sections. Keeping in mind the nature of sound, however, one should not apply this law to a surface the radius of curvature of which is not very large compared to the acoustical wavelength.
Very often, even from the home cinema design stage, we come across curved walls that are spherical or cylindrical segments or they can be approximated by such surfaces. Then we can apply the laws of rays reflected at a concave or convex mirror, known from optics (1/a +1/b = 1/R, where ‘R’ is the mirror’s radius, ‘a’ the source distance, ‘b’ the distance of the focus).
We can now draw some practical conclusions, useful when designing a home cinema system. A concave mirror may concentrate the impinging sound energy in certain regions, but it may also be an effective scatterer which distributes the energy over a wide angular range. Whether the one or the other effect dominates depends on the positions of the home cinema speakers and the listeners.
For this reason, home cinema rooms with elliptical floor plans often suggested by architects are plagued by quite unequal sound distribution even if neither the speakers nor the listeners are in a geometrical focus. We can now see why curved wall surfaces in home cinema installations are avoided, because it requires a lot of knowledge and understanding of the Physics involved, in order to avoid the rather disturbing acoustical effects caused by them.
Modelling a dedicated home cinema around a rectangular room is a much easier task and is always recommended.