Controlling Reflections in a Control Room
October 5, 2011
An audio control room ideally should provide a neutral (but not “dead”) environment to critically evaluate audio content. This is done by ensuring that the room itself and systems within it don’t add coloration of their own to sound being monitored.
Achieving this goal involves many facets of acoustical and audio systems design. One such aspect is controlling specular reflections which interfere with the direct sound from the monitor loudspeakers.
Specular reflections of acoustical energy occur at frequencies where sound acts more like a ray; that is, when they hit a flat surface, they reflect off it, with the angle of incidence equal to the angle of reflection.
Many surfaces in a control room can offer fairly flat surfaces for reflections, walls and ceilings being obvious ones, but also audio console control surfaces and console tops, and video monitors, to name a few.
Reflections reaching the listening position are delayed from the direct sound. When the two combine, the total frequency response ends up with dips and peaks at intervals determined by the difference between the arrival times of the direct and reflected sound energy.
Under these conditions, a listener can no longer hear an accurate representation of the sound being monitored and will find it difficult to create a balanced mix. This not only applies to the frequency response, but also affects the ability to accurately perceive directional cues.
Specular reflections can be controlled through room geometry, where they can be directed away from critical listening positions, and perhaps into diffusers to spread out the energy in time and space. This needs to be done correctly to avoid creating other problems.
Reflections can be reduced or eliminated through the use of appropriate absorbing materials. Absorption is especially effective placed at the specific locations where the reflections occur, rather than along an entire surface. This takes some acoustical analysis, but it’s worth it to avoid overly deadening a room.