Tech Talk - Optimizing Cabinet-Mounted Speakers
By David Smith
These days homeowners want invisible loudspeakers. For applications like distributed audio, in-wall speakers are the norm. But for higher performance, the home theater will often have non in-wall systems hidden away in wall units and equipment cabinets. Many high performance loudspeakers may sound great when mounted on stands and well away from the room boundaries, but special steps are required to get the best sound when a speaker will end up mounted in a cabinet.
There are several issues that must be taken into consideration when cabinet-mounting a loudspeaker:
• Boundary effects cause a heavier balance than with free space mounting.
• The cavity inside the cabinet has resonances due to its dimensions.
• The cabinets themselves can have rattles and panel resonances.
• Cabinet mounting may prevent aiming the system toward the listener.
• Doors and grilles over the speakers can block the drivers.
• Perforated screens will cause reflections and reduce treble significantly.
Most speakers are designed to have a flat response for some type of mounting condition. They can be designed for a flat response in free space when placed well away from adjacent walls or floors, or for a flat response in half space when mounted flush into a wall. If they have been designed for free space mounting, placing them into a cabinet will increase the level of bass and lower midrange, typically by 3 to 4 dB. This leads to unnatural sounding dialog and music. Manufacturers can deal with this issue either with multiple inputs or switches to adjust the crossover network for the boundary conditions, or even specifically balance the system for cabinet mounting only.
Worse than the raised midrange that the cabinet's extra boundary provides, the interior of the cabinet will have narrow band resonances due to its dimensions. Typical cavity dimensions lead to resonances in the 200 to 400Hz range. These give a chesty or "hooty" character to voices and can sound distinctly unnatural and resonant. Lining the interior with foam or fiberglass, ideally with a thickness of 2 inches or greater, will effectively dampen these resonances. Treatment will be more effective if the dimensions of the cavity are reduced, which will raise their resonance frequencies.