Ideally the wall for flush-mounting limits the radiation from the monitor to the front hemisphere only. Flush mounting studio monitors into a wall offers also other important advantages such as eliminating unwanted secondary sound radiation from the monitor cabinet's edges and nearly idealizing the radiation space. The result is minimization of diffraction effects, improved transient response and imaging.
Low frequencies are radiated omni-directionally (equally to all directions). The underlying principle of wall construction is that the larger the wall mass, the less energy transmission there will be through the wall. Therefore, the wall should ideally be made of heavy materials, such as bricks or concrete. Any volume behind the wall should be filled with acoustically absorbing material, such as rockwool.
Suitable materials to make a monitor wall:
This is the best material as it is the heaviest and stiffest. Unfortunately, it is not always possible to build concrete walls into existing rooms. No acoustic treatment (rockwool) is needed behind a concrete wall if the wall is air tight. The surface can be finished with wood, soft cloth, etc.
Bricks (breeze blocks or normal bricks):
This is also a very good material as it is heavy and can produce very stiff walls. A brick wall is easier to build. No acoustic treatment (rockwool) is needed behind a brick wall if the construction is air tight. The surface can be finished with wood, soft cloth, etc.
Two to three layers of gypsum board are needed to increase the wall mass and to lower the wall resonant frequency sufficiently. It is possible to insert other materials, such as sand bags, wood and lead sheets, between the layers to add mass. It is better to put sound absorbing material, such as rockwool, behind the wall as some sound energy may leak through the wall into the enclosed volume due to the relatively low wall mass. These walls are typically constructed using steel frames. Use of wooden frames usually does not result in equally good wall characteristics.
Wooden walls are not recommended as these are typically not lossy and stiff enough and the unit mass of a wooden wall remains rather low. It is good to put some sound absorbing material, such as rockwool, into the cavity volume or the volume behind the wall due to the relatively high chance of sound energy transmission due to the low wall mass. When the wall is used for flush mounting the monitors, they should be mounted on a separate heavy stand built into the wall: a brick foot under the monitor stand is also a good idea to reduce sound energy transmission.
An important issue is to make sure the enclosures are installed exactly flush with the front wall without leaving any gaps or edges between the enclosure and the room wall:
Flush-mounting the monitors into the wall:
To reduce structural vibrations due to mechanical conduction of the vibrational energy, the monitors should be mounted on rubber pads so that a resonant frequency of 2–8 Hz can only result. This de-couples the monitors mechanically from the wall and avoids structural vibration transmission:
For some monitor models, Genelec offers wall mount kits. The kit can be built into the wall and allows the monitor to be installed later as well as removed for servicing. The kits have been designed so that they ensure correct low frequency radiation while providing the acoustical benefits of flush mounting.