The ideal soffit wall is built to limit the energy radiation to the front hemisphere only, i.e. towards the listening position and not backward & away from the listening position. Remember that wavelengths at the lowest frequencies, 20Hz to 100Hz, are some 3m to 20m long so there must be a very high and solid mass to stop this low frequency energy that is travelling in all directions – low frequencies are omnidirectional. If this is not the case, then the low frequency will have serious cancellation dips and comb filtering irregularities in the frequency response.
There is a wide choice of materials that can be used to construct a soffit mount wall but the underlying principle is that the larger the mass, the less energy transmission there will be to volume behind the soffit wall. Therefore the soffit wall should ideally be made from heavy materials such as bricks or concrete. Note that when the soffit wall is well made any volume behind the wall does not actually belong to the room anymore so there is no need to fill it with any acoustic treatment, i.e. rockwool. The materials you can use to make the soffit wall are:
This is the best material as it is the heaviest and very sturdy. Unfortunately it is not always possible to build concrete walls into existing rooms. No acoustic treatment (rockwool) is needed behind such a soffit wall due to the high mass. The concrete surface can be decorated as you wish with wood, soft cloth, paint, etc.
* Bricks (breeze blocks or normal bricks)
This is also a very good material as it is as heavy as concrete and still very sturdy. A brick wall is easier to build and no acoustic treatment (rockwool) is needed behind such a soffit wall. The brick surface can be decorated as you wish with wood, soft cloth, paint, etc.
* Gypsum Board
Two to three layers of board are needed to increase the wall mass and lower the resonant frequency. It is possible to insert other materials, such as sand bags, wood and lead sheet, between the layers to add to the mass. It is better to put some rockwool behind the soffit wall as some sound energy may leak into the enclosed volume due to the relatively low wall mass. Additional heavy wood bracing of the wall is highly recommended to reduce resonances. Even so, this type of wall is still quite easy to build into an existing room. The gypsum surface can be decorated as you wish with wood, soft cloth, paint, etc.
Wooden walls are difficult to realise because large quantities of wood are required to ensure that there is no vibration transmission. Heavy bracing is also required to reduce unwanted resonances. It is good to put some rockwool into the volume due to the relatively high chance of sound energy transmission due to the low wall mass. The monitors should be mounted on a separate very heavy stand that is built into the soffit wall: a brick foot below the monitor stand is also a good idea to reduce sound energy transmission. Surface finish with wood is probably the most appropriate.
Mounting the monitors into the soffit wall
The monitors should be mounted onto rubber pads with a resonant frequency of 2 – 8Hz to de-couple them from the wall and avoid structural vibration transmission – in practical terms the wall should not vibrate when the monitors are played at high level.
An alternative way to mount the monitor cabinets into the soffit wall cut out is to use the side mounting M10 threaded inserts (normally covered by plastic caps) – be sure to use bolts that do not extend more than 30mm into the cabinet from the surface otherwise damage may result.