The previous parts of this series have dealt with various ways of improving the sound isolation of an existing recording room without going to unreasonable expense. In professional circles, however, it's often necessary to build a 'room within a room' to obtain the level of isolation desired. This is obviously expensive, and it will significantly reduce the amount of space available, but it does allow the control room shape to be designed with acoustic considerations in mind -- for example, the front of the room may be shaped to avoid any strong early reflections from the loudspeakers reaching the mix position. The reason I'm covering the room-within-a-room concept is not to provide detailed building instructions, but rather to explain the general principles involved, and even though few project studio owners are likely to go to these lengths, I have known people with large garages employ these techniques quite successfully on a DIY basis.
A room-within-a-room starts with a floating floor strong enough to bear the full weight of the inner room, the latter of which is invariably built on a wooden framework and then panelled. The studding frame is generally covered, outside and in, with layers of plasterboard, chipboard and fibreboard. A world-class professional studio might use a concrete inner shell built onto a floating concrete floor (definitely not a DIY project), but the majority of pro studios are designed around a timber-framed inner shell. If the void between the inner and outer room is small, due to tight space constraints, it may be better to apply all the plasterboard to the inside face of the inner room, to maintain as large an air gap as possible between the inner and outer walls. However, if the air gap is larger than nine inches or so, applying plasterboard to both the inner and outer faces of the floating room will give better isolation.
While a double-room structure offers good isolation, compared to a single structure with uprated walls, the void between the inner and outer rooms needs to be as wide as possible to minimise low-frequency leakage. In some circumstances, especially when the inter-room space is narrower than might be desirable, it may also be useful to introduce an absorbent material, such as Rockwool, into the void. Of course, the double structure brings with it the problem of designing doors and windows which will not compromise the sound isolation you've just worked so hard to achieve.
The inner room should be accessed by a double-door system. The door in the existing wall is normally arranged to open outwards, while the door in the inner room opens inwards. These doors should be heavy and should have well-fitting seals. Clearly, the space between the two doors will need to be isolated from the inter-room void, but if you were to make this space into a solid-walled tunnel you'd run the risk of structurally-borne sound escaping. Instead, arrange the door-frames so that they are each extended into the inter-door space, but that a small gap is left between them. You can fill this gap with mastic to prevent structurally-borne sound crossing the void. Expanding polyurethane foam filler may also be useful in these areas, as it acts as both an adhesive and a gap filler, while remaining a poor conductor of sound. However, it is very light and so should only be used to fill small gaps. There are various strategies for building the 'airlock', but the main requirements are that there should be no direct, solid path between the inner and outer rooms, and no gaps left unsealed. Figure 2 shows a possible way of fitting double doors.
Windows, too, should be isolated from the inter-room void, so as well as having separate windows in the inner and outer walls, you'll also need to extend the frames into the space in the same way as for doors. Because the outer building shell is much heavier than the inner room, the doors and windows in this outer shell must be made as heavy as possible. Commercial installations use thick plate-glass windows, but standard double-glazing units in both the inner and outer walls also gives good results.
As an alternative to the extended frame structure just described, some designers prefer to isolate the window and door apertures from the the rest of the inter-room void by using barrier mat to build a flexible tunnel around the openings that join the inner and outer walls. This may be a better approach where the space between the two rooms is quite wide, and a similar strategy can be a
"...sound travels faster in solids than it does in air, and without isolation, structurally-borne vibrations could arrive at the engineer's ears before the direct sound."
Care must be taken to ensure that the whole structure is airtight (other than via any properly installed air-conditioning ducts), so any joints in the plasterboard should be sealed with mastic. Other than the obvious leakage problems associated with doors and windows, there's potential for leakage wherever cables enter the room. It only takes a small hole to compromise your sound isolation, so after the cables have been fitted it's usual to seal the entrance points with mastic or expanding foam.
Because a soundproof room is airtight, you need to consider how fresh air will be introduced. Air conditioning may keep the room cool, but it won't bring in fresh air, so unless you have a separate air circulation system, you'll have to open the doors at regular intervals.
The room-within-a-room approach to construction, sometimes also known as a floating room, is used both in control room design and in studio design. The main difference is that the control room is likely to be more elaborately shaped than the studio, to assist with the acoustic design.
Control rooms may also feature alcoves or recesses to hold equipment or monitor loudspeakers. It's important, when designing for built-in monitors, to isolate them from the structure of the floating room in some way, or the sound quality could be compromised. This is because sound travels faster in solids than it does in air, and without isolation, structurally-borne vibrations from the speaker enclosures could arrive at the engineer's ears before the direct sound from the speakers. If this is allowed to happen, overall sound quality suffers and stereo imaging is adversely affected.
A simple way to overcome this is by isolating the monitor from the surface upon which it stands, using blocks of neoprene foam. It may also be useful to place a concrete paving slab (cut to size if necessary) beneath the loudspeaker, with neoprene between the slab and the speaker cabinet. The added mass of the concrete will help to decouple structural vibrations, and a little extra isolation can also be achieved by floating the concrete slab itself on further blocks of neoprene. Loudspeakers may also be placed on MDF boxes filled with dry sand (an empty box will tend to resonate). Alternatively, there are tubular metal speaker stands on the market which may be filled with sand to add mass and damp resonances. I'll cover choosing and installing monitors in detail later in this series.