In this final instalment, we present a practical overview of the 'room‑within‑a‑room' principle which is used in the construction of virtually all major studios.
The methods so far described in this series for improving the sound isolation of doors and windows are based on the same fundamental principles as those used in professional studio installations, but the methods I've discussed for uprating floors, walls and ceilings would almost certainly be inadequate for use in a large commercial studio where high sound levels are involved, unless the studio happens to be be located well away from neighbours or sources of external noise.
The ideal way to build a studio is to start off with a sufficiently large, solid‑walled building and then to construct additional rooms inside. These inner rooms should be self‑contained structures built upon a suitable floating floor, and should not be in physical contact with the main shell of the building other than via the supports for the floating floor and possibly via Neoprene isolation blocks. The greater the space between the inner room and the outer shell, the better the low frequency isolation — an 8‑inch gap should be considered a minimum.
The reasons why the room‑within‑a‑room approach is so effective should be apparent to anyone who has read the preceding parts of this series — by building one room inside another, we have created a double‑walled construction and, at the same time, have virtually eliminated structurally‑borne sound by isolating the inner room from the outer shell. The degree of isolation achieved depends largely on the type and construction of the floating floor.
As a rule, the more massive the floating floor, and the greater the space between it and the 'true' floor below, the more effective it will be at low frequencies. In mechanical terms, the inner room can be considered as a mass supported by a spring, and if the resonant frequency of the system is lower than the lowest sound frequencies being generated, the degree of isolation can be extremely high.
While lightweight floating floors can be approached by the DIY enthusiast with relative safety, the kind of massive concrete floor on springs used in a large commercial studio requires the services of a specialist architect and builder and is inappropriate for all but the most serious commercial studios.
For smaller studios, edit suites or video‑post facilities fortunate enough not to have acute problems with outside noise or nearby neighbours, a relatively lightweight inner room construction is adequate and follows the same general principles as the double‑skinned studding wall described earlier in the series. The timber framework for the walls is made from 4 x 2‑inch timber built directly onto the floating floor and the ceiling joists are fixed to the wall frames. Ceiling joists may need to be 2 x 6 inches or deeper depending on the span to be covered, but it is advisable to consult an architect over the constructional details before starting work.
An alternative form of ceiling construction favoured in broadcast is to use pre‑screeded, channel‑reinforced woodwool slabs which are plastered on the underside after construction. Woodwool slabs are made from strands of compressed wood mixed with cement and, because of the air trapped during manufacture, the material is relatively light and quite strong; it's also highly fire‑resistant. It is manufactured in panels, with metal channel fixed to the edges, enabling a wider ceiling to be assembled. The channels also provide a degree of rigidity. Once again, your supplier or builder's merchant should be able to provide information as to how wide a span can safely be covered by this method. Where more sound isolation is required, two layers of woodwool slab may be used with an air‑gap between them.
It is vitally important that the floating floor is properly supported and is rigid enough to support the weight of the walls and ceilings. Failure to ensure this will result in the floor 'crowning' as the floor edges are pushed down, leaving a bulge in the centre. Proper support of the floor often involves using more support material around the edge of the floor, neoprene isolation blocks being a popular choice for smaller installations. If you intend to use a proprietary floating floor material such as Lamella, it is advisable to discuss your requirements with the supplier, who should be able to advise on loading and support. It helps if several layers of flooring chipboard are screwed and glued over the basic floor structure before the wall construction starts, as this will increase both the mass and stiffness of the floor. Figure 1 (above) shows a room-within-a-room built directly onto a floating floor.
An alternative approach, and one that might be safer from a DIY point of view, is not to build the wall frame directly onto the floating floor at all, but instead to support it on felt or neoprene strips laid on the true floor. The floating floor may then be built inside the room with felt or neoprene isolation strip between the edges of the floor and the wall partitions. This approach might be more practical when the room size dictates heavy ceiling joists. Although the theoretical isolation is compromised to some extent because of the increased coupling between the walls and the true floor, this tends not to be serious, though it is wise to ensure that sound‑producing equipment is mounted on the floor and is not in contact with the walls. Figure 2 shows this type of construction. If the underlying floor is not solid, it is imperative you consult an architect to determine whether the floor will safely carry the weight of your inner room.
Though a Lamella floor may again be used in a room of this type, if the underlying floor is solid, it is possible to build a concrete floating floor by using a layer of 30mm Rockwool (around 150kg/cubic metre density) covered by 70mm of lightweight, reinforced concrete, a polythene membrane being laid over the Rockwool before the concrete is poured. The floor may be finished using a layer of concrete screed, as shown in Figure 3.
This is relatively cheap to build and has a higher mass than most simple chipboard floors, which helps improve low frequency isolation. A further advantage is that plastic ducts can be set into the concrete to carry cables from one side of the room to the other.
Where a doorway is required, there is an obvious weak spot in the sound isolation. A double-door construction is essential, one door in the inner room and one in the outer shell, and it is advisable to use barrier mat to help isolate the space between the doors from the void between the inner and outer rooms. Figure 4 shows how this may be achieved.
Either one or both of the doors should be fitted with compression latches and proper door seals fitted, as described in Part 2 of this series. The heavier the door construction, the more effective the sound isolation is likely to be. Similar precautions should be taken when building windows ‑ the inner and outer frames may come close to each other but it is essential that there be some gap between them. Figure 5 shows a practical way of arranging this.
Building a 'room‑within‑a‑room' need not be beyond the scope of the DIY enthusiast, but professional consultation is advised at the planning stage, to ensure the structural safety of the new room as well as of the floor upon which it stands, and that it complies with building regulations. The wider the gap between the inner and outer rooms, the better the low frequency isolation, and if you can integrate any corridors into the studio design in such a way that they also double as air gaps, isolation can be improved even further.
Where two inner rooms must be built inside the same outer room, sound leakage from one room to the other can be a problem. One solution is to use an airtight curtain of barrier mat between the rooms to provide some degree of isolation between the void surrounding one inner room and the void around the other. An alternative approach is to use barrier mat to seal off the void at the point where the two rooms meet, as shown in Figure 6. This allows the adjacent walls to be uprated with extra layers of plasterboard or chipboard to improve inner‑room isolation.
What is important is not that any of the ideas discussed here are carried out to the letter, but that you understand the basic principles involved in sound isolation. Most studio designs use a mixture of ideas and methods to achieve their aims, but the laws of physics always remain the same. Once you have sketched out your initial design, inspect it to find out where the weak spots are, and the chances are that you'll be able to adapt one or more of the techniques I've explained in this series to improve the situation.