The design and construction of control room observation windows (together with doors) has always proved to be one of the most difficult aspects of studio construction. Windows and doors are the weakest parts of any studio design due to the simple fact that it is not possible to build them from the same materials as the walls, consequently they exhibit an inferior sound isolation characteristic. One sheet of glass is pretty hopeless as a sound attenuator. Sound hits the glass, the glass bends in sympathy and so the sound travels straight through only slightly attenuated.

Sound may be described in its most simple physical form as just a movement of air, therefore it follows that sound cannot be transmitted in a vacuum. Unfortunately, it isn't feasible for the majority of people to construct a studio window containing a vacuum. A far more practical approach, is to build a window from two pieces of glass, these being positioned a few centimetres apart to create an air gap, which forms an 'air spring' and so effectively decouples the direct air movement from one room to another.

The result of this type of construction is to reduce the sound level transmitted to the other side of the window by an acceptable amount but any window you place in a wall will, not unnaturally, degrade the sound attenuation factor. Window size is important; the smaller the window the better the overall performance of the wall. It is, however, quite feasible to construct a 6' x 4' window but square windows should be avoided.

Techniques

Well that's enough of the theory. In practice there are several building techniques which achieve varying amounts of sound attenuation. Each produces a usable window and each design has its place within everybody's budget. Before deciding which design you are going to opt for, however, you must first ask yourself two golden questions: (1) How loud will the sound be on the other side of the control room window? (2) What is the wall surrounding the window made from?

Window design, and therefore cost, can vary enormously, and is dependant more or less on these two factors. If, for example, you wanted to construct a voice overdub studio, then the window need not be too extravagant, so a fairly simple double glass construction would be satisfactory. However, a studio used for recording loud amplified sounds, such as rock music, would require the best possible window construction, and would aim to provide at least 50 dB of sound attenuation.

The materials used for the construction of the wall into which the window is to be built are of primary importance and need to be taken into account. It's no good having a plasterboard wall with a triple glazed window in it! Similarly, the benefits of a double-leafed heavy mass wall structure would be wasted if you installed a simple window made of 6 mm thick glass. A balance between efficiency and cost must be sought. It's the old story of horses for courses.

Design

Once you have decided what or which type of materials your wall is built from you can then proceed to select the correct window design. These generally can be categorised into neat packages in terms of both efficiency and cost.

Figure 1 shows a single sheet of glass in a frame, mounted within a single leaf wall construction. This type of window will only provide approximately 25 dB attenuation and is of little use in studio design.

Figure 2, the double glazed window, probably the most commonly used design, is constructed from two sheets of glass with a 200 mm air gap between them. You can expect about 40 dB attenuation, from such a design.

Figure 3, the final design, is the triple glazed window. Three sheets of glass are used, each being spaced from the other by 200 mm. This design should be used when the best possible attenuation factor is needed, and a triple glazed window should give around 55 dB of sound attenuation. This is about the best figure that can be achieved without going into a very costly design involving a larger space between the double leafed walls.

The attenuation figures discussed are for a best case situation. The windows should be tightly sealed and for those containing air spaces, sound absorbent material should be placed around the inside edges of the frame, known as reveals. The whole of the wood frame should float on a rubber bed and not touch the wall. Ordinary domestic window glass should be avoided, the best results will be obtained from using 9, 10 or 12 mm thick glass. In windows of two and three sheet construction the panes of glass must not be parallel and should be of different thicknesses. It's also common practice to angle the piece of glass on the studio side downwards slightly, which stops the view through the window from being obscured by reflections from studio lights and helps to prevent the build-up of standing waves between the sheets of glass.

Construction

There are several factors common to all the window designs. The wood used to build the frame should be of a 'hardwood' type, this is because hardwood has normally been kiln dried and therefore its moisture content will be similar to that of the air in a normal warm studio environment. Softwoods, like Pine or Deal, have a high moisture content and would over a period of time dry out and warp.

The glass itself should be thick, as previously mentioned, 9, 10 or 12 mm. The larger the window the thicker the glass. For low budget designs you could use 6 mm glass but only in small windows; performance will, however, be affected.

The spacing between the sheets of glass should be around 200 mm, but this may be difficult to achieve when the wall itself isn't very thick. In situations like this, you will either have to reduce the gap between sheets, and hence the attenuation performance, or you may build out from the wall (Figure 4). When installing the glass you should seal all the edges around the frame with a soft mastic window sealing compound. An air tight seal is crucial to the performance of the window.

Two and three sheet glass construction should be installed on double-leafed walls. For double glazed designs, each pane of glass should also have a separate frame, one on each wall. Triple glazed construction is the same but the third pane of glass should be positioned at an angle spanning the two walls.

Having installed a wooden frame, the next step is to fit the glass. There are many ways of doing this, the one described here will give you a guide to the sort of method you should use, remember though that any compromise will probably result in a less efficient performance. Figure 5 shows a detailed section of the design. The key to this method is the use of rubber isolators (the rubber should be of a solid type and exhibit very little compression), these take the weight of the glass and form a cushion for it to float on. Any low bass frequencies which would normally have caused the window to vibrate are effectively dampened by the rubber isolators. If you have difficulty in obtaining the rubber surround for the glass, try using the rubber seal found on car windscreens.

Between The Glass

The space between the double-leaf wall which is seen when the glass is in place, must have some form of filling and finishing to make it presentable and this should obviously be attended to before installing the actual glass. The simplest way to deal with this is to stuff fibreglass wadding into the gaps around the edge, then using a rubber solution glue secure fibreboard or acoustic tiles in place to provide an attractive finish. As you are constructing an airtight seal, the atmosphere inside the window will change little compared to that of the room. Many people advise that you place silica gel crystals inside the window to absorb any moisture, as it is practically impossible to build an absolutely air tight window and the room temperature air will still circulate. It is debatable whether or not this is necessary, but it can, however, do no harm.

That concludes this article. It's well worth experimenting with a design, but if you base your ideas on those shown here you should be able to arrive at a window construction that satisfies both your budget and isolation specifications.