This month, we examine doors and windows, the most vulnerable areas when it comes to sound leakage.
In the case of the home studio, or even a small commercial studio, a 'room within a room' style of construction is generally out of the question, either on the basis of space or of cost. In this case, the only option is to 'uprate' the existing walls, floors, ceilings, doors and windows to achieve an acceptable degree of soundproofing. Of course, this must be coupled with realistic expectations; if you live in a residential area and want to be able to record live music at all hours of the day and night, 'room within a room' construction is pretty essential. But most private studios don't operate 24 hours a day and tend not to use live drums on a regular basis. In this case, uprating the existing structure should be adequate.
The weakest points in a typical room are always the doors and windows, so check these first to see which pose the greatest leakage problems. When all the weak spots have been identified, start by tackling the worst leaks first so that you can get some idea of the degree of improvement as each stage is completed.
Firstly, check to see how well the doors and windows fit when closed. They need to be airtight — it only takes a small gap to allow sound to leak through. Doors can be checked by shining a torch around the edge of the door to see if light leaks through anywhere. In the case of windows, you can usually spot the leaks by feeling for draughts.
Using proper seals to eliminate gaps is a help in reducing airborne sound, but is still only a partial solution; the mass of doors and windows tends to be much lower than that of the walls, which means that sound generated inside the room will cause them to vibrate. And as we know, a vibrating solid object radiates sound. Some form of modification is invariably required to bring about a significant increase in sound isolation.
Internal doors are often very light and offer something like 15dB of sound isolation, which makes them around 30dB worse than a typical wall. Even off‑the shelf fire doors are disappointing from the sound isolation point of view unless used in pairs. The solution is to replace the door with a heavier one or add a heavy layer to the existing door. Two layers of three‑quarter‑inch ply, chipboard or plasterboard fixed to one side of the door will help, but ensure that the frame and hinges will stand the additional weight.
Alternatively, you can make your own doors from scratch, using two or more layers of thick ply or chipboard separated by a small gap, which may be filled with Rockwool, fibreglass or softboard to help deaden any vibration of the panels. Chipboard is not a structurally strong material so ply is a better choice, but chipboard is viable so long as hardwood inserts are used to take the hinges and the latch mechanism. Figure 1 shows two approaches to building your own door. Once the door has been hung, ensure that the seal is airtight all the way round by fitting a proper sealing gasket.
Rather than using commercial draught excluders, the most effective way to seal a door is to use a half‑round neoprene gasket fitted to the door frame (including the bottom), in conjunction with a pressure latch. These work using a tapered plastic wedge screwed to the door frame — as the handle is turned, the door is forced against the seal.
Even the best fitting, heavily‑built double door is unlikely to be as effective as a solid wall in keeping sound in or out; a double door structure fares better. As with walls, the larger the air gap, the better the sound isolation, but where space is limited, it is quite acceptable to use two outward opening doors each side of a single wall. This doesn't take up any more space than a single door, and a typical wall will leave room for an air gap several inches across. If you can arrange for a larger gap by building a vestibule‑style airlock, the low frequency isolation will be even better. If the door spacing is more than a couple of feet, you can use a spring closer or conventional door latch on one of the doors rather than a pressure latch, as the slight reduction in sealing efficiency will have less of an effect than if the doors were very close.
Because sound travels through solid materials, it's best to use separate door frames rather than one wide one. For the very best results, frames should be isolated from the surrounding brickwork and from each other using Neoprene sheeting or pads of compressed Rockwool. This isn't necessary in most domestic environments, but should be considered if you're building from scratch. Figure 2 shows a practical double-door system built into a single wall.
Start with a plain door frame and hang the door in it, using substantial hinges. The door jambs may then be cut to size (the corners are best mitred), and the sealing strip glued on with contact adhesive before fitting the door. Don't forget to use a jamb at the bottom of the door too, to ensure all‑round sealing. With the latch half closed, the jambs should be lightly pinned into place so that the sealing gasket just touches the door all the way round. When the latch is fully closed, the seal will compress just enough to form a good seal. If the gasket is fitted too tightly, the door will be difficult to close and may warp.
When you're happy that the door jambs are in the right place, additional nails or screws are used to make the job permanent. Figure 3 (above) shows how the seals are fitted.
Double‑glazed windows are more effective at keeping out sound than the old wooden sash type, though if retaining the original windows is important, you could consider fitting secondary glazing on the inside or fitting a shutter to block the window entirely. In my early home recording days, I used a removable shutter which was built up from panels in the same way as a DIY door and clamped in place with sash window thumbscrew fasteners so that it could be removed easily. Some form of gasket is essential, and I used ordinary foam draught excluder at the time, as that's all I could get hold of; the half‑round neoprene used for the doors is better. The inside face of the shutter may be covered with carpet or acoustic foam to make it visually attractive and less reflective. Constructional details of this type of shutter are shown in Figure 4.
If you decide to use secondary glazing, use the heaviest grade of glass you can get and leave the largest possible air gap between the original window and the secondary glazing. A viable alternative is to fit a completely new double‑glazed window on the inside of your existing window opening, leaving the original window intact on the outside. However, if you don't mind replacing the window, you could do a lot worse than fitting double‑glazed windows to both the inside and the outside of the window opening. When buying double‑glazed units, ensure that the air gap between the two sheets of glass is at least 25mm. If you use real drums and amplified guitars close to other residents, this approach is worth serious consideration, even though it can be costly to implement.
A typical studio installation makes use of heavy plate glass windows, usually built up from three separate sheets of glass separated by large air gaps. Ideally the glass should be of different thicknesses to prevent resonances and the panes should be mounted so that they are not quite parallel. This latter consideration helps prevent standing waves, though choosing different weights of glass is more important than angling the glass. Some form of acoustic absorber — Rockwool or acoustic foam — will often be incorporated into the window frame to absorb any energy that enters the air gap. The glass should also be mounted on a neoprene gasket to isolate it from the wooden frame. Figure 5 shows how a typical studio window could be built.
Control room windows are usually made up from two or three panes of glass and are constructed in a similar way to the triple‑glazed windows previously described. Plate glass is the preferred choice; it should be mounted on Neoprene in a non‑parallel configuration and with as large an air gap as is practical.
Some studios dispense with the traditional control room window altogether and instead use commercial, double‑glazed patio doors to divide the studio and the control room. This serves a dual role in that it provides a means of access and leaves a large viewing area. Two sets of double‑glazed patio doors must be used, with an air gap between them of at least two feet to provide reasonable low frequency isolation. The walls within the airlock thus formed should be lined with Rockwool or acoustic foam to a depth of at least two inches and covered in fabric such as hessian. Because the walls are likely to receive impacts as equipment is brought in and out of the studio, the underside of the cosmetic fabric may be reinforced with wire mesh or expanded metal grille. Figure 6 shows a patio door air lock.
Even after much work and expense, the leakage through even well designed doors and windows is still likely to be greater than through the walls. This is especially true at low frequencies where both mass and air gaps are required in order to be effective. Even so, the improvement can be dramatic, so even if you have a limited budget for soundproofing, attend to the doors and windows before you do anything else.
- Doors need to be as heavy as possible, with an airtight fit. Even a small gap can let a lot of sound in or out.
- Double door systems significantly outperform single doors.
- Sound also travels through solids, so neoprene sheet should be used to isolate structures such as door and window frames from walls.
- Rockwool stuffed into air gaps will help to damp panels and also absorb a little of the airborne sound trying to cross the gap.
- Aerosol polyurethane foam is useful for filling small gaps and holes. It also doubles as an adhesive, making it ideal for fixing frames in place while maintaining a degree of sound isolation.
- Double‑ or triple‑glazed windows are best with as large an air gap as is possible. Individual panes should be isolated from their frames using a suitable glazing gasket material.
Neoprene sheeting, neoprene half‑round gasket compression latches and many other specialised soundproofing materials may be obtained from: Siderise www.siderise.com
General acoustics materials and kits are sold by many retailers, including Studiospares in London www.studiospares.com