ACOUSTICAL CONSTRUCTION CONCEPTS

(This Section, with some editing, is taken from the Audible Landscape, FHWA¹.)

Noise can be intercepted as it passes through the walls, floors, windows, ceilings and doors of a building. Examples of noise-reducing materials and construction techniques are described in the pages that follow.

To compare the insulation performance of alternative constructions, the Sound Transmission Class (STC) is used as a measure of a material’s ability to reduce sound. Sound Transmission Class is equal to the number of decibels a sound is reduced as it passes through a material. Thus, a high STC rating indicates a good insulating material. It takes into account the influence of different frequencies on sound transmission, but essentially the STC is the difference between the sound levels on the side of the partition where the noise originates and the side where it is received. For example, if the external noise level is 85dB and the desired internal level is 45dB, a partition of 40 STC is required. The Sound Transmission Class rating is the official rating endorsed by the American Society of Testing and Measurement. It can be used as a guide in determining what type of construction is needed to reduce noise.

WALLS

Walls provide building occupants with the most protection from exterior noise. Different wall materials and designs vary greatly in their sound insulating properties. Figure 20.520-2 shows a sample of wall types ranging from the lowest to the highest sound insulation values.

Remember that the effectiveness of best wall construction will be substantially reduced if vents, mail slots or similar openings are permitted in the walls. If vents are permitted the ducts must be specially designed and insulated to make sure noise does not reach the inside. The best approach is simply to eliminate all such openings on affected walls

WINDOWS

Sound enters a building through its acoustically weakest points, and windows are one of the weakest parts of a wall. An open or weak window will severely negate the effect of a very strong wall. Whenever windows are going to be a part of the building design, they should be given acoustical consideration. Figure 20.520-2 illustrates the effects of windows on the sound transmission of walls. For example, if a wall with an STC rating of 45 contains a window with an STC of 26 covering 30 % of its area, the overall STC of the composite partition will be 35, a reduction of 10dB.

The first step in reducing unwanted sound is to close and seal the windows. The greatest amount of sound insulation can be achieved if windows are permanently sealed. However, operable acoustical windows have been developed which are fairly effective in reducing sound. Whether or not the sealing is permanent, keeping windows closed necessitates the installation of mechanical ventilation systems. The smaller the windows, the greater the transmission loss of the total partition of which the window is a part. Reducing the window size is a technique that is used because: (a) it precludes the cost of expensive acoustical windows; and, (b) it saves money by cutting down the use of glass. The problems with this technique are: (a) it is not very effective in reducing noise; e.g., reducing the proportion of window to wall size from 50 % to 20 % reduces noise by only 3 dB; and, (b) building codes require a minimum window to wall size ratio. If ordinary windows are insufficient in reducing noise impacts in spite of sealing techniques, thicker glass can be installed. In addition, this glass can be laminated with a tough transparent plastic that is both noise and shatter resistant. Glass reduces noise by the mass principle; that is, the thicker the glass, the more noise-resistant it will be. A 1/2” thick glass has a maximum STC rating of 35dB compared to a 25dB rating for ordinary 3/16” glass. However, glass thickness is only practical up to a certain point, when STC increases become too insignificant to justify the cost. For example, a 1/2” glass can have an STC of 35; increasing the thickness to ¾” only raises the STC to 37. However, a double glass acoustical window consisting of 2- 3/16” thick panes separated by an airspace will have an STC of 51 and can cost less than either solid window.

In addition to thickness, proper sealing is crucial to the success of the window. To prevent sound leaks, single windows can be mounted in resilient material such as rubber, cork or felt.

Double-glaze windows are paired panes separated by an airspace or hung in a special frame. Generally, the performance of the double-glazed window may be increased with: (a) increased airspace width; (b) increased glass thickness; (c) proper use of seals; (d) slightly dissimilar thickness of the panes; and, (e) slightly nonparallel panes.

In general, the airspace between the panes should not be less than 2-4” if an STC above 40 is desired. If this is not possible, a heavy single-glazed window can be used. The use of slightly nonparallel panes is a technique employed when extremely high sound insulation is required, such as in control rooms of television studios.

The thickness of double-glazed panes may vary from 1/8” to ¼” or more per pane. Although thickness is important, the factors that most determine the noise resistance of the window are the use of sealant and the width of the airspace.

As in the case of all windows, proper sealing is extremely important.

DOORS

Acoustically, doors are even weaker than windows, and more difficult to treat. Any door will reduce the insulation value of the surrounding wall. The common, hollow core wood door has an STC rating of 17dB. Taking up about 20 % of the wall, this door will reduce a 48 STC wall to 24 STC. To reduce noise, a hollow-core door can be replaced by a heavier solid-core wood door that is well sealed and is relatively inexpensive. A solid-core wood door with vinyl seal around the edges and carpeting on the floor will reduce the same 48 STC wall to only 33dB.

The alternative solution to doors is to eliminate them whenever possible from the severely affected walls and place them in more shielded walls.

In any case, no mail slots or similar openings would be allowed in exterior doors.

ROOFS

Acoustical treatment of roofs is not usually necessary unless the noise is extremely severe or the noise source is passing over the building. The ordinary plaster ceiling should provide adequate sound insulation except in extremely severe cases. An acoustically-weak roof which is likely to require treatment is the beamed ceiling. Beamed ceilings may be modified by the addition of a layer of fiberglass or some other noise resistant material. Suspended ceilings are the most effective noise reducers but they are also the most expensive.

REFERENCES

1 U.S. Department of Housing and Urban Development, A Study of Techniques to Increase the Sound of Insulation of Building Elements, Report NO. WR 73-5, Washington, D.C., June, 1973.

2 Ibid.

3 D.E. Bishop and P.W. Hirtle, "Notes on the Sound Transmission Loss of Residential-Type Windows and Doors," Journal of the Acoustical Society of America, 43:4 (1968).

4 U.S. Gypsum, Sound Control...p. 100.

5 Ibid p. 15.