The Sound Transmission Class (STC)
A certain parameter has to be used in order to compare the performance of different construction materials. One such parameter is the sound transmission class(STC). The sound transmission class can be defined as the numerical value equal to the number of decibels in terms of reduction of sound when it passes through a material, which is intended to have some insulating property. This can make one understand that a high value of STC implies the material possess a high insulating property. It basically works on the influence of external frequencies of the areas of the partition where the sound originates and where the receiving of the sound takes place. This can be explained by an example. Let us assume the desired level of sound internally is 45 decibels and the external level of noise is 85decibels, then we require a partition material that has a sound transmission class value of 40 STC for adequate acoustic comfort. STC is a sound transmission class rating which is put forward by the American society of Testing and Measurement. This acts as a parameter to guide an architect to know which material would suit the most, to have an essential acoustic feature by the reduction of noise.Construction Techniques in Acoustic Planning of a Building
The structural elements and their construction for controlling noise in buildings are mentioned below.Construction of Walls for Noise Control in Buildings
Wall are an important structural element in all kind of buildings, that provides protection from the noise externally as well as internally. The usage of different wall materials or the usage of different design for the wall would bring variation in the insulating properties of the element. The figure below shows the difference in sound attenuation with variation in wall design. The methods employed for noise control in buildings are explained in the following:1. Wall Mass and the Thickness are Increased
The massiveness of a material is an efficient parameter that resists noise. Hence concrete walls are more insulating than wooden walls. Another way of increasing the insulating property is to add more thickness for the walls. The increase of thickness of walls would result in more mass which in turn increase insulation. A reduction of 6 decibel sound happens by this method of construction. But the techniques must be carried out keeping in mind the cost and economy. Wall construction that undergoes vibration under huge frequencies of sound has to be avoided.
Fig.1. Different Wall Design Techniques Showing the Variation of Sound Attenuation
2. Use of Cavity Partition in Buildings for Noise Control
The sound transmission can be resisted by the usage of airspace between the two partition walls. The air space can also be placed in between two or more layers. This concept is more effective than a single wall of equal weight, which is found to be more economical.3. Increase Airspace Width of Walls
The increase in airspace will obviously increase the noise insulation property. But the huge increase in the width of air space is difficult to design and consumes more space.4. Increasing the Stud Spacing
It is found by a study that the spacing between the studs would increase the sound transmission capacity of the room. Say an increase of 2 to 5 decibel STC is determined for a stud spaced 24 inches than those spaced at 16 inches.5. Usage of Studs in a Staggered Manner
The arrangement of studs in a staggered manner as shown in figure-1, where studs are placed alternatively would help in noise absorption, thus reducing noise intensity.
Fig.2. Figure Shows Variety of Wall Types, their Cost Comparison and STC Values
6.Studs and Panels held together by Resilient Materials
Making use of inexpensive resilient layers like glass or fiber board, or semi-resilient attachments which are inert in nature, will help in reducing the STC rate by two to five decibels.7. Panels Used are Dissimilar
Using different thickness and materials for panels would help in reduction of noise, thus increasing the sound insulating quality of walls.8.Sound Absorbing Blankets Used in the Airspace
The sound absorbing blankets are also called as isolation blankets which are placed in the airspace arrangement, that are provided between the panels. This blanket enables an increase in sound attenuation. Mineral or rock wool, wood fibers or fiberglass are some the materials used to make these blankets. These blankets have an attenuation capability up to 10 decibels. The method is more effective where lightweight construction is more prominent.9.The Cracks and Edges are Sealed
The full advantage of a high-performance wall can be brought out only when it is properly sealed and crack free. The perimeter of the wall must be properly sealed. Formation of cracks or holes would affect the insulation property of the wall. It has been observed that a hole of the 1-inch square will result in a reduction of STC of the wall by 10.
Fig.3. Depicts Before and After the Sealing of Cracks
Construction of Windows for Acoustic Control in Buildings
Windows are one of the weakest elements of a building. Their inappropriate position or open condition would affect the performance of insulating walls. Therefore, it is recommended to have acoustical consideration in the arrangement of windows. The graph below shows the variation of STC values of the wall, for the different area occupied by the windows which are shown in percentages.
1.Windows can be Closed
Permanent sealing or closing of the windows are the best measures to reduce the direct effect of noise. Permanent sealing becomes essential when an air conditioning system must be enabled. So, sealing acts as a constant solution for noise. The masking of noise is an effect that is facilitated by the air conditioning system, which is discussed in the following topics.2. Windows Size can be Reduced
The loss of contribution of total partitions can be reduced by making the window size to small. Small windows do have certain other advantages like:- Expensive acoustic windows can be excluded
- Usage of glass is reduced