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Acoustic transmission occurs when sound waves pass through a barrier or material, resulting in noise on both sides of the barrier. Even seemingly solid or dense materials provide plenty of openings for sound waves to pass through. This phenomenon can be good or bad, depending on the application. For example, unwanted noise from neighbors can be an annoyance, while acoustic transmission in a recording studio may be desirable in terms of creating the best sound quality.
Engineers and builders often incorporate special techniques and materials into their projects to control acoustic transmission levels. These professionals rely on sound transmission class (STC) ratings, to help them achieve the desired level of noise reduction. The STC rating of a material or structure measures the transmission loss as sound waves pass through at a specific level of frequency. Many cities require walls of homes and buildings to meet a minimum STC rating in order to maintain a comfortable environment for residents.
The higher the STC rating of a material, the more effective it will be at controlling acoustic transmission. Manufacturers test products to provide information of the STC rating, though these ratings only hold if materials are used correctly. Stone materials like concrete and brick offer some of the highest STC ratings for walls, while insulation can help reduce acoustic transmission through wood or metal-framed walls. Even doors and windows have their own acoustic transmission ratings, though special seals or weatherstripping can help control noise transfer.
Both in construction and other fields, designers have three basic ways to control acoustic transmission. The first is to use materials that absorb sound waves, such as cotton or fiberglass insulation. Another method is to use mass to deaden sound. An example of this involves using solid concrete blocks to build a home rather than standard wood framing. Finally, designers can use an air space to separate two areas, which helps to reduce the amount of acoustic transmission between the two regions.
Acoustic transmission poses a number of problems for people in many applications. In homes and businesses, noise from traffic or mechanical systems can interfere with sleep, meetings, or other activities. Even noise transfer from one room to the next within a building can hinder privacy, which pose can significant problems in health care and business. In schools, noise can interfere with teaching and learning. Finally, even noise in theaters and music studios must be controlled to minimize the impact of unwanted sounds in surrounding spaces.