Acoustical Engineering - Environmental Acoustics

Environmental Acoustics

Environmental acoustics are concerned with the control of sound and vibrations in an outdoor environment. This includes sounds generated by traffic, aircraft, industrial equipment, and anything else that might be considered a nuisance or a safety concern. Engineers concerned with environmental acoustics face the challenge of determining an acceptable level of noise and how noise can be controlled.

Accurately determining appropriate criteria for evaluating noise is often difficult due to the unsteadiness in level and spectral content of most environmental noises. Although qualitative assessments such as speech interference evaluations by trained speakers and listeners can be useful, accurate quantitative measurements are often more difficult to obtain. To address this problem, a wide variety of rating systems that simplify analysis are used for different applications. Common systems include:

• Equivalent Level
• Day–Night Average Level
• Community Noise Equivalent
• Noise and Number Index
• Noise Exposure Forecast (for aircraft noise)
• Composite Noise Rating (for aircraft noise)

Most systems simplify analysis by taking mean sound levels and by relying on what is known as the A-weighted sound level, which assigns to each frequency a weight that is related to the sensitivity of the human ear. These standardized measurements are often sufficient. Otherwise the analysis specific to a given situation is usually complex.

Environmental noise control often involves the creation of noise barriers and use of buffer zones. An Example of noise barriers are the walls built between highways and residential areas. Barriers are less effective in reducing high frequency noise due to the way sound diffracts around walls, but can reduce the overall loudness of traffic noise by as much as one half. Noise buffer zones are also an effective and simple method of noise control. Buffer zones group areas of high-level noise together and surround them with areas of lower-level noises. For example, an airport would not be surrounded immediately by residential properties, but rather by industrial and commercial properties. These two methods are perhaps the most common examples of outdoor noise control.