Noise 

Noise is defined as any sound that is undesirable because it interferes with communication, is intense enough to damage hearing, diminishes the quality of the environment, or is otherwise annoying. Human response to noise varies by the type and characteristics of the noise source, distance from the source, the person’s own sensitivity, and time of day. Noise can be intermittent or continuous, steady or impulsive, and it may be generated by stationary or mobile sources. Sound levels are expressed in decibels (dB), usually weighted for human hearing (dBA). To present “average” sounds on a 24-hour basis, the day-night sound level (DNL) method is used. The DNL metric is recommended by the U.S. Environmental Protection Agency and is the Department of Defense’s accepted metric to measure environmental noise (USACHPPM 2006). DNL is used by most Federal agencies as a land-use planning tool for predicting areas of potential annoyance both inside and outside an Installation.

DNL describes the average daily acoustic energy over an entire year, meaning that the whole spectrum of sound, from quiet to loud noises, is averaged across the year. The DNL metric also incorporates a “penalty” for nighttime noise (normally 10:00 p.m. to 7:00 a.m.) when loud sounds are more noticeable and annoying. The weighted measurements screen out the very high and low sound frequencies that cannot be heard by humans.  A-weighted noise measurements reflect what people hear, noted as dBA or ADNL.  A-weighting is typically applied to measuring noise for small arms activities. For low-frequency sounds that can cause vibrations, a C-weighting metric is used, noted as dBC or CDNL. Many find that these lower-frequency sounds like artillery and explosions are more annoying than other noises so that is taken into account in this metric.  Occasionally, “unweighted” peak sound levels or maximum sound levels are used to assess maximum noise levels during single-noise events such as large-caliber weapons deployment. Unweighted peak measurements, with no time averaging, are a good predictor of complaints (USACHPPM 2006); however, they are not typically used in land-use planning nor do they indicate noise exposure over a given time.

To assist communities outside Post boundaries in land-use planning and zoning, the Army has identified three planning categories or zones associated with noise level contours: Zone I, Zone II, and Zone III. The paragraphs and the table below present these zones and the types of activities that are considered compatible within these zones (USACHPPM 2006).

• Zone I – includes all areas around a noise source in which DNL is less than 65 dBA or 62 dBC. This area is usually suitable for all types of land-use activities (e.g., homes, schools and hospitals).
• Zone II – consists of an area where the DNL is between 65 and 75 dBA or 62 and 70 dBC. Exposure to noise within this area is normally incompatible with noise-sensitive land uses (residences, hospitals, churches, educational facilities) and use of the land within the zone should normally be limited to activities such as industrial, manufacturing, transportation, and resource production (e.g., industrial parks, factories and highways).  In situations where noise-sensitive land uses occur within Zone II, guidance recommends noise level reduction features be incorporated in design and construction.
• Zone III – is an area around the source of noise in which the DNL is greater than 75 dBA or 70 dBC. The noise level within this zone is considered incompatible with noise-sensitive land uses, such as churches, schools, parks, playgrounds, residences, and hospitals.

Noise Zones and Compatibility

The affected environment for the noise analysis includes those areas exposed to IBCT construction noise as well as noise generated from on-Post sources (such as training ranges).  The figure below illustrates the C-weighted existing noise contours (in CDNL or dBC) at Fort Stewart (Coastal Georgia Regional Development Center 2005); currently residential areas are affected by these impulsive Noise Zone II levels and under all alternatives (including no action), most of the facilities and ranges would be established within the existing Noise Zone II contours.

Existing Fort Stewart Noise Contours