Noise – Far Field
Noise is defined as unwanted sound that interferes with normal human activities or otherwise diminishes the quality of the environment. Noise associated with military systems (e.g., aircraft, tanks, ships, generators, auxiliary power units, etc.) is one of most pervasive environmental noise to communities surrounding DoD installations. Although many other sources of noise are present in the surrounding area(s), noise is readily identifiable depending on the type of system and the proximity of its operations to personnel and the community.
Noise impacts associated with military systems and their operations are analyzed from both physiological and behavioral perspectives. Common impacts from noise include annoyance, speech interference, sleep disturbance, and effects on domestic animals and wildlife. This is most evident, for example, with aircraft operations. If the acquisition of a system is likely to generate noise during operation, then determining the noise footprint of the system is critical as part of the design and systems engineering process, especially during technological development, and engineering and manufacturing development. The system’s noise attributes and footprint are used in hazard/risk identification (far-field) and National Environmental Policy Act/Executive Order 12114, Environmental Effects Abroad of Major Federal Actions, analyses.
Noise Metrics
Noise from individual events of a system’s operation, as well as cumulative sound levels, can be important in determining the effects of noise. Measurements and descriptions of noise (i.e., sounds) are usually based on various combinations of the following factors:
- the vibration frequency characteristics of the sound, measured as sound wave cycles per second (Hertz [Hz]); determines the pitch of the sound
- the total sound energy radiated by a source; usually reported as a Sound Power Level (SWL)
- the actual air pressure changes at a particular location; usually measured as a Sound Pressure Level (SPL); the frequency characteristics and SPL combine to determine the loudness of a sound at a particular location
- the duration of a sound
- the changes in frequency characteristics or pressure levels through time
The measurement and human perception of sound involves two physical characteristics–intensity and frequency. Intensity is a measure of the strength or magnitude of the sound vibrations and is expressed in terms of pressure – the higher the sound pressure, the more intense the perception of that sound. The frequency of the sound is the number of times per second the sound oscillates. Low frequency sounds are characterized as a rumble or roar, while sirens or screeches typify high frequency sounds. Sound intensity is measured in terms of sound levels ranging from zero Decibels (dB), which is approximately the threshold of hearing, to 130 dB, which is the threshold of pain for humans.
Far-Field Noise Considerations
The cumulative effects of far-field noise can create potentially adverse or even hazardous noise levels at nearby noise-sensitive receptors (e.g., residential neighborhoods, churches, parks, graveyards, schools, etc.). Depending on the extent of the environmental impact from noise generated during system operations, funding for noise mitigation to reduce the exposure risk for nearby noise-sensitive receptors can increase the cost of the acquisition.
Far-field noise is analyzed by calculating noise exposure contours for system operations (e.g., military airspace operations, construction operations, tank maneuvers, firing range activities, etc.). To quantify these effects, the DoD typically uses three noise measuring techniques, or metrics
- first, a measure of the highest sound level occurring during an individual noise event
- second, a combination of the maximum level of that single event with its duration
- third, a description of the noise environment based on the cumulative operational activity
Far-field noise levels can be measured using single-event noise levels for a system that is used infrequently or for limited durations (described as SPL or maximum sound level). Another measure of instantaneous level is the peak SPL. When the operation of a system is on a more sustained basis, then the following metrics are used – (1) Day-Night Average Sound Level (Ldn or DNL) or (2) Community Noise Exposure Level (CNEL) for operations conducted in California. As used in environmental noise analyses, a metric refers to the unit or quantity that quantitatively measures the effect of noise on the environment. These cumulative noise metrics (Ldn and CNEL) consolidate an average day’s noise into a single noise level for an area, while “penalizing” operations that occur at night due to higher annoyance levels during night-time hours. Results are expressed using a dB scale. Potential impacts (e.g., land use designations, populations including ethic and poverty considerations, acreage, biological resources, etc.) can then be assessed based on the noise sources.
Recommendations and considerations in the 1992 Federal Interagency Committee on Noise (FICON) report, Federal Agency Review of Selected Airport Noise Analysis Issues, are typically used in determining far-field noise impacts. Examination of noise levels between 60 dB and 65 dB Ldn/CNEL is performed based on the FICON recommendations. The FICON screening components are as follows:
1) noise exposure contours at the 75 dB, 70 dB, and 65 dB Ldn/CNEL; additional contours optional and considered on a case by case basis
2) analysis within the proposed alternative 65 dB Ldn/CNEL contour to identify noise sensitive areas where noise would increase by 1.5 dB Ldn/CNEL; increases of 1.5 dB introducing new noise sensitive areas to exposure levels of 65 dB or more are included in the analysis
3) analysis within the 60 to 65 dB Ldn/CNEL contours to identify noise sensitive areas where noise would increase by Ldn/CNEL 3.0 dB; only when documented increase of 1.5 dB Ldn/CNEL within the 65 dB Ldn/CNEL contour
The acquisition program manager should consult with ESOH practitioners when estimating the noise footprint of a system given the potential complexity of the analysis process. If significant environmental impacts to the community and noise sensitive receptors are identified, then potential mitigation options will need to be evaluated and resourced. In some instances, options may be eligible for Federal funding. The program office needs to engage with the Command-level Environmental, Public Affairs, and Legal Offices for any decisions regarding noise mitigation measures associated with far-field noise impacts and for communication with the surrounding communities at DoD installations that may be affected by noise-producing operations of acquisition systems.
