§ 256.10 - Air installations compatible use zone noise descriptors.

(a) Composite Noise Rating (CNR) and Noise Exposure Forecast (NEF) values as previously required by Sections III., IV., and V. of DoD Instruction 4165.57, “Air Installations Compatible Use Zones,” July 30, 1973 1 will no longer be used.

(b) Where CNR 100 (or the quietest boundary of CNR Zone 2 if otherwise computed) or NEF 30 would previously have been used, data shall be collected sufficient to permit computation of Ldn 65 noise contours and these noise contours shall be plotted on maps accompanying AICUZ studies.

(c) Where CNR 115 (or the boundary of CNR Zone 3 if otherwise computed) or NEF 40 would previously have been used, data shall be collected sufficient to permit computation of Ldn 75 noise contours and these noise contours shall be plotted on maps accompanying AICUZ studies.

(d) Where previous studies have used CNR or NEF, for meters of policy, noise planning and decisionmaking, areas quieter than Ldn 65 shall be considered approximately equivalent to the previously used CNR Zone 1 and to areas quieter than NEF 30. The area between Ldn 65 and Ldn 75 shall be considered approximately equivalent to the previously used CNR Zone 2 and to the area between NEF 30 and NEF 40. The area of higher noise than Ldn 75 shall be considered approximately equivalent to the previously used CNR Zone 3 and to noise higher the NEF 40. The procedures shall remain in effect only until sufficient data to compute land values can be obtained.

(e) When computing helicopter noise levels using data collected from meters, a correction of +7db shall be added to meter readings obtained under conditions where blade slap was present until and unless matters are developed which more accurately reflect true conditions.

(f) Noise contours less than Ldn 65 or more than Ldn 80 need not be plotted for AICUZ studies.

(g) Since CNR noise levels are not normally directly convertible to Ldn values without introducing significant error, care should be exercised to assure that personnel do not revise previous studies by erroneously relabeling CNR contours to the approximately equivalent Ldn values.

(h) Where intermittent impulse noises are such as are associated with bombing and gunnery ranges are of importance such noises will be measured using standard “C” weighing of the various frequencies to insure a description most representative of actual human response.