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Page 115
Suggested Citation:"Definitions." National Academies of Sciences, Engineering, and Medicine. 2016. Evaluating Methods for Determining Interior Noise Levels Used in Airport Sound Insulation Programs. Washington, DC: The National Academies Press. doi: 10.17226/23473.
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Page 115
Page 116
Suggested Citation:"Definitions." National Academies of Sciences, Engineering, and Medicine. 2016. Evaluating Methods for Determining Interior Noise Levels Used in Airport Sound Insulation Programs. Washington, DC: The National Academies Press. doi: 10.17226/23473.
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Page 116

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115 A-Weighted Sound Level: A standard frequency weighting that filters the microphone signal in a manner that compares relative loudness of various sounds. A-weighting is standardized by ANSI. A 10-dB increase in sound level is generally perceived to be approximately twice as loud. All noise data in this report are A-weighted. Community Noise Equivalent Level (CNEL): A metric used by the state of California for the 24-hour A-weighted average noise level. The CNEL accounts for the increased sensitivity of people to noise during the evening and nighttime hours. From 7 pm to 10 pm, sound levels are penalized by 5 dB; from 10 pm to 7 am, sound levels are penalized by 10 dB. Day–Night Average Noise Level (DNL): A metric established by the U.S. EPA to describe the average day–night level with a 10 dB penalty applied to noise occurring during the night- time hours (10 pm to 7 am) to account for the increased sensitivity of people during sleeping hours. Decibel (dB): A logarithmic unit used in acoustics to describe the magnitude of a sound with respect to a reference sound level. The term “sound level,” “noise level” and “SPL” all imply a standardized reference level near the threshold of human hearing (0 decibels). Hertz (Hz): The rate or frequency of air pressure fluctuations called sound. One hertz is equivalent to one complete cycle of pressure variation per second. One kilohertz (kHz) is 1,000 cycles per second. Noise Reduction Coefficient (NRC): A measure of the acoustical absorption performance of a material, calculated by averaging its sound absorption coefficients at 250, 500, 1000 and 2000 Hz, expressed to the nearest integral multiple of 0.05. Octave band: An octave band is a frequency band where the highest frequency is twice the lowest frequency. For example, an octave filter with a center frequency of 1 kHz has a lower frequency of 707 Hz and an upper frequency of 1.414 kHz. OITC: Outdoor-indoor transmission class as outlined in ASTM E1332. The OITC spectrum (curve) is based upon an average of aircraft takeoff, train, and vehicular noise sources. Resonance: A resonance occurs at a certain frequency when the system response at that fre- quency is significantly higher than at other frequencies. It is somewhat like the squeal heard when a microphone is brought too close to a loudspeaker. In a room, one way to create resonances is for the distance between two parallel walls to be half the wavelength or integer multiples of the wavelength of the sound. Definitions

116 Evaluating Methods for Determining Interior Noise Levels Used in Airport Sound Insulation Programs Sound Transmission Class (STC): The sound transmission class is a single number rating describing the attenuation of sound through building partitions. Sound attenuation properties called TL are measured at a minimum of 16 continuous frequency bandwidths in one-third octaves, primarily through the speech range. The STC rating is derived by fitting a standard curve to the measured data as prescribed by ASTM Standard E413. Spatial Average: Spatial Average refers to the act of manually moving the microphone in front of the façade so as to measure the sound field across the plane of the building façade (i.e., at all points).

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TRB's Airport Cooperative Research Program (ACRP) Report 152: Evaluating Methods for Determining Interior Noise Levels Used in Airport Sound Insulation Programs provides guidance for selecting and implementing methods for measuring noise level reduction in dwellings associated with airport noise insulation programs. The report complements the results of ACRP Report 89: Guidelines for Airport Sound Insulation Programs.

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