Metropolitan Sytems Information Statement 1989

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BASIC CONCEPTS IN AIRCRAFT NOISE Noise is commonly defined as "unwanted sound." Aircraft noise is unwanted sound emanating from aircraft operations. DEFINITIONS Decibel: This is a measure of relative sound pressure in the atmosphere referenced to an'arbitrary standard pressure. Decibel is abbreviated as "dB". Frequency Spectrum: Sound energy occurs at a wide range of frequencies, with those normally perceptible by the human ear between 20 Hz (cycles per second) and 20,000 Hz. A separate sound level is associated with each of these separate frequencies. Combining these into "weighted" decibel scales provides a measure of overall sound pressure. A -Weighted Decibel: This is a weighted sum of spectral sound energy in which each frequency is given a weight similar to that perceived by the human ear. Thus, the A -weighted decibel (or dBA) is commonly used as the measure of community noise impact. Addition of decibels: This is a logrithmic ratio of measured to reference sound pressure. The addition of two equal sound levels yields a level 3 decibels greater; a sound level 10 decibels below another contributes nothing to the overall sound level. Noise Level Reduction (NLR): This term expresses the effectiveness of a structure in reducing exterior noise level. Generally speaking, it is the difference between noise outside of a building and noise inside the building during the same time period. EFFECT OF NOISE Noise can have a wide range of impacts on persons exposed to levels that are not wanted or anticipated. The three primary effects normally used to establish acceptable interior noise levels are: sleep interference, sleep awakening and annoyance. The acceptable interior levels established by the Metropolitan Council (see Table C-1) are intended to provide reasonable protection against these effects. THE SOUND SPECTRUM Room absorption differs from sound transmission loss in that it dissipates rather than stops acoustical energy. A material that is excellent for sound transmission loss (e.g., steel door) is a very poor absorber. A material that is an excellent absorber (e.g., acoustical fiberglass) has a very low sound transmission loss. The effect of absorption in a receiving room is that it increases the effective sound transmission loss of a partition because the energy passing through the partition is partly absorbed. For example, the same wall located in a living room (with greater -than -average absorption) and a kitchen (with less -than - average absorption) could have a difference in effective sound transmission loss of as much as 4 dB. 40