One of the major issues that the general public frequently raises regarding aircraft operations is the potential for noise impact. The definition of noise is as follows: provided as well as the various ways that sound levels can be measured. In addition, the potential areas of public impact due to elevated sound levels are described. Lastly, various means to mitigate aircraft noise are discussed. Noise and Sound The terms noise and sound are used interchangeably. Aircraft noise is unwanted sound; noise level is defined in the same way as sound level. Decibels Sound level is defined in terms of mean square sound pressure L = 10 lg[p2rms/p2ref] where L = sound level, decibels (dB) lg = base 10 logarithm p2rms = mean square sound pressure pref = reference pressure = 2 × 10-5 Pa Human perception of sound varies according to the frequency content. This effect can be accounted for by using A-weighting and measuring and predicting sound levels in A-weighted decibels (dBA). A-weighting A-weighting is based on typical human hearing of soft sounds. It discounts low-frequency sounds. It may sometimes underestimate the effect of aircraft noise, partially due to the ability of longer waves to bend and penetrate walls and barriers. That is, A-weighting is not perfect. However, slow-setting (~1 second rise time) A-weighting is almost universally accepted. In most cases it correlates well enough with actual transportation noise impact. Day-Night (Sound) Level (Ldn or DNL) Day-night (sound) level is a descriptor that recognizes the added impact of nighttime noise. It is a 24 hour Leq based on A-weighting with 10 dBA added between the hours of 10:00 p.m. to 7:00 a.m. DNL is an accepted descriptor of environmental noise when sleep-interference is a factor. Community noise impact is commonly described by DNL contours. See Appendix 1 for the calculation of: Day-night (sound) level based on measured or predicted noise levels Day-night (sound) level based on sound exposure levels Combining Noise Contributions Noise energy is proportional to mean square pressure. To combine the noise contributions from several sources noise levels are converted to mean square pressure. The noise level due to N contributions is Lcombined = 10 lg[10L1/10 + 10L2/10 + …… +10LN/10] where L1, etc = contributions of individual sources. When two equal noise contributions occur simultaneously, the combined noise level is about 3 decibels higher than the level of a single contribution. Equivalent Sound Level (Leq) When noise levels vary with time, they are averaged as follows: Leq = 10 lg[(1/T) ò 0T 10L/10dt] where Leq = equivalent sound level (dBA), the energy average of A-weighted sound over a given time interval L = time-varying noise (sound) level (dBA) T = averaging time Equivalent sound level is the appropriate metric for examining the impact of noise on schools and similar receptors where sleep-interference is not a factor. Note that noise events are not to be compared with averages. Sound Exposure Level Sound exposure level (SEL) is an energy-time integral used to represent an aircraft flyover. SEL predictions are based on representative noise measurements from aircraft flyovers corrected for altitude, aircraft type, modifications (re-engining, Hushkits), and operation (takeoff or landing). |