ARCHIVED REPORTS_2011 - Laserfiche WebLink

Home Browse Search

A P P E N D I X A FUNDAMENTAL CONCEPTS OF ENVIRONMENTAL NOISE This section provides background information to aid in understanding the technical aspects of this report. Three dimensions of environmental noise are important in determining subjective response. These are: a) The intensity or level of the sound; b) The frequency spectrum of the sound; c) The time-varying character of the sound. Airborne sound is a rapid fluctuation of air pressure above and below atmospheric pressure. Sound levels are usually measured and expressed in decibels(dB),with 0 dB corresponding roughly to the threshold of hearing. The "frequency" of a sound refers to the number of complete pressure fluctuations per second in the sound. The unit of measurement is the cycle per second(cps) or hertz (Hz). Most of the sounds which we hear in the environment do not consist of a single frequency, but of a broad band of frequencies,differing in level. The name of the frequency and level content of a sound is its sound spectrum.A sound spectrum for engineering purposes is typically described in terms of octave bands which separate the audible frequency range(for human beings,from about 20 to 20,000 Hz) into ten segments. Many rating methods have been devised to permit comparisons of sounds having quite different spectra. Surprisingly,the simplest method correlates with human response practically as well as the more complex methods. This method consists of evaluating all of the frequencies of a sound in accordance with a weighting that progressively de-emphasizes the importance of frequency components below 1000 Hz and above 5000 Hz. This frequency weighting reflects the fact that human hearing is less sensitive at low frequencies and at extreme high frequencies relative to the mid-range. The weighting system described above is called "A"-weighting, and the level so measured is called the "A-weighted sound level" or"A-weighted noise level." The unit of A-weighted sound level is sometimes abbreviated "dBA." In practice,the sound level is conveniently measured using a sound level meter that includes an electrical filter corresponding to the A-weighting characteristic. All U.S. and international standard sound level meters include such a filter. Typical sound levels found in the environment and in industry are shown in Figure A-l.