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Appendix A Acoustical Terminology Acoustics The science of sound. Ambient Noise The distinctive acoustical characteristics of a given space consisting,of all noise sources audible at that location. In many cases,the term ambient is used to describe an existing or pre-project condition such as the setting in an environmental noise study. Allenuation The reduction of an acoustic signal. A-Weighting Afiequency-response adjustment of a sound level meter that conditions the output signal to approximate _ human response. Decibel or dB Fundamental unit of sound,A Bell is defined as the logarithm of the ratio of the sound pressure squared over the reference pr sswc:squared. A Decibel is one-tenth of a Bell. CNEL Community Noise Equivalent Level. Defined as the 24-hour average noise level with noise occurring during evening hours(7-10 p.m.)weighted by a factor of three and nighttime hours weighted by a factor of 10 prior to averaging. Frequency The measure of the rapidity of alterations of a periodic signal,expressed in cycles per second or hertz Ldu Day/Night Average Sound Level. Similar to CNEL but with no evening weighting. Leq Equivalent or energy-averaged sound level. Lmax The highest root-mean-square(RMS)sound level measured over a given period of time. L(n) The sound level exceeded a described percentile over a measurement period.For instance,an hourly L50 is the sound level exceeded 50%of the time during the one hour period. Loudness A subjective term for the sensation of the magnitude of sound. Noise Unwanted sound. Peak Noise The level corresponding to the highest(not RMS)sound pressure measured over a given period of time. This term is often confused with the"Maximum'level,which is the highest RMS level. RT„ The time it takes reverberant sound to decay by 60 dB once the source has been removed. Sabin The unit of sound absorption. One square Coot of material absorbing 100'/6 of incident sound has an absorption of 1 cabin. SF:I, A rating,in decibels,of a discrete event,such as an aircraft Flyover or train passby,that compresses the total sound energy into a one-second event. Threshold of I learing The lowest sound that can be perceived by the human auditory system,generally considered to be 0 dB for persons with perfect hearing. "I reshold of fain Approximately 120 dB above the threshold of hearing. Impulsive Sound of short duration,usually less than one second,with an abrupt onset and rapid decay. Simple'fone Any sound which can bejudged as audible as a single pitch or set of single pitches. i-c. brennan & associates 1�consultants in acoustics