Sound is mechanical energy transmitted by pressure waves in a compressible medium such as air.  Noise is generally defined as unwanted or excessive sound.  Sound can vary in intensity by over one million times within the range of human hearing.  Therefore, a logarithmic scale, known as the decibel scale (dB), is used to quantify sound intensity and compress the scale to a more manageable range.

Sound is characterized by both its amplitude and frequency (or pitch).  The human ear does not hear all frequencies equally.  In particular, the ear deemphasizes low and very high frequencies.  To better approximate the sensitivity of human hearing, the A-weighted decibel scale has been developed.  A-weighted decibels are abbreviated as “dBA.”  On this scale, the human range of hearing extends from approximately 3 dBA to around 140 dBA.  As a point of reference, the figure below includes examples of A-weighted sound levels from common indoor and outdoor sounds. 


Typical Outdoor and Indoor Noise Sources


Using the decibel scale, sound levels from two or more sources cannot be directly added together to determine the overall sound level.  Rather, the combination of two sounds at the same level yields an increase of 3 dBA.  The smallest recognizable change in sound level is approximately 1 dBA.  A 3-dBA increase is generally considered perceptible, whereas a 5-dBA increase is readily perceptible.  A 10-dBA increase is judged by most people as an approximate doubling of the perceived loudness.

Two of the primary factors that reduce levels of environmental sounds are increasing the distance between the sound source and the receiver and having intervening obstacles, such as walls, buildings or terrain features, that block the direct path between the sound source and the receiver.  Factors that act to increase the loudness of environmental sounds include the proximity of the sound source to the receiver, sound enhancements caused by reflections, and focusing caused by various meteorological conditions.

Brief definitions of the measures of environmental noise are:

·         Equivalent Sound Level (Leq):  Environmental sound fluctuates constantly.  The equivalent sound level (Leq), sometimes referred to as the energy-average sound level, is the most common means of characterizing community noise.  Leq represents a constant sound that, over the specified period, has the same sound energy as the time-varying sound.  The WLAC noise monitors currently measure sound in 15 second intervals and these are used to calculate the 1-hour Leqs.

·         Day-Night Sound Level (Ldn):  Ldn is basically a 24-hour Leq with an adjustment to reflect the greater sensitivity of most people to nighttime noise.  The adjustment is a 10-dB penalty for all sound that occurs between 10 p.m. and 7 a.m.  The effect of the penalty is that, when calculating Ldn, any event that occurs during the nighttime is equivalent to ten of the same event during the daytime.  Ldn is the most common measure of total community noise over a 24-hour period.

·         Community Noise Equivalent Level (CNEL):  CNEL is similar to the Ldn but includes additional adjustments to reflect the greater sensitivity of most people to noise during the evening (7 PM to 10 PM).  In addition to the 10-dBA penalty for noise between 10 p.m. and 7 a.m., the CNEL includes a 5 dBA penalty for noise from 7 p.m. to 10 p.m.  The effect of the penalties are that in the calculation of CNEL, an event that occurs during the evening hours is equivalent to three of the same events during the daytime hours and an event in the nighttime hours is equivalent to ten of the same event during the daytime hours. 

·         Work Hours Sound Level:  The work hours sound level is effectively a sound level based on the hours that construction is expected to be active.  For weekdays Monday through Friday, it consists of the Leq for the period between 8 a.m. and 6 p.m.  For Saturdays, it consists of the Leq for the period between 9 a.m. and 5 p.m.  Construction does not normally take place on Sunday.