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A-Weighted Sound Level 
The sound pressure level in decibels as measured on a sound level meter using the A-weighting network.  A-weighted sound levels are designated dB(A) or dBA.
Day-Night Average Sound Level (Ldn)
The 24-hour energy average of the A-weighted sound pressure level, with the levels during the period 10:00 p.m. to 7:00 a.m. the following day increased by 10 dBA before averaging.
Decibel (dB)
A unit for measuring the volume of a sound, equal to 20 times (the logarithm to the base 10 of the ratio of the pressure of the sound measured to the reference pressure, which is 20 micropascals (20 micronewtons per square meter).
Equivalent A-Weighted Sound Level (Leq)
The constant sound level that in a given situation and time period, conveys the same sound energy as the actual time-varying A-weighted sound. 
Impulsive Sound
Sound of short duration, usually less than one second, with an abrupt onset and rapid decay.  Examples of sources of impulsive sound include explosions, drop forge impacts, and the discharge of firearms.
Any sound which annoys or disturbs humans or which causes or tends to cause an adverse psychological or physiological effect on humans.
Pure Tone
Any sound which can be distinctly heard as a single pitch or a set of single pitches. 
RMS Sound Pressure
The square root of the time averaged square of the sound pressure, denoted P(rms)
An oscillation in pressure, particle displacement, particle velocity or other physical parameter, in a medium with internal forces that causes compression and rarefaction of that medium. The description of sound may include any characteristic of such sound, including duration, intensity and frequency.
Sound Level
The weighted sound pressure level obtained by the use of a sound level meter and frequency weighting network, such as A or C, as specified in the American National Standards Institute specifications for sound level meters (ANSI S1.4), or the latest approved revision thereof. 
Sound Level Meter
An instrument which includes a microphone, amplifier, RMS detector, integrator or time averaging device, output meter, and weighting networks used to measure sound pressure levels.
Sound Pressure
The instantaneous difference between the actual pressure and the average or barometric pressure of a given point in space, as produced by sound energy.
Sound Pressure Level
20 times the logarithm to the base 10 of the ratio of the RMS sound pressure to the reference pressure of 20 micropascals (20x10µN/m). The sound pressure level is denoted Lp, or SPL and is expressed in decibels.
An oscillatory motion of solid bodies of deterministic or random nature described by displacement, velocity, or acceleration with respect to a given reference point.