Minnesota Department of Transportation

511 Travel Info

Noise Analysis

people planting a tree

Noise glossary

A-Weighted Sound Level The sound level obtained using the “A” weighting characteristic of a sound meter. The “A” weighting approximates the ear’s sensitivity to sounds of various frequencies.
Acoustics The branch of physics dealing with sound and sound waves.
Ambient Noise All-encompassing noise associated with a given environment; usually a composite of sounds from many sources, near and far.
Attenuation Any decrease in sound level; can be caused by increased distance, diffraction around a barrier, etc.
Cost effectiveness criteria Limits used to determine when a noise barrier is economical in terms of tangible benefits produced by money spent.
Daytime The hours from 7 a.m. to 10 p.m.
Decibel (db) A measure used to express the relative level of a sound in comparison with a standard reference level. The higher the sound level in decibels, the more intense or loud the sound.
Design Year The future year used to estimate the probable traffic volume for which a highway is designed. A time, 10 to 20 years, from the start of construction is usually used.
Diffraction The bending of sound waves around an obstacle, such as a barrier. The process reduces the effectiveness of the shadow zone. (See also Shadow Zone)
Existing Noise Levels See ambient noise.
Freeway or expressway A divided, controlled-access highway with four or more lanes.
Frequency The rate of vibration expressed in number of cycles per second (hertz or Hz). Frequency corresponds roughly to pitch in the human perception of sound
L10 The sound level that is exceeded 10 percent of the time (the 90th percentile) for the period under consideration.
L10(h) The hourly value of L10.
Leq The equivalent steady-state sound level which in a stated period of time contains the same acoustic energy as the time-varying sound level during the same period.
Leq(h) The hourly value of Leq.
Nighttime The hours from 10:00 p.m. to 7:00 a.m.
Noise Undesirable or unwanted sound; as defined by Minnesota Pollution Control Agency Regulations (NPC-1), "any sound not occurring in the natural environment, including but not limited to sounds emanating from aircraft and highways, and industrial, commercial and residential sources."
Noise Wall or Barrier Any natural or man-made objects such as noise walls, berms or mounds, buildings etc. that affect the sound propagation by interrupting the direct line of sight between the noise source and receiver.
Shadow Zone As it relates to barriers, an area of decreased sound energy governed principally by the properties of diffraction and transmission loss.
Substantial Change A substantial increase or decrease in noise level is a change of 5 dBA or more.
Tire/Pavement Noise The noise produced by the interaction between the rolling tires of a vehicle and the pavement, and separate from other sources (engine, exhaust, etc.)
Traffic Noise Impacts Impacts occur when the predicted noise levels or when measured traffic noise levels exceed the MPCA noise level standards, FHWA noise abatement criteria, or when predicted traffic noise levels substantially exceed the existing noise levels.
Type I Projects A proposed federal or federal-aid highway project for the construction of a highway on new location or the physical alteration of an existing highway which significantly changes either the horizontal or vertical alignment or increases the number of through-traffic lanes.
Type II Projects A proposed federal or federal-aid highway project for noise abatement on an existing highway.