In the area surrounding Phoenix, Arizona, it was observed that traffic noise from freeways was sometimes substantially higher than expected at distances of one-quarter mile or farther from the freeway. Although it was evident that these higher than normal sound levels were the result of atmospheric conditions, the exact causes and mechanisms were not well understood. Furthermore, specific policy was needed in order to respond to these issues. In response, the Arizona Transportation Research Center initiated project SPR 555, “Evaluate the Atmospheric Effects Associated with Highway Noise Propagation.” ATS Consulting led a team with several technical partners to find the answers to the following questions: What are the atmospheric conditions in the Phoenix Valley that contribute to higher than normal sound levels? Are the conditions unique to the Phoenix Valley? Can these atmospheric effects be anticipated?

Prior research has shown that the primary atmospheric parameters that affect sound propagation are: (1) the absolute temperature and humidity that affect the amount of atmospheric absorption, (2) temperature and wind-speed gradients that cause refraction of sound, and (3) turbulence that causes scattering of the sound and short-term temporal variations in the sound levels. These atmospheric parameters can result in large fluctuations in sound level. The results of our study for the Arizona Department of Transportation (ADOT) confirmed previous measurements and anecdotal information that the Phoenix area is particularly prone to high sound levels because of strong temperature inversion conditions that occur on clear, calm nights.

The physics behind wind and thermal gradients causing higher than normal sound levels are well-known. However, the differential equations that describe sound propagation under realistic atmospheric conditions must be solved numerically, and it is difficult to obtain the detailed atmospheric data required for accurate predictions. It is only recently that researchers have been successful in developing accurate projections of some common sound-focusing effects. With modern computing power and numerical methods of solving the differential equations plus new equipment that facilitates detailed atmospheric measurements, it is becoming more and more feasible to develop realistic projections based on measured atmospheric conditions.

The primary components of our research study for ADOT were:

A review of the literature relevant to how atmospheric conditions affect sound propagation
Detailed noise measurements in a Scottsdale neighborhood along the East Loop 101 Freeway (Pima Freeway)
Computer modeling of sound propagation under various measured and inferred atmospheric conditions
Noise measurements before and after installation of an asphalt rubber-friction course (ARFC) on the Pima Freeway
A pilot study investigating parametric models of tire/pavement noise

Technical members on the ATS team for this project included the National Center for Physical Acoustics, Navcon Engineering, Sanchez Industrial Design, Louis Southerland, Consultant in Acoustics, and Robert Bronsdon, Consultant in Acoustics. Following is a list of presentations and technical papers related to this research project:

Highway Noise Levels in a Suburban Environment, ForumAcustium 2005

Atmospheric Effects on Propagation of Highway Noise, Arizona DOT, Presentation, TRB Committee ADC40 Summer Meeting, July 2005.

Highway Noise Levels in a Suburban Environment Under Inversion Conditions, Presentation, TRB Committee ADC40 Summer Meeting, July 2005.

Papers & presentations related to this project >>>