Road Science Tutorial
Better Roads Staff
“For longitudinal tining,” the authors write, “the nominal spacing of the tines is 3/4 inch. For transverse tining, nominal spacing of 1/2 inch is specified. The nominal depth of the tined grooves in the plastic concrete is 1/8 inch.”
The Tech Brief may be downloaded from the CPTC at cptechcenter.org/publications/surface_char_specs_tech_brief.pdf and the individual guide specs can be found at cptechcenter.org/projects/surface-characteristics/index.cfm
New Ways to Measure Noise
Integral to concrete’s new attack on noise is a shift from measuring sound pressure remotely, to sound intensity directly at the source, the tire-pavement interface.
The key recommendations developed by the Concrete Pavement Surface Characteristics Center are largely based on tire-pavement noise tests conducted worldwide using microphones right at the tire moving on the pavement, write Rasmussen, Sohaney and Wiegand in a companion Tech Brief issued in May, Measuring and Reporting Tire-Pavement Noise Using On-Board Sound Intensity (OBSI).
“OBSI measures tire-pavement noise at the source using microphones in a sound-intensity probe configuration mounted to the outside of a vehicle, near the tire-pavement interface,” they say. “Measurements are performed while the test vehicle drives across the pavement of interest.”
Sound and noise can be a relative experience. A rock concert produces sound levels at about 110 decibels. A quiet night actually produces some 30 decibels of sound. But only levels above 85 to 90 decibels are thought to pose health risks.
Sound levels are measured exponentially. One expert describes it this way: Assuming two planes with the same individual sound level (3 decibels) are added for every double. If a plane taking off creates 100 decibels of noise, two planes would make 103 decibels, four planes 106 decibels, and eight 109. Highways, roads and streets routinely produce decibel readings of from 65 to 85 decibels.
When a sound level (such as 90 dBA) is reported, it is most often a measure of the amplitude of sound pressure changes, the authors add. “Sound intensity is different from sound pressure in that it has both amplitude and an associated direction,” they write, making it a more meaningful criterion for analyzing noise emissions.
Benefits of OBSI include:
• The directional characteristic of the probe makes it better-suited for measuring a specific noise source, while attenuating sounds from other sources in other directions (such as engine or exhaust noise);
• Sound intensity is much less contaminated by “random” noise, such as wind noise generated as the vehicle is moving; and
• Because sound intensity measures the acoustic energy propagating away from the source to the roadside, it correlates well with sound measured at the roadside (known as pass-by or wayside measurements).
The new Tech Brief may be downloaded atcptechcenter.org/publications/surface_char_specs_tech_brief.pdf
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