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Quieter Pavement Test Results: Southbound I-5 Through Lynnwood

Results through August 2010

The vertical bar on the left side of the graph above represents the initial reference measurement that we collected on the old asphalt before it was ground and repaved with new asphalt. The horizontal lines represent measurements on the four pavement types:

  • older asphalt (aged)
  • new hot mix asphalt, or control, pavement
  • new polymer-modified quieter asphalt
  • new rubberized quieter asphalt

As with any scientific test, we used a control pavement against which we can compare the new pavement types and their acoustical performance.

What are decibels and Hertz?

We measure the intensity of sound in decibels, which indicate a sound’s relative loudness, and Hertz, which is the tone or pitch of sound that people hear.

Decibels (dBA) from roadway noise are measured in A-weighting, which allows our noise meters to imitate the same frequencies that can be heard by the human ear.  The decibel scale (pdf 334 kb) ranges from 0 decibels, the threshold of human hearing, to 140 decibels, where serious hearing damage can occur.  The average human ear can only distinguish between two sound levels that are at least three dBA different in loudness. 

Traffic noise typically ranges between 55 and 80 dBA along a highway right of way line.  According to the Federal Highway Administration, roadway noise impacts start at the 67 dBA level.  This is when people standing a few feet apart have trouble hearing each other in normal communication.  It should be noted everyone has a different reaction and tolerance to various noise levels.

Most people only discern a difference in noise level when there is a three or more decibel change.  This is equivalent to doubling traffic volumes, such as an increase of 1,000 to 2,000 vehicles on the road.

A Hertz (Hz) is the measurement we use when studying the tone or pitch of sound that people hear.  The higher a noise's Hz level, the higher it sounds.  People generally hear the range between 20 and 10,000 Hz.  Around the 1,000 Hz frequency are sounds that many people cannot ignore: crying babies, rushing streams and traffic noise.