California Department of Transportation
District 6 Director Sharri Bender Ehlert
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1352 W. Olive Avenue
P.O. Box 12616
Fresno, CA 93778-2616


  • (559) 488-4082
  • (559) 444-2409
  • 711 - TTY

(559) 445-6259




Noise & Vibration Specialists FAQs

Why does Caltrans perform noise and vibration studies?

Highway noise analysis and abatement/mitigation requirements stem from the following state and federal environmental statutes:

  • California Environmental Quality Act (CEQA)
  • National Environmental Policy Act (NEPA)
  • Title 23 United States Code of Federal Regulations, Part 772 “Procedures for Abatement of Highway Traffic Noise and    Construction Noise” (23 CFR 772)
  • Section 216 et seq. of the California Streets and Highways Code

How are noise levels measured?

Noise specialists measure levels by decibels or dBA — a unit of relative loudness. This unit of relative loudness closely approximates the human ear’s response to sound. On this scale, normal breathing registers about 10 dBA, a clothes dryer at 10 feet registers about 50 dBA, and a pickup truck going 50 miles per hour and 50 feet away registers about 70 dBA. Adding 10 decibels doubles the apparent noise level.

When are soundwalls warranted?

Soundwalls are necessary in locations adjacent to the highway where peak-hour noise levels are greater that 65 decibels. A soundwall will be proposed if it can reduce measured noise levels along the highway by 5 decibels.

To be effective, how high do soundwalls have to be?

Heights of walls are based upon their ability to lower the level of traffic noise. If a soundwall height is reduced to a point where it no longer achieves this measured noise reduction, it will no longer qualify for federal funding (and it will not be built). An effective soundwall normally blocks the line of sight from a 5-foot-tall receptor to the 11.5-foot truck exhaust stacks on the highway.

What if I don’t want a soundwall built?

A soundwall will not be built if more than 50% of the first-row property owners (adjacent to the project area) do not want it.

Do soundwalls block dust, dirt, and soot (particulate matter) produced by freeway traffic?

Barriers provide some limited protection from highway particulate matter as evidenced by concentrations often found at the base of the highway side of a soundwall. Soundwalls likely have minimal effect in stopping airborne soot and other aerosol particulate matter and vehicular emissions.

Can asphalt absorb sound?

Yes, rubberized asphalt has been reported to reduce noise levels by up to 3 decibels. However, rubberized asphalt requires special conditions to cure properly and can be placed only in select areas.

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