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The Noise and Vibration Unit, from left to right:
Aye Htoon, Quyen Tran, Roland Cerna, Samia Soueidan, Irene Wong, Jin Lee, Arnold Parmar. Inderjit Dhaliwal is missing from the photo.

The Sound and the Fury: Noise Unit Keeps Construction Clatter (and Complaints) in Check
by  Kelly Markham
Issue Date: 06/2011

If there’s construction, there’s construction noise – and probably noise complaints. The Noise and Vibration Unit ensures that the din doesn’t exceed allowable levels. The key number: 86 decibels.

[Click on photos to enlarge and read captions.]

You can’t see it, you can’t feel it, and it won’t make you late for work the way a closure might. And yet, it’s the source of one of the most common construction-related complaints District 7 receives. This fleeting, intangible irritant is noise. There is no construction without construction noise – much to the chagrin of those who live near freeway projects and find themselves listening to heavy equipment at odd hours. But what many people don’t realize is that construction noise is subject to countless federal, state and local regulations aimed at keeping it in check.

In District 7, the Noise and Vibration Unit is responsible for ensuring that Caltrans projects comply with those regulations – and many more related to traffic noise.

“Most projects come through our branch as part of the environmental process. We determine what noise studies are necessary, perform the studies, and determine feasible and reasonable solutions to mitigate or abate noise impacts,” said Senior Transportation Engineer Jin Lee, who has headed up the eight-person unit for more than 10 years. “We take measurements and create models that predict how much noise a freeway improvement might generate in 25 to 35 years.”

And that's the unit's primary focus: performing traffic noise impact studies and designing abatement and migitation measures, mostly for future projects. But the unit is also responsible for handling noise complaints stemming from projects currently under construction.

Generally speaking, every project has a specification that addresses construction noise. Usually the specification states that, to comply with federal and state regulations, construction equipment must not exceed 86 decibels from 50 feet away between 9 p.m. and 6 a.m. To give you a sense of how loud that is, a pickup truck going 50 miles per hour at 50 feet away registers about 70 decibels. Adding 10 decibels doubles the apparent noise level.

More complex projects may have a more detailed specification. For example, a contractor may be required to have trained staff on hand to take noise measurements on a pre-set schedule, ensuring that the maximum allowable levels are not exceeded.

Certain construction operations are more likely to generate complaints than others because they’re particularly loud, like pile driving and jackhammering, or particularly annoying, like backup alarms bleating into the predawn darkness. Demo work can also be problematic, as can projects with multiple noisy operations occurring simultaneously.

If Lee receives a noise complaint, he sends a member of his team to the construction site during work hours with a portable noise instrument to take measurements. The instrument looks a bit like a large cell phone with a microphone on top, and is mounted on a tripod at ear level (see photo). Typically, the instrument will take a noise reading every 10 seconds for 24 hours.

When the investigation is complete, the construction office is given a report describing the results. If the contractor is not in compliance, Lee’s team will suggest mitigation measures, such as using newer (and less noisy) equipment, installing muffler systems, turning down the volume on backup alarms, or using sound blankets, which are heavy cloths hung around particularly loud operations to attenuate noise. Once the contractor implements the mitigation measures, Lee’s team revisits the site to verify compliance. If necessary – and very rarely – a project can be shut down until a contractor addresses problematic noise issues.

Some noise complaints stem more from an error of perception than excessive decibels. This happens occasionally when construction requires the removal of trees or other vegetation. Residents may complain that freeway noise is louder because the removed vegetation served as a sound buffer. But for vegetation to reduce noise, at minimum, the landscaping would need to have 16-foot evergreen trees (not deciduous) growing 100 feet deep, which doesn’t exist anywhere in District 7 and is almost unheard of on urban freeway systems.

“Typical freeway landscaping doesn’t reduce noise, though it does have a secondary psychological benefit – out of sight, out of mind,” said Lee. “People believe that if they can’t see it, they don’t hear it. But we’ve taken measurements before and after vegetation removal. There is no difference.”

So whether people actually hear excessive noise or just think they do, Lee and his team are on the job, keeping noise in check and putting ear drums at ease. He can’t guarantee a silent night, but he can keep it to 86 decibels.

A noise instrument is  mounted on a tripod at ear height near I-405 in the community of Bel Air Crest (part of the I-405 HOV design-build project). Transportation Engineer Arnold Parmar takes noise measurements at a resident's property in the City of Diamond Bar near SR-57 (part of the SR-57/SR-60 connector project).