Thursday, March 13, 2008

How Noisy is the Proposed Rail System for Honolulu?

Here is a terrific post from the Honolulu Advertiser, by a member of the transit project team, explaining the the noise impacts from a metal rail system vs. the noise from a rubber tire or bus system. Apparently, metal rail is less noisy than even buses.


The Honolulu Advertiser article of March 11 ("Steel rail cheaper but noisier option") reminded me how difficult it is to explain the "rocket science" mathematics of acoustics.

As a member of the city's project team working on noise analysis, I think it would be beneficial to understand a few basic concepts regarding transit noise.

Noise levels are measured on the A-weighted decibel scale (dBA) according to how sensitive the human ear is to sounds of different frequencies. Sound levels in typical libraries are about 40 dBA. Noise levels in a busy store like Macy's or Sears would be about 60 dBA.

The newspaper article included a table of pass-by noise levels for various technologies. If we adjust the values to 50 mph at 50 feet for each, the two rubber tire technologies are reported as 74 and less than 80 dBA. The three steel wheel values become less than 75, 77 and 77 dBA. These numbers are consistent with the FTA-provided values and demonstrate there is almost no difference between the technologies in the noise generated.

With a modern rail system, nearly all of the noise is generated where the wheel contacts the guideway. This is very different from city buses, where most of the noise comes from the engine and exhaust system.

If the vehicles are operating on an overhead guideway, a very short barrier 2 to 4 feet high will block the wheel noise generated by rail transit. A rubber-tired guided bus would require an 8- to 12-foot barrier to mitigate the noise it generates. For people standing on the ground, the noise level would be reduced by 5 dBA or more by these barriers.

The impact calculations quoted in the article from the Alternatives Analysis did not include any noise-reduction measures. Discussing noise impacts without considering these mitigation measures is akin to talking about the noise generated by a car without a muffler.

Preliminary results from the environmental impact statement analysis suggest that with mitigation measures that will be included in the project, all of the noise impacts from rail transit would be reduced to moderate or completely eliminated.

Lawrence Spurgeon
Supervising environmental engineer, Parsons Brinckerhoff


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