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Freeway noise has been a problem from the opening of the first freeway. Paul S. Venekassen, for whom I worked in the early 70s, recommended 3m(?) high sound barriers made of concrete panels. They provide an 11dB or greater reduction in Sound Pressure Level towards the start of the noise path into residential neighborhoods. By now such panels have been installed along most of the urban freeways adjacent to residential neighborhoods. If the reading were 90 dB on the freeway shoulder that would translate into 79 dB on the neighborhood side, falling away at approximately 6dB per doubling of distance. But even two or three miles away from a freeway the noise will often be audible on quiet evenings. Then there are airplanes, helicopters, ambulances and police sirens in addition to street traffic. Cities are noisy places. Part of why I moved close to the middle of nowhere, only to find I still have the occasional siren and helicopter.

There are now available commercial concrete sound blocks that have a slot opening that is to be oriented towards the source and a cavity that can be filled with absorbent material. This forms a Helmholtz resonator with a very broad Q. Were such materials used in sound barriers considerable further reductions would be possible. What is absorbed is neither  transmitted, reflected nor refracted over the barrier.    

"It is not necessary to have hope in order to persevere."

by ARGeezer (ARGeezer a in a circle eurotrib daught com) on Fri May 31st, 2013 at 06:14:19 PM EST
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