On Soundwalls and Rockridge: Sound

Saturday, June 4, 2011

Since the 1970s, the transportation system comprised of Highway 24 and BART that bisects Rockridge has been both a boon and a bane to the neighborhood: It's convenient for getting around but often a negative in terms of both air pollution and noise pollution. Could the latter problem be addressed with the construction of noise barriers, or soundwalls?As noted in the May issue of The Rockridge News, the settlement with CalTrans providing for mitigations of the construction effects of the 4th Bore of the Caldecott Tunnel includes funds to study the need for soundwalls at points along Highway 24. Soundwalls can be controversial for their esthetics, and often generate debate over their effectiveness and their potential to affect scenic views.decisions to place soundwalls along a roadway involve the wishes of people living near the road. RCPC and The Rockridge News present this and subsequent articles in the newsletter that will attempt to clarify the issues for those who may ultimately be affected by soundwall construction. Because of the topic's complexity, this month's article is presented in Q&A format.

*Q: What is the difference between "sound" and "noise," and how are they measured? A: Sound is vibrations in the air, and, basically, noise is any unwanted sound, obviously a subjective distinction. Sound vibrations move in waves through air and other materials. Sound is measured in decibels (dB), a logarithmic unit. Here are the measures of some common sounds: Pin dropping, 15 dB; normal conversation, 60 dB; hairdryer, 80 dB; a car passing at 10 feet, 80 dB; front seat at a rock concert, 130 dB. Government health guidelines suggest ear damage can occur at as low as 85 dB.The decibel scale differs from the more familiar numerical scale used in weights and measures. For example, if a 100-pound person gains 10 pounds, his or her weight has gone up 10 per cent. By contrast, when a 100 dB sound increases to 110 dB, its intensity has gone up ten-fold. On the other hand, your ear will hear it as being twice as loud.

*Q: How do we characterize sound? A: Sounds have direction, intensity (loudness), and frequency. Sound waves typically travel in many different direc-tions simultaneously, as does light from a light bulb. A sound's intensity is a measure of how much air pressure changes as the sound passes. Frequency measures how fast the variation in air pressure occurs, corresponding to what musicians call pitch.

*Q: What sounds make up freeway noise, and how does it change with distance from the source? A: The hum of tires, the growl of engines and transmissions, the hiss of air moving past vehicles, and the vibration of the roadway make up the cacophony. Sound traveling in open air from a single car drops by about 6 dB, a noticeable difference, when the car doubles its distance from the listener. A decrease of 10 dB makes a sound seem about half as loud. However, factors other than distance also have effects.

*Q: How can weather affect perception of freeway noise? A: Weather conditions, such as wind, can shift noise upwards or downwards.

*Q: How do surfaces affect perception freeway noise? A: Like light, sound can be absorbed by some solid materials but bounce off and be redirected by other materials. Sound can also "bend" as it passes through holes or around obstacles. These and many other factors help determine the usefulness and construction of a soundwall.

*Q: Are soundwalls the solution to freeway noise? A: Not always. Besides technical factors, other considerations arise, such as costs and visual impacts. These and additional issues will be addressed in articles to come.