Noise Walls

Q UESTIONS AND A NSWERS

   by Mike O'Connor

   For big projects, such as a new freeway or a substantial upgrading of several miles of a roadway, there is often a lag of several years between the initial planning and the actual construction. For example, beginning in 1986, mo'c Physics Applied was the noise consultant for a certain major roadway project in Santa Clara County, California, but... it wasn't constructed until the late 1990's. During the long planning process there are usually several opportunities for affected citizens to attempt to influence the outcome, and noise walls are naturally one of the principal concerns of the wayside residents.

     Noise-barrier walls are sometimes controversial. Although the walls may block about 90% of the energy in the traffic noise, that 90% reduction in energy is heard as only about a 50% reduction in loudness. That's the way human hearing works. Thus, many find that the mitigation that is offered by noise walls is inadequate. Other residents regret the fact that noise walls block the view to the horizon and give the roadway a 'channelized' look, and sometimes business owners or even the police object to the blocking of lines of sight. There even are those who harbor the view that noise walls don't really work well acoustically and are therefore detrimental overall.

     Well, that means that we have lots to talk about and think about. The idea behind this particular page is to provide some helpful information about noise walls without being too technical. Those who want more complete explanations should click on some of the underlined words which are links to additional pages. All of the questions are derived from my own professional experience and are loosely based on recollections of actual queries by affected parties, but in no case is the description of the setting or the wording of the question so accurate as to allow identification of the person.

A NSWER   There are really two independent elements to this question: reflections off walls and sound absorption by plantings. Notice also that the circumstances are that a wall was 'retrofit' to a pre-existing roadway; it's not that an entirely new roadway was constructed. I'll start with reflections.

     Noise walls do reflect sound, but that usually causes only a slight increase in noise levels along the wayside of the roadway that is opposite to the wall. However, there are rare circumstances in which a receptor who was previously shielded from the roadway by some obstruction becomes exposed to reflected sound as the result of a new roadside wall having been constructed. In some such circumstances, the change in exposure to the traffic noise could possibly be significant.

     The matter of absorption of sound by plantings needs careful discussion. It's a bit like the subject of reflections off of walls in that one cannot say that there is no effect at all. Rather, the question is really about the significance of what is usually a small effect.

     In general, it takes either a considerable depth of plant material or a considerable density of it to attenuate sound. Clearly the only hope for reducing reflections off noise walls by growing plants on them or near them would be to provide density, as there is no depth to be had in a plant which clings to a wall and little to be had by planting a narrow strip in front of a wall.

     (Perhaps the reader is familiar with fiberglass mats that are used to provide thermal insulation in walls— they are called 'batts'. A fiberglass batt is an example of a dense fibrous material that can also attenuate sound.)

     To return to wayside noise walls, the climbing vines that are sometimes planted so as to grow on noise walls are leafy plants and, as such, every leaf must have its own exposure to the sun. Thus they are not dense, but airy. Sound can easily pass around their leaves without being absorbed. In order to get a significant reduction of the reflected sound with plants it would be necessary to somehow get a very dense mossy plant to grow on the side of the wall. In almost any climate it would be difficult to get a dense mossy plant to grow on the side of a masonry wall.

     Just as was said for reflections, this is not to deny that there would be any effect at all. Such climbing plants might diminish some of the higher pitched components of the reflected tire noise a bit, but the overall reduction in the loudness of all of the reflected noise would have been insubstantial.

     To finally answer the question, the particular roadway authority in charge of the wall in this resident's neighborhood did not make a planting because the plants that can feasibly be grown on or in front of noise walls are not dense enough to substantially reduce reflections, and, because a decision was made to avoid the cost of putting in, tending and irrigating decorative plants. The climbing vines that some other roadway authorities have planted on wayside noise walls are there mainly to beautify, and to discourage graffiti.

A NSWER   One should exercise caution before blaming apparent increases in noise levels on recent installations of noise-barrier walls. For example, if a roadway has been improved by adding new travel lanes to an existing alignment, then, that same capacity increase which lead to the construction of new noise walls caused increased traffic volumes and allowed the traffic to speed up during commute periods. More traffic means more noise and faster-moving traffic is noisier than slower-moving traffic. Under some such circumstances it would be possible for the environment at some locations to be noisier than it was before the roadway and walls were both improved.

     For example, if this resident's home is not actually on the freeway but is partly shielded from it by intervening obstructions, such as rows of other homes, then the new wall of the edge of the freeway could not have added much in the way of additional attenuation for this resident: it could only block sound paths that were already partially blocked. If that is the case, the new noise walls might not have reduced freeway noise levels to below what they were before, but it would at least be slightly quieter than it would have been had the roadway been improved and had no new walls been constructed.

     When mere improvements to existing roadways are given noise wall mitigation the environment will usually be quieter than it was before the roadway was improved, especially on a daylong basis. In particular, residents along the immediate wayside who had no shielding from the unimproved roadway get the full benefit of the new noise walls. For them, the roadway is surely quieter. (See the next question and answer for related information.)

A NSWER   In the main, it is not possible to entirely eliminate noise impacts from roadways by constructing wayside noise barrier walls or by depressing the roadway. Both measures are effective at reducing noise levels but the most effective measures may reduce the loudness of the traffic noise to 30 to 40% of what it would have been had no mitigation measures been undertaken; reducing the loudness to half of what it would have been is more typical of noise wall performance. Thus, one would definitely expect a new freeway to noticeably raise noise levels in adjacent communities.

     The fact that the sound seems to come from a mile or so down the road has an interesting explanation. It's probably due to something called a temperature inversion. Inversions are most usually present under windless conditions at night or at dawn— the very times at which one expects quietude. Inversions allow sound to leapfrog over obstacles and travel distances of a mile or more without the usual amount of diminution. In a sense, there may even be a kind of amplification. Constructing noise walls or depressing roadway segments doesn't necessarily reduce vehicular noise that comes from such distances when an inversion is present, because the inversion makes paths for noise which pass over the tops of the wayside walls but eventually reach ground possible. Thus the early morning commute period may now seem much louder to this resident, given that there is a new freeway in that part of town.