Basic Noise Wall Facts

  by Mike O'Connor

  
 
                 This simple conceptual sketch illustrates several important facts about the performance of outdoor noise walls. On it are shown 'rays' which represent types of paths that the sound can take between the source and receptor. We will discuss each type of path in turn.

   One path is shown going straight through the wall. The sound that goes directly through the wall is usually called the transmitted sound. Another path that the sound can and will take is a bent path over the top of the wall. That's right, some of the sound will be deflected downward as it goes over the wall. Sound that has been deflected downward in passage over an edge such as the top of a wall is referred to as having been diffracted. Finally, there is reflected sound which bounces off of the wall. Not shown on the sketch is absorbed sound. The term refers to the elimination of sound via conversion of the energy that is in the sound into heat. Some amount of absorption occurs on each of the three types of paths which are shown.
                             The percentages which are shown on the sketch pertain to the energy content in the sound. We will first discuss the relative importance of each of the three kinds of paths. We will then add up the percentages to get, on each side of the roadway, the total energy content of the noise from the automobile, as affected by the wall. We will then determine the overall significance of the tallied effects of the wall, especially in regard to loudness.

   Note first that the transmitted sound is shown as amounting to a virtually negligible 1% of all of the sound that goes out from the passing car toward the house. For most walls, the amount transmitted through the wall would be even less than that. Thus we can almost always ignore the transmitted sound. Likewise, no allowance has been made for absorption because the only walls that absorb a significant amount of sound are those that have deliberately been made to be absorptive, such as by covering them with dense absorptive materials. The atmosphere absorbs sound, but over such a short distance of a hundred feet or so, as is depicted here, the absorption by the atmosphere is also negligible.
                                          Thus, at the residence we are left with the diffracted sound and a small contribution from the transmitted sound, which add up to just 10% of the incident sound. (Of course 100% represents all of the sound that would reach the receptor at the house were there no wall at all. We're calling that the 'incident' sound.)

   Across the way, on the left side of the sketch, we see that the reflected sound is not the 91% of the incident sound which hits the wall minus the 1% that is transmitted through the wall, but quite a bit less— just 50%. The reduction from 90% to 50% is due to the fact that sound is diminished with distance it spreads out from a source (simply because of the spreading out) and the distance from the car to the wall and back across the roadway is greater than the direct distance from the car to the left side of the roadway. So, receptors on the left side of the roadway would receive 150% of the sound that would be incident there without the wall.

   To summarize the results thus far, we have found that, with a wall, the receptor at the house on the right side of the road would receive only 10% of the noise that would be there if there were no wall at all; on the left side of the road any receptors there would be exposed to 150% of the noise that would be there if there were no wall at all. We now turn to the most important question, that of the significance of these findings.
                                                       It's important to understand that one doesn't hear sound as having a loudness that is simply proportionate to the amount of energy in the sound, the way that the price of a new roadway may be found to be approximately proportional to its length. For example, if a sound is changed so that it has, either half (or twice) the energy that it had before, then that would amount to just a noticeable decrease (or increase) in loudness. It would take a 10-fold decrease (or increase) in the amount of energy in the sound to halve (or double) the loudness.

   Thus, at the house on the right side of the road passing automobiles would sound approximately half as loud with the wall as they would without the wall; receptors on the left side of the road would sense a small and barely noticeable increase in loudness due to the wall. (Opinions about the percentage increase in loudness that is produced by a 50% increase in energy content are likely differ substantially from one person to another, but it is to be expected that most would acknowledge an increase in loudness of roughly 10 to 15%.)
                                                                    Readers should understand that this is only an example. The actual performance of noise walls differs from site to site due to a number of factors: the height of the wall; the amount of background noise from sources other than vehicles on the roadway; the amount of heavy truck traffic on the roadway; the horizontal extent of the wall; the distances from the car to the wall and from the wall to the receptor; and, the elevations of the roadway and the receptor relative to the top of the wall.
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