Masonry Magazine January 1973 Page.18
Definition of Terms
Ambient Noise Level The noise level existing in an area before a proposed roadway. This quantity is measured in dBA and expressed as L10 or L50 ambient noise levels.
Decibel A logarithmic value used to express sound levels. A sound judged to be twice as loud as another would be rated about 10 decibels (dB) greater.
dBA The sound pressure level in decibels measured with a frequency weighting network corresponding to the "A" scale, which tends to suppress frequencies below 1,000 Hz. In much of the published technical literature the terms dB and dBA are used interchangeably.
Frequency The number of times per second that a sine wave of sound repeats itself. The unit for the cycles per second is called the hertz (Hz).
Level A logarithmic notation of decibels.
L10 Sound level exceeded 10% of the time.
L50- Sound level that is exceeded 50% of the time.
Factors Bearing on Highway Noise
Noise which results from highway traffic, as observed at any given point, depends on a great number of variables. Not all of these variables can effectively be controlled in a program of noise abatement. Those variables affecting highway noise include the height and distance of the reception point in relation to the noise source, the distortion of sound waves due to wind and ground effect, the absorbtion of sound by air, the influence of interfering objects, and the nature of the sound source itself.
Noise orginating on a highway results from a combination of items such as the composition of vehicular traffic at different times, number of traffic lanes, highway gradient, and type of pavement. The percentage of diesel truck traffic is significant both because of the inherent loudness, and because the exhaust stacks are typically about eight feet above the level of the pavement.
Erection of a highway noise barrier, such as a wall, does not provide an absolute acoustical "shadow" on the side away from the noise source. This is so because sound energy spills over the top of the barrier due to diffraction. Figure 1 shows sound levels anticipated at various distances from a highway when several different noise abatement measures are used. From this it seems clear that some type of barrier will be required when structures such as schools and residences are located within 400 feet of major highways, if the Federal Highway Administration's proposed noise standard are to be met.
Estimating Highway Noise Reduction
In 1971 the Highway Research Board, a private organization, prepared an extensive document on noise abatement. In its report 117, "Highway Noise, A Design Guide for Highway Engineers," it evaluated several theoretical and empirical techniques for estimating highway noise at the source, and the noise reduction from several abatement measures. Calculations were presented for the effects of elevated and depressed highways, shielding by structures, noise reduction from plantings, and noise abatement by barriers. The report contained a design guide identifying variables in traffic noise prediction, and the effects of abatement measures. Examples and work sheets were also included.
In the case of both elevated and depressed highways, the maximum achievable noise abatement was determined to be 15 dBA attenuation because of limitations from refraction and environmental factors. The attenuation was arbitrarily reduced by 5 dB for trucks because of the higher location of the noise source. Shielding by structures, as afforded by two or three rows of houses, was considered to be about 3 to 5 dB, but could reach a maximum of 10 dB if no direct line of sight existed between the sound source and point of measurement. Contrary to popular opinion, little attention could be given to bushes, trees, and other foliage planted in widths less than 50 to 100 feet. A design value of 5 dB reduction could be used for every 100 feet of plantings if the trees are at least 15 feet tall, and dense enough to form a visual barrier between the roadway and the observer. The total attenuation attributed to plantings should not be over 10 dB.
Design values for shielding by solid, acoustically opaque roadside barriers may be estimated from the plot shown in Figure 2. It is based on the assumption that the barrier extends over the entire length of the noise-producing highway. In order to use the graph, the "equivalent lane" must be determined. This is the center line of a lane on