aAssumes divided 8 lanes with 9.1 m median.

bBased on observed 1.5 m above grade.

Install Double Wall for Optimum Sound Isolation

Avoid Structural Contact Between Walls

Isolate Source From Enclosure

Avoid Structural Contact Between Walls

Isolate Source From Enclosure

Acoustical Lining Reduces Noise Buildup

Solid Wall Construction n-"

Install Lined Ducts if â–ºVentilation or Cooling is Needed

Airtight Seal Needed to Preserve Isolation

Spring Mount

FIG. 6.7.3 Enclosures for controlling noise. (Reprinted from National Bureau of Standards Handbook 119, 1976.)

from around the ends may compromise, or short-circuit, barrier attenuation. The required barrier length depends on the total net attenuation needed. When 10 to 15 dB attenuation is needed, barriers must be long. Therefore, to be effective, barriers must not only break the line of sight to the nearest section of the source, but also to the source far up and down the line.

Of these four paths, the noise diffracted over the barrier into the shadow zone represents the most important parameter from the barrier design point of view. Generally, determining barrier attenuation or barrier noise reduction involves only calculating the amount of energy diffracted into the shadow zone. The procedures presented in the barrier nomograph used to predict highway noise are based on this concept.

Another general principle of barrier noise reduction is

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