Frequency Band

the relation between noise attenuation expressed in (1) dBs, (2) energy terms, and (3) subjective loudness. Table 6.7.1 gives these relationships for line sources. As indicated in the loudness column, a barrier attenuation of 3 dB is barely discerned by the receiver. However, to attain this reduction, 50% of the acoustical energy must be removed. To cut the loudness of the source in half, a reduction of 10 dB is necessary. That is equivalent to eliminating 90% of the energy initially directed toward the receiver. As previously indicated, this drastic reduction in energy requires very long and high barriers. In summary, in barrier design, the complexity of the design increases with the need for attenuation as shown in Table 6.7.2.

The design of roadside barriers sometimes uses the barrier nomograph in reverse order. A set of typical solutions is summarized in Table 6.7.3. The noise reduction at 152 m is less than that at 30 m because the barrier does not cast as large a shadow at a distance. The effectiveness of the barrier is reduced for trucks because of the elevated nature of the source.

Transmission Loss

When the position of the noise source is close to the barrier, the diffracted noise is less important than the transmitted noise. If the barrier is a wall panel that is sealed at the edges, the transmitted noise is the only concern.

The ratio of the sound energy incident on one surface of a panel to the energy radiated from the opposite surface is called the sound transmission loss (TL). The actual

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