and the resulting levels combined at the receiver location. Reflections from nearby buildings can produce flanking paths where the sound travels around a barrier. Examples can be found in Fig. 5.5.

A high building located behind a barrier can scatter sound back into the shadow zone, particularly where there is an overhanging roof, thereby reducing the barrier's effectiveness. Measured data for these types of conditions are shown in Fig. 5.6. A local reverberant field set up between a barrier and a building can decrease barrier effectiveness, especially at low frequencies. Low-frequency reverberation can also generate an increase in noise level compared with the no-barrier condition.

When a wind is blowing from the source to the receiver and the distance is sufficiently long, the barrier effectiveness is much reduced due to the downward bending of the sound waves. This effect occurs when the barrier is relatively far (say A and B > 100 m) from the source and receiver.

The construction of sound walls on top of berms presents a curious dilemma. Due to the interaction of sound with the top of the berm, an additional attenuation of about 3 dB is achieved over that which would be obtained from a wall. If a 0.3 m (1 ft) wall is built on top of a berm the attenuation would increase about one dB for the extra height and decrease about 3 dB due to its being a wall, yielding a net increase in sound level. Thus walls on top of berms must be sufficiently high to offset the loss of the berm effect. In practice, although berms are more efficient attenuators, they are difficult to build very high. Most berms must be constructed with a 2:1 slope, so they end up being four times wider than they are high, and space constraints limit their use.

Ground attenuation, which occurs when sound waves graze or skim across the ground, is reduced when sound diffracts over a high wall since the barrier changes the angle of approach. Barrier attenuation may be partially offset by the loss of ground attenuation. By doing a few sample barrier calculations one can quickly discover that barriers of a given height are most effective when they are located close to the source or receiver and least effective when they are positioned half-way in between.

Line Source Barriers

When a barrier is constructed along a line source, such as a highway, the geometry and thus the Fresnel number changes for each angle of roadway covered by the barrier. Figure 5.7 illustrates this condition and Fig. 5.2 gives the sign convention for the angle segments.

Figure 5.6 Sound Attenuation by a Barrier in Front of a Reflecting Surface (Sharp, 1973)

Figure 5.6 Sound Attenuation by a Barrier in Front of a Reflecting Surface (Sharp, 1973)

-O 6' w Configuration I

Frequency, Hz Source Height = 0.5 Feet

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