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Why are low-frequency sounds difficult for highway noise barriers to block?

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Low-frequency sounds are difficult for highway noise barriers to block primarily due to the phenomenon of diffraction, which is the tendency of waves to bend around obstacles.12

The effectiveness of a noise barrier is limited because sound waves readily spread into the shadow region behind the barrier. This process is highly dependent on the relationship between the sound's wavelength and the dimensions of the obstacle. Because low-frequency sounds have longer wavelengths, they diffract more easily around barriers than high-frequency sounds.123

Since road noise—particularly from large vehicles—often contains a significant amount of low-frequency energy, these sounds can effectively "bend" over or around the barrier to reach the observer. To mitigate this, barriers must be positioned as close as possible to the noise source or the observer to maximize the degree of diffraction required for the sound to reach the listener, while also being constructed to limit the amount of sound transmitted directly through the barrier material.1