Why must highway noise barriers be placed close to the road or nearby homes to work effectively?

Highway noise barriers must be placed as close as possible to either the noise source (the road) or the observer (nearby homes) to maximize their effectiveness by managing the diffraction of sound waves.1

Because sound waves have wavelengths comparable to the size of everyday objects, they do not travel in perfectly straight lines like light; instead, they readily bend around corners and obstacles, a phenomenon known as diffraction. Road noise, particularly from large vehicles, often contains low-frequency sounds with longer wavelengths that are especially prone to diffracting over the top of a barrier.231

By positioning the barrier near the source or the observer, the path that sound waves must take to reach the listener is forced to bend more sharply. This geometry maximizes the diffraction required for the sound to reach the observer, thereby increasing the barrier's ability to create an acoustic shadow and reduce the intensity of the noise reaching the protected area. Additionally, the barrier must be constructed to limit the amount of sound transmitted directly through it.13