Sound or noise barriers, such as walls or screens, are designed to create an acoustic shadow by blocking the free flow of sound waves. The reduction in sound level within this shadow zone behind the barrier is dependent on frequency. At high frequencies, the effect of the barrier is clear, when in low frequencies (long wavelength) the shadow effect is diminished.
The sound barrier location has to be well chosen. To work best, the barrier must make the sound travel as far as possible from the direct route and change direction by the greatest angle. The best effect of a screen is if it is placed close to the noise source. Half way between these two is the worst position. The attenuation of a screen is mainly determined by the effective screen height, H , and the width of the screen compared to the size of the noise source (see the picture below).
Screens inside buildings must always be combined with a sound absorbing roof to avoid reflection. Normally, the effect of a screen inside a building is about 5 – 10 dBA. The sound reduction index for the screen has to be about 20 – 25 dBA.