Increasing the insertion loss of noise barriers using an active-control system

In recent studies on active noise control in open space, a multichannel active-control system was used to create the largest quiet zone with the least increase of total sound-power output when its sensors and actuators were optimally arranged. This control system was also applied to increase the insertion loss of noise barriers. The control system was arranged to create quiet zones on the top of the barrier; thus, the sound diffraction along the barrier top was reduced and the insertion loss of the barrier increased. It was found that the;control system significantly increases the insertion loss of the noise barrier at low frequencies when the ground on both sides of the barrier is totally nonreflective. In practice, a nearby reflective ground may deteriorate the performance of the active noise-control system. The effects of ground reflection an the effectiveness of the active-noise barrier are considered in this paper. Both simulation and experimental results show that the reflective ground significantly reduces the effectiveness of the active noise-control system on the barrier. Reflection from the ground on the noise-source side has the greatest influence on the control efficiency. This suggests that passive reduction of the reflection from the ground on the source side should be incorporated in the design of the active-control system. Experiments were made for noise with multiple-frequency components in a low-frequency band. The results suggest that the active-control system can be effective at improving the performance of the noise barrier for noise with multiple-frequency components. (C) 1998 Acoustical Society of America. [S0001-4966(98)04412-9].