Index of content:
Volume 104, Issue 6, December 1998
- NOISE: ITS EFFECTS AND CONTROL 
104(1998); http://dx.doi.org/10.1121/1.423924View Description Hide Description
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 on 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.
104(1998); http://dx.doi.org/10.1121/1.423925View Description Hide Description
This paper presents an active noise control system with a combined feedback–feedforward configuration, which may be useful when the location and directivity of the primary source in a room are not known. Using the combined system, the need to choose between a feedforward or a feedback system configuration beforehand is avoided. The optimum performance of feedforward, feedback, and the combined active control systems have been calculated from measurements taken in a room with various primary source positions. Internal model control (IMC) together with optimal Wiener filtering are used in the design of the optimum controllers, which was performed for each primary source position and also included robust stability to guarantee the stability of the controllers in face of plant uncertainty. The performance of the feedforward controller alone was much more dependent on the position of the primary source than that of the feedback controller alone, but the combined system always performed better than either of those systems. The results also suggested that the combined system could obtain good performance even if the response of its feedback controller was fixed, independently of the primary source location, but the response of its feedforward controller was adapted for different primary source positions.
104(1998); http://dx.doi.org/10.1121/1.423926View Description Hide Description
A train is considered to be a homogeneous line source; therefore, the concept of the linear density of the A-weighted sound power, is used. The process of noise generation on a bridge is characterized by the product, and that beyond a bridge by where Geometrical spreading is considered to be the only wave phenomena that governs noise propagation. The theory presented here requires measurements at two locations in the vicinity of the bridge in question and makes possible the calculation of the sound exposure level, The obtained experimental values are consistent with the theoretical values.
104(1998); http://dx.doi.org/10.1121/1.423927View Description Hide Description
This article presents synthesis curves for the relationship between DNL and percentage highly annoyed for three transportation noise sources. The results are based on all 21 datasets examined by Schultz [J. Acoust. Soc. Am. 64, 377–405 (1978)] and Fidell et al. [J. Acoust. Soc. Am. 89, 221–233 (1991)] for which acceptable DNL and percentage highly annoyed measure could be derived, augmented with 34 datasets. Separate, nonidentical curves were found for aircraft, road traffic, and railway noise. A difference between sources was found using data for all studies combined and for only those studies in which respondents evaluated two sources. The latter outcome strengthens the conclusion that the differences between sources cannot be explained by differences in study methodology.