Volume 134, Issue 1, July 2013
Index of content:
- NOISE: ITS EFFECTS AND CONTROL 
134(2013); http://dx.doi.org/10.1121/1.4808079View Description Hide Description
Dependence of the performance of feedforward active duct noise control on secondary loudspeaker parameters is investigated. Noise reduction performance can be improved if the force factor of the secondary loudspeaker is higher. For example, broadband noise reduction improvement up to 1.6 dB is predicted by increasing the force factor by 50%. In addition, a secondary loudspeaker with a larger force factor was found to have quicker convergence in the adaptive algorithm in experiment. In simulations, noise reduction is improved in using an adaptive algorithm by using a secondary loudspeaker with a heavier moving mass. It is predicted that an extra broadband noise reduction of more than 7 dB can be gained using an adaptive filter if the force factor, moving mass and coil inductance of a commercially available loudspeaker are doubled. Methods to increase the force factor beyond those of commercially available loudspeakers are proposed.
134(2013); http://dx.doi.org/10.1121/1.4808175View Description Hide Description
Trees in urban spaces surrounded by buildings may be effective in dispersing sound energy, and this could affect sound level distribution and street canyon reverberation. To quantify this effect of trees with a view to including it in numerical predictions, this paper examines sound scattering from a single tree in open field by means of reverberation time (RT). Five trees of different species and crown sizes were considered. The influence of ground condition, receiver height, crown size and shape, foliage condition, and source-receiver angle and distance has been assessed. The results show that RT20 is proportional to the tree crown size, which is the most important factor. The maximum RT20 measured was 0.28 s at 4000 Hz for the studied trees when in leaf (with foliage). The presence of leaves increased RT20 at high frequencies, typically by 0.08 s at 4000 Hz. It was also demonstrated that the source-receiver angle can affect the characteristics of decay curves significantly. With increasing source-receiver distance within 40 m, RT20 was slightly changed. It was shown that ground condition and receiver height affect the decay curves, especially at low and mid frequencies, where sound scattering is of relatively limited importance.