Pollution Research Paper

STUDIES ON FOLIAR SOUND ABSORPTION CAPACITIES OF SOME URBAN TREES BY IMPEDANCE TUBE METHOD

AMBIKA JOSHI, VISHWAS DESHMUKH , NITESH JOSHI AND PAYAL RANE

Abstract

Noise pollution is a common problem in urban areas with ever increasing vehicular density. Urban trees help in noise attenuation. Various methods are employed to measure sound absorption coefficients of substances .Sound absorption coefficient is normally measured using expensive instruments and calculating absorption coefficients. The absorption coefficient of a substance falls within a scale from 0 to 1. The concept of this absorption coefficient was established by Sabine. In the current work sound absorption ratios of leaves of some tree species are derived using a impedance tube, function generator, standard audio amplifier, omnidirectional microphone and oscilloscope connected together. A sound absorption ratio was derived to measure sound absorption on a relative scale in a closed system calculate sound absorption ratio Sabine’s formula was used and absorption ratio was derived. Sound absorption ratio towards unity indicates poor sound absorption capacity. Sound absorption ratio of leaves of different tree species was studied. Observations were made using oscilloscope connected to a long impendence tube and a speaker at different frequencies like 500Hz, 700Hz etc. In designing the tube 500Hz – 1000Hz was taken as operating frequency for the reverberation testing and used in sound absorption test. To reduce the effect of attenuation due to walls of tube it was desired to have a large diameter of tube and long length so that at least two minima and one maximum could be found. An omnidirectional microphone was used to study standing pattern. Readings were taken with and without samples so that every time when samples were replaced by the other the initial setting was not disturbed. Initial setting wave pattern was maintained by adjusting i/p power (i.e. current & voltage) of the speaker. At 500Hz the noise levels are closer to audible range, hence it was used as reference and for comparison of absorption by plants at other frequencies. According to the study, leaves of different plant species showed different absorption at different frequencies. Barringtonia acutangulaGoerb, Eucalyptus globulusLabill, Ficus benghalensisL, Ficus glomerataRoxb, Ficus religiosa L, Mangifera indicaL, Polyatlhia longifoliaThev, Sterculia urensL and Thespesia populnea Soland are good sound absorption the method is relatively inexpensive and easy to use to measure sound absorption of foliar surfaces.