Disposal of waste gases from industrial sites are generally exhausted to the ambient air through exhaust (chimney) stacks. As well as exhausting waste gas into the atmosphere these stacks will often enable the emission of noise generated by the process equipment.
Increasing the distance from a noise source normally results in the level of noise getting quieter, due primarily to geometrical spreading, however, this relationship does not always stand true when predicting the noise from exhaust stacks and it is not unusual for exhaust stacks to result in higher noise levels at greater distances from the stack whilst being less audible at ground level close to the base of the stack.
This is in part because exhaust stacks have a relatively high degree of directivity with most noise being released upwards into the atmosphere rather than to the sides or below the lip of the stack. This is a simplified assessment and the actual mechanism is a more complex relationship between the frequency of the sound, stack exit dimensions, exhaust gas velocity and the temperature of the exhaust gas.
Additional variables are introduced by meteorological conditions, in particular wind, with noise levels downwind of a source being louder than upwind. Wind velocity generally increases with height due to friction of the airstream closer to the ground creating a gradient that refracts sound waves upwards in an upwind direction and downwards in a downwind direction. Additional refraction effects may be caused by local turbulence, wind direction, topography and temperature gradients in the atmosphere. Ground affects and atmospheric absorption will also introduce an attenuation relationship with frequency into the prediction of noise levels at greater distances from the exhaust stack.
Noise from exhaust stacks, carried over a greater distance are more likely to vary in level according to changing wind speeds and in frequency (tonal) characteristics due to changes in directivity, and refraction effects. This may create a characteristic non-steady noise within the environment which is, subjectively, likely to be more noticeable and may result in a greater level of annoyance.
This article based upon an initial report by Simon Stephenson, ‘Predicting environmental noise from industrial stacks’ and published in the Acoustics Bulletin January/February 2013 pp.38 – 41.