In situations involving turbulent, corrosive or high temperature exhaust gas, active control of duct-borne random noise might be more conveniently applied beyond the duct outlet. Near field attenuators are examined to disclose those configurations which radiate least noise to the far field, since it is usually there that the most serious complaints originate. Theoretical investigations of simple dipole, tripole and quadrupole source arrangements reveal that near field systems may prove applicable where the wave number kd (d- outlet diameter), is less than 0.73.
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