There has been a growing interest in the use of natural ventilation in buildings to supplement orreplace mechanical air supply systems. However, for buildings in busy urban areas the potentialto use natural ventilation can be limited by excessive noise entering through natural ventilationopenings such as windows and trickle ventilators. Such openings tend to have large open areasto enhance air flow while offering a very low resistance to the transmission of external urbannoise. Traditional treatments for controlling the ingress of noise through an opening tend toincrease significantly the air flow resistance of the opening, thereby making natural ventilationnon-viable. This paper describes some experimental studies of alternative forms of noise controlthat do not create an increased resistance to air flow. The first alternative was a passive systemconsisting of a panel containing sound absorbtent material fixed in front of a ventilationopening. The second alternative investigated was an active noise control system whichattempted to sample the urban noise spectra close to the inlet opening and neutralise the noise byinjecting the inverse spectra ('anti-noise') in to the air supply. Finally, a hybrid systemincorporating both the passive and active techniques was tested. Initial results suggest that thishybrid approach may yield a design strategy that allows urban buildings to benefit from naturalventilation whilst maintaining an acceptable internal acoustic environment.