The historical reason for conducting noise measurement in transonic wind tunnels was to attempt an explanation for differences in aerodynamic data caused by boundary layer transition. More recently noise measurement at transonic speed has become useful in its own right. There are two problems that severely hinder this measurement: high levels of background noise caused by the wind tunnel; and the presence of the turbulent boundary layer. Wind tunnel noise is controlled by appropriate design or modification of the tunnel configuration. This allows measurement of noise levels down to the level of pressure fluctuations produced by the turbulent boundary layer. Further processing using a microphone array can identify and separate noise from the turbulent boundary layer fluctuations allowing noise measurement several dB below that which can be measured with a single microphone.