This work investigates the development of a novel multiple microphone technique for analysis of the acoustical behaviour of brass instruments under playing conditions. In the current work, multiple microphones are deployed within a cylindrical section of the bore of a brass instrument. The technique allows for measurement of the instrument during playing, or while excited by a loudspeaker signal, and allows for a more robust analysis of the transfer of acoustic energy between microphones. The long term goal of this study is to allow wave separation under playing conditions in order to better understand the nature of the lip reed and the strong coupling between player and instrument. Preliminary measurements of transient behaviour in brass instruments are presented and discussed.