Experimental techniques for the visualization of the acoustic fields radiating from brass musical instruments

Radiation from brass instrument bells under artificial excitation can be visualized using a number of experimental techniques. In this work, we present two different approaches: Schlieren optical imaging using a high speed camera and acoustic 2D scanning using a linear microphone array. The potentials and limitations of both methods, as well as their methodological requirements, are discussed. Examples of measurements on several instruments are presented. Schlieren optical imaging is applicable for the detection of large amplitude wavefronts formed by nonlinear wave steepening and is this is particularly sensitive to the high frequencies that form within these waves. The 2D scanning linear microphone array on the other hand can, in principle, measure all frequencies and the use of the exponential sine sweep method allows any nonlinear behaviour (in the loudspeaker and microphone or in the air) to be removed to get an accurate measurement of the linear radiation field for any frequency of choice within the measurement bandwidth.