Microphone Arrays consist of many microphones recording a musical instrument. At the Institute of Systematic Musicology 128 microphones record simulteneously with a sample frequency of 48 kHz.
Propagating the radiated sound-field back to the radiating surface, the vibration of the instruments can be measured. Furthermore, radiation characteristics can be investigated. This works with steady-state sounds as well as with transients and therefore performance characteristics can be seen, too.
The back-propagation method used with the examples here is the Minimum Energy Method (MEM). It assumes as many radiation points on the geometry as there are microphones. Each radiation point has a certain radiation characteristics which might be brighter or narrower. The characteristics is not known beforehand, but trying many of them, the one with the minimum reconstruction energy is correct. The method is very robust and very suitable for near-field recordings. As musical instruments have evanescent waves, those vanishing in the distance, such near-field recordings are necessary to measure the instruments vibrations.
Sketch of a microphone array (left) with radiation points (right). The radiation points are the points on the musical instrument radiating sound.
Guitar in front of a microphone array.
Eigenmodes of a drum as measured and back-propagated with a microphone array shown above.
Radiation from a vihuela, a Spanish Renaissance guitar.
Bader, R., Münster, M., Richter, J, & Timm, H.: Microphone Array Measurements of Drums and Flutes. In: Bader, R. (ed. / Hrsg.).: Musical Acoustics, Neurocognition and Psychology of Music / Musikalische Akustik, Neurokognition und Musikpsychologie. Hamburger Jahrbuch für Musikwissenschaft 25, 13-54, 2009. Bader2005_AkustischeKamera
Bader, R.: Characterizing Classical Guitars Using Top Plate Radiation Patterns Measured by a Microphone Array. Acta Acustica united with Acustica 97, 830-839, 2011.