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Institute of Turbomachinery and Fluid Dynamics
Logo Leibniz Universität Hannover
Institute of Turbomachinery and Fluid Dynamics
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Radial Mode Analysis (RMA)

Radial Mode analysis (RMA) is a well-known evaluation technique that computes the complex amplitudes of higher order acoustic modes propagating through flow ducts. This experimental technique has been developed to study the transmission of the discrete tonal noise components. The acoustic field inside a turbomachine can be understood as a superposition of several sound pressure waves. This method decomposes the transmitted sound field in dominant acoustical modes at specific frequencies. Subsequently, the sound field is decomposed into radial and azimuthal acoustical modes.

Field of Application

  • to gain a detailed understanding of the sound source mechanisms
  • to study of sound transmission of discrete tonal noise components in turbomachinery and cylindrical ducts
  • to quantify the sound power radiated by turbomachinery
  • to analyze several acoustical structures under no flow, uniform flow and with swirling flow conditions
  • to describe the impact of modern sound reduction concepts (liner-segments)

Measured Quantities

  • sound pressure frequency
  • blade passing frequency
  • azimuthal mode amplitudes
  • radial mode amplitudes
  • phase
  • azimuthal and radial mode orders


Acoustic measurements in turbomachinery are required for the study and consequent understanding of their internal sound-propagation mechanisms. RMA enables the calculation of the acoustic power radiated in and against the flow direction, although at high frequencies it requires the acquisition of the sound field at a large number of positions in the flow duct. After recording the acoustical data at the specific excitation frequencies at the different azimuthal positions, the RMA is carried out in order to quantify the dominating sound field. Afterwards, each signal is analyzed with a time Fast Fourier Transformation (FFT). The quality of the analysis results is very sensitive to the arrangement of the microphone array design, the frequency and the flow parameters.


Akif Mumcu, M.Sc.