Transfer Function Measurement (TRF)

Finally, post-processing provides the time delay (zero delay plane), minimal phase, group delay and several frequency-time transformations (e.g. cumulative spectral decay and spectrogram). A new measurement technique is provided for detecting low energy, impulsive disturbances, such as clicks and Rub & Buzz effects.
It reveals distortion of a much lower level and with a considerably higher temporal resolution (“instantaneous distortion”) than the traditional Fourier analysis, which shows mean values only. In addition to the common 2D representation (distortion vs. frequency), the distortion can be visualized in a 3D plot. Here, the crest factor of the impulsive distortion is mapped vs. frequency and vs. one of the measured signals (voice coil displacement). This is quite useful to localize and identify the cause of the disturbance.

The figure to the left illustrates the measurement of the equivalent input distortion using the TRF module. The harmonic distortion are measured at different points in the sound pressure field (left diagram) are transformed to the input of the loudspeaker system using the inverse transfer function H(f,r_i)^-1. This transformation shows that the harmonic distortion has a common source close to the input of the loudspeaker system, and the linear system response performs only a post-shaping of the distortion component. The measurement of the equivalent input distortion is a powerful concept to identify the physical cause of the distortion, to reduce redundancy of the measured data and to simplify the interpretation of the results.