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Cepstral Techniques for Transducer Measurement

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We discuss the use of the cepstrum in the measurement of electro-acoustic transducers, especially loudspeakers. The cepstrum, defined as the inverse Fourier transform of the complex logarithm of the system's transfer function, is implemented digitally by interfacing a dual-channel FFT analyzer with a microcomputer. The importance of the cepstrum is that it holds out the possibility of enabling one to deconvolve cabinet and room reflections from a transducer measurement even at low frequencies. This would enable the production of anechoic (free-field) low-frequency measurements indoors as well as allowing the investigation and assessment of transducer diffraction effects. The power, complex and phase cepstra as well as two new cepstra - the minimum-phase and all-pass cepstra - are introduced. The processing required to calculate these cepstra and their envelopes is described in some detail with the help of flowcharts, and illustrated with synthetic as well as actual loudspeaker measurement data. It is shown how limited but significant success in deconvolution is possible by means of cepstral editing. This paper can be considered a continuation of preprint 2050.

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