MEASURING THE LINEAR AND NONLINEAR PROPERTIES OF ELECTRO-ACOUSTIC TRANSMISSION SYSTEMS

Competitive audio consumer products require not only cheap signal processing hardware but, also low-cost analog equipment and sound transducers. The nonlinear distortions produced by these electro- acoustic transmission systems cannot be described and analyzed by standard methods based on linear systems theory alone. In order to take the nonli- near properties into account, we present a measure- ment method for the linear and nonlinear transmis- sion characteristics of almost arbitrary systems and show its application to the analysis of electro-acoustic systems. Examples demonstrate the measurement of the impulse response of a loudspeaker-enclosure- microphone-system with cheap analog equipment. 1 INTRODUCTION Real-time signal processing used to be the most ex- pensive part of acoustic echo and noise control app- lications. The additional cost of complementing the digital hardware with good quality audio equipment was tolerable and consequently, nonlinear effects of sound transducers could be neglected. The situation is changing with the availability of cheap computing power for real-time applications. Competitive audio consumer products require not only cheap signal processing hardware but also low- cost sound transducers. Software-only solutions of speech communication features for desktop compu- ters have to rely on built-in microphones and spea- kers, whatever their quality may be. Therefore, non- linear distortions have to be taken into account in the design of low-cost electro-acoustic systems. But also the digital transmission of speech or au- dio signals is subject to nonlinear effects. Subband- coding with the least possible number of bits assigned to each band is a standard technique. Also low-cost analog-digital-converters produce distortions, which cannot be described by linear effects only. Many acoustic echo and noise control appli- cations require to measure or estimate the pro- perties of the loudspeaker-enclosure-microphone- system (LEMS) including any digital pre- and post- processing. The nonlinear effects described above may show up in many places of this electreacoustic transmission chain, so that it is not always possible to consider them by a proper theoretical analysis. Also measurements of the distortion factor of isolated com- ponents do not give a complete picture of the nonli- near behaviour of the overall system. On the other hand, common measurement methods for the room impulse response record only the linear transmission characterics and are blind for nonlinearities. This contribution presents a measurement method for the linear and nonlinear transmission characteri- stics of almost arbitrary systems and shows its ap- plication to the analysis of electro-acoustic systems. The theoretical foundations of the method are de- scribed. Examples for the measurement of a room impulse response with cheap sound transducers show how nonlinear effects can be detected and evaluated.