Practical aspects of implementing car interior active noise control systems

When implementing real-world, close-to-production active noise control (ANC) systems for car interiors, many aspects far beyond textbook theory have to be taken into consideration -- many of which might also be left out for first demonstrator systems. Due to the predominant role of robustness and reliability, any kind of uncertainty in the system has to be considered carefully. Among others, the uncertainties and changes of the acoustical environment, e.g. due to temperature changes, number of passengers, open windows and also of system components, e.g. loudspeakers and microphones have to be measured and/or modeled. This also gives some new insight into the acoustical environment inside cars at low frequencies. In addition, the quality of input data (e.g., the update rate for rpm information) is of major importance for the acoustical performance in terms of noise reduction. Finally, stability analyses and robustness calculations must be extended to incorporate uncertainties as well as time domain effects even for more or less freqency domain problems like engine noise. This requires to re-formulate feed-forward systems as feed-back systems and calculate system responses. Measurement results on the interior acoustics (and its uncertainty) as well as additional developments on ANC system robustness will be presented.