Shield grounding assessment for audio systems in the aircraft environment

A study was made to quantify the degradation in noise rejection in an aircraft audio communication system when implemented with optimum high intensity radiated fields (HIRF) and lightning wire shield configurations. The intent was to take advantage of shields already incorporated in the system as a standard audio wiring practice, to raise the RF and lightning immunity by grounding the shields at each end. Measurements were performed on representative audio circuits and wiring in a shielded room under wire-to-wire coupling and ground potential disturbances. Various options for shield grounding were implemented, and the noise response of the system measured and recorded. Several circuit types utilized in the aircraft were duplicated, including balanced, single-ended, and isolated sources. It was found that shield grounding at both ends did not always compromise the noise rejection within the audio frequencies. On the other hand, there was a reduction in noise interference at the higher frequencies when both-end grounding was imposed, not to mention the common-mode protection obtained in the case of lightning transients. However, for ground potential interference sources, there was a significant increase in noise coupling. This would be a problem at aircraft power frequencies and related harmonics. It was also found that some of the grounding practices currently in use may be detrimental. Recommendations are made for design improvement.