Aviation Investigation Report A98H0003
1.16.4 Analysis of Cockpit Sounds Recorded on the CVR
During the playback of the CAM channel on the CVR, numerous "click" sounds were heard, many of which were subjectively identified as typical events in a cockpit, such as radio keying, book binders opening or closing, cutlery striking a plate, and the cockpit door opening or closing. Other clicks could not be readily identified, and an attempt was made to determine whether any of the clicks could be the sound of a CB tripping.
Several tests were conducted in an MD-11 cockpit during which CBs at various locations on several CB panels were tripped in a noise environment similar to in-flight conditions. The audio recordings from this testing, and the unknown "clicks" from the SR 111 CVR were digitized and analyzed at the Industry Canada Communications Research Centre. A cross-correlation study was used to detect any similarities between the "click" segments of each digitized file. The analysis of the spectra of the original signals and the cross-correlations revealed that the background noise environment had a significant effect on the signal. Attempts to remove the effects of the background noise environment from the "click" by notch-filtering, linear prediction, spectral subtraction, and median smoothing were unsuccessful.
The results of the Industry Canada Communications Research Centre study were inconclusive. Subjectively, some of the sounds recorded on the SR 111 CVR were similar to the CB sounds recorded during the test in the MD-11 cockpit; however, none were scientifically proven to be similar.
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