CI 611 Cockpit Voice Recorder

[canuck slf]

After reading about the CI 611 CVR initial findings I am curious as to whether CVRs pick up noise via the structure as well as air borne noise?
If they are fairly rigidly mounted to the structure this would enable them to pick up noise which is normally inaudible due to the background ambient noise.
It would also be interesting to know if the Aloha "convertible"
737 CVR recorded similiar noises as the skin failed.

Thanks in advance
Regards

[PickyPerkins]

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canuck slf posted 24th June 2002 22:30
CI 611 Cockpit Voice Recorder After reading about the CI 611 CVR initial findings I am curious as to whether CVRs pick up noise via the structure as well as air borne noise? If they are fairly rigidly mounted to the structure ................
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Seems like you may right, and that structure bourne noise may cause the automatic gain control on the CVR to squelch the slower-travelling airborne sound, so that a loud sound clearly heard by several cockpit crew can be missing on the CVR record. I found the following in a RTO report (http://www.bst.gc.ca/en/reports/air/1995/a95h0015/a95h0015.asp).

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The loud bang heard by the crew and other witnesses was not evident on the CVR. The only unusual sounds recorded occurred two seconds after the V1 call, when the first of a series of 21 "thuds" was heard. A loud bang would certainly contain significant frequency components well within the CVR bandwidth (200-5,000 hertz). The lack of a pronounced loud bang on the CVR was likely the result of the wave transmitted through the aircraft structure causing the automatic gain control on the CVR to squelch the structure-borne signal, thereby masking the slower-travelling airborne sound. The series of thuds was considered similar to the sound of repeated compressor stalls.
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