Originally Posted by
d747
Regarding the CVR and FDR pingers, Is it possible that the frequency may have changed they are transmitting on?
Conceivable but irrelevant.
A radio transmitter typically transmits on one frequency that will only be picked up by a receiver tuned to receive signals on that frequency.
A sonar receiver is a broadband receiver, like a radio hams airband broadband receiver. It will be detecting soundwaves across the entire frequency spectrum.
The beacon transmission frequency was posted earlier. There is one problem with underwater sound and that is ray path propagation. My memories of oceanography are fading except to say that the soundwaves do not travel in straight lines. They will curve and slow as they approach the surface. They will be reflected from the thermocline at about 300-450 feet depending on surface temperature anr previous weather conditions. They will descend before curving back toward the surface. These give rise to convergence zone detection many miles from the source and no immediate clue as to distance if they are detected in the 2nd or 3rd convergence zone, typically a band 4 miles or so wide 30 odd miles apart.
Once near the source then direct path will lead to ultimate location.
The propagation distance deoends, amonst the other criteria, on the emitter frequency with lower frequencies propagating further.
I have copied a piece from #770 from the earlier thread:
[quote]At the frequency of the DFDR/CVR pinger (37.5khz) the average absorption rate in sea water is between 6-10 dB/km. This does not take into account thermal layers. The relationship between absorption and frequency is more on a log scale rather than linear. Lower frequencies can travel further than higher ones, not unlike the audio range in air./[quote]
Combined this suggests that a short range, direct path detection is more likely that a long range convergence zone detection. It returns the issue to a needle in a haystack situation.