Originally Posted by
JulioLS
Because it has a lot further range than audible frequencies, and detectors are readily available.
Next...
Actually, no, the latest research indicates that an 8.8kHz pinger has much greater underwater range...
"Following the two simulation exercises practiced in this study, it is quite clear that the decision made by the aeronautical administrators to adopt the use of a low-frequency ULB in the fuselage of airplanes was justified. Despite the differences between the two methods, both agree on higher pressure values at longer distances for the 8.8 kHz pings, as expected. The numerical model indicates the possibility of detection (RL > 10 dB [12]) at distances of ~8 km for the 8.8 kHz signal, while for the 37.5 kHz signal the detection can be considered not possible since the water depth (3790m) was bigger then the range where RL = 10 dB."
See:
https://www.researchgate.net/publica...ropical_waters
Note: The ULBs fitted in
Air France Flight 447, which crashed on 1 June 2009, were certified to transmit at 37.5 kHz for a minimum of 30 days at a temperature of 4°C. Investigating the crash, the BEA recommended that FDR ULBs' transmission period be increased to 90 days and for "airplanes performing public transport flights over maritime areas to be equipped with an additional ULB capable of transmitting on a frequency (for example between 8.5 kHz and 9.5 kHz) and for a duration adapted to the pre-localisation of wreckage" (i.e. with increased range).
- GY