Vinnie Boombatz:
This topic has been covered before in the AF447 threads and, if I recall, in the MH370 threads:
http://www.pprune.org/tech-log/39510...ml#post5683946
While it is true that there is a noise minima in the deep ocean in the 40KHz band, that is not especially relevant to maximizing detection range. For long ranges, the attenuation at 37kHz is a far more severe limitation than the background noise (and is part of the reason why there is less noise in that band). While the choice of 37kHz for pinger locators was made many decades ago, I expect that the choice was based on the smaller size/weight compared with a low frequency device, and the fact that the short range is adequate for the more common near-shore accidents.
For deep water work we commonly use transmitters in the 7-15kHz band, yielding ranges beyond 10km; 20-30kHz can yield 3-5km; and 40kHz 2-3km. These are not necessarily comparing equal sound pressure level, equal energy/ping, or any other "normalized" comparison, but are ranges practically obtained by traditional oceanographic systems. In this case, typical transmit levels in the 7-15kHz band are 200-210db re 1uPa @ 1m, and the typical aircraft pinger locators transmit at 160db re 1uPa @ 1m.
The problem with low frequency pingers is that they are significantly larger and heavier. A typical 7-15kHz transducer is 7-10cm in diameter and length, compared to less than 2cm for the 37kHz pingers. Certainly a lower frequency pinger or transponder could be carried by an aircraft, but everyone hates to add a kilogram of mass without reason. Also, the 37kHz pinger, being smaller, is much easier to design for shock resistance; that design has been highly optimized over the years. Shock protecting the larger, ceramic transducers might present an issue.
As also noted before, the use of transponders, rather than pingers, would extend the battery life. A transponder does not transmit unless it first receives an interrogation. Thus it only needs to operate its receiver until someone arrives to search, and it can then start transmitting (then also providing range via travel time). It is also possible to further save power by only listening intermittently until an interrogation is heard, and then to listen continuously for a while. All these extra features were complicated decades ago, but are now trivial to implement.