There seems to be some confusion on the black box pinger.
The pinger on the black boxes use sound waves at 37.5 kHz, and require sonar detectors, which are essentially sophisticated microphones. Even though this falls in the LF band of 30kHz to 300kHz, the LF designation is for radio, or rf, signals, not sound. 37.5 kHz is ultrasound to us humans.
Sound signals are compression waves - they vibrate the molecules back and forth in the direction they are traveling. Radio, or light, waves are called transverse waves. They consist of an electric, or E field, and a magnetic, or H field, which are both at right angles to each other and to the direction they are traveling.
As they propagate out from a sound source, both sound and radio waves are attenuated by 1/(4*pi*r^2), which is the surface area of a sphere - this is the main reason why sounds or the brightness of lights get weaker the further one moves away from the source. In addition, in water, it appears that sound at 37.5 kHz is attenuated by an additional 0.1 dB/kilometer due to absorption, meaning that sound waves are barely affected by the water. Radio waves at 35 kHz are attenuated by about 6000 dB / kilometer due to the conductivity of salt water, which says the radio wave are essentially totally absorbed.(Underwater Radio Communication by Lloyd Butler VK5BR). This is why they use sound waves instead of radio waves.
However, this does not take into account thermoclines, which, from what I understand, can cause almost total reflection of a sound wave.
Hope this helps.