Below is a representation of the human hearing threshold which gives us a good idea as to why a 3 kilohertz (kHz) beacon signal is a good signal to use for echolocation.
This chart is plotting the intensity in decibels that is audible by the human ear at different frequencies along the audible spectrum of sound. Additionally, the different lines represent different signal "loudness levels". At lower frequencies, and lower loudness levels it becomes increasingly difficult to differentiate between tones. The lower tones, near 20 Hertz are only audible once they reach approximately 75 decibels (dB), which means that a low tone needs to be far more powerful to be perceived.
There is a consistent dip around 3kHz where it is indicated that softer sounds are perceived just as well as other frequency tones of the same loudness. As signals get higher in frequency, it is required again that they be of greater loudness in order to be perceived at the same level.
This has to do with the construction of the human ear canal. A 3kHz frequency resonates nicely in a 2.4cm tube at body temperature, which is the average size of an ear canal.
We have to remember however, that a single frequency tone is not entirely optimal due to other circumstances. For instance, a 3kHz tone may be completely absorbed by an object. Fiberglass boards appear to absorb these frequencies very efficiently, as well as upholstered benches according to this absorption coefficient chart. For this reason, it is important to choose a signal that consists of a broad spectrum of frequencies.