The resonant frequency of a room is a concept often discussed in acoustic design, such as the layout of an auditorium or music studio. It could also be a topic of discussion when installing a home stereo system.
Essentially, most rooms are rectangular or square, meaning that they have parallel walls across from each other. These parallel walls make it possible for sound waves to get "trapped" or resonate back and forth between one wall and the other. Like the infinite image you might see if you look into a mirror and also have a mirror behind you. And it starts getting really interesting when you realize that there are particular frequency sound waves that "match" the size of the room.
For example, if a room measures 11.3 feet wide in one direction, that means that a 50Hz tone will bounce perfectly back and forth through the room. Since the pressure wave continually peaks at the same point in the room it will amplify that frequency in that location.
No, you probably won't actively use this concept during echolocation, but, yet again, an interesting little side note about how sound works and some of the effects you might encounter on a daily basis while practicing active echolocation.