The recording equipment I use is sensitive enough to record bats that are flying some distance away from the microphone (in some cases more than 20-30 meters if the conditions are just right). However, the recordings that you make in such cases are limited in usefulness because all you know is that at least one bat was flying around the area. There has been some success identifying bat species by their echolocation sounds, as well as counting bats using recordings only, but to do this, you need to have good recordings from bats that you can identify so that you can tell if your procedure is working. This means you need to have captured bats that you can record so that you can correlation their echolocation sounds with their age, sex, species, etc.
Unfortunately, bats are notoriously difficult to catch - talk to anyone who has tried to capture a bat that is flying around inside their house if you want the details! Since echolocating bats are so good at detecting small objects, it is hard to trap them in a net. To get around this problem, you have to use a special type of net called a mist net, which is made up of very fine fibers that a bat can miss if it's not paying attention to its sonar. You also generally put the nets up in an area the bats fly through regularly, so they won't be expecting it. The bats sometimes fly by memory, so they might not notice a net until they've already hit it.
There is one important limitation to using these nets - you're generally required to have a permit filed with an organization like the state Department of Natural Resources or another agency before you're allowed to catch bats in this manner. If you're catching endangered species, you also need permission from the US Fish and Wildlife Service. Below are some pictures of a bat that I caught in Georgia a few years ago.
Here's the bat sitting on my hand. You can see the thin, black threads of the mistnet. Also notice something VERY important - I never handled these bats with bare hands! Bats are wild animals, and like all wild animals they will bite if they are threatened or provoked. A bat that has been caught in a net is both frightened and provoked, so I can guarantee it will try to bite. By wearing leather gloves, I can guarantee that the bat won't be able to bite me, leading to concerns about disease or infection from any bites. This also means I wouldn't have to get a bat tested for rabies (which requires killing it) so it's better for me and the bat to wear gloves.
Here's a closer shot of the bat - this particular one got very tangled in the net, but she was relatively calm after the first minute or so.
Here's another bat that got caught in a net from a house in Georgia. This bat was living in an attic, and we caught it by putting a net up over the attic vents near the eaves of the house as shown here:
Here's the bat after being released. She was agitated, but I was able to record her and release her unharmed, so I got some data and the bat got some free mealworms before I released her, so everyone benefited! :)
The next question is what do you do with a bat once you've caught it? Read on for more information about that.