There are several ways of capturing impulse responses of acoustic spaces. The simplest is to produce a loud, quick sound within a space and record it. A balloon pop, starter pistol, or even hand clap can do the trick. This method is easily corrupted by ambient noise, however, and will only work well in spaces which are unusually quiet (recording studios, concert halls, etc).
A more sophisticated method employs a swept sine wave or ‘chirp’, which slowly rises from the lowest to highest register of human hearing (20Hz to 20kHz). Analyzing how a room responds to this chirp will create a good impulse response, but only if used with expensive speakers and microphones in a reasonably quiet space.
The EchoThief software provides a novel technique which can create high-definition impulse responses even in noisy environments. This website contains impulse responses gathered in unusually clamorous places: windswept canyons, rush hour underpasses, and noisy skateparks, among others.