OK, I know it’s only a comic book movie, but still: sonar cell phones?
Work out the details. Let’s assume it is possible to force anyone’s cell phone to become an echo transponder (it maybe possible for all I know). The ultrasonic beep would have to be generated on the phone—cell phone transmissions have a notoriously low range: less than 10 kiloherz (kHz). The human ear can usually hear up to 20 kHz, and some people beyond that.
If the cell phone speaker can manage to generate the ultrasonic beep because Batman’s software completely took over the codec, then you still got problems. The sound goes out, and we’ll be nice and pretend it actually forms a nice sweeping cone, but when it comes back, it’s still coming through a crappy microphone and digitized by a cheap ADC. I’ll play nice and assume Batman was really smart and came up with a nifty software hack to perform the real-time compression of ultrasound. How does he know which direction the echo came from?
Sonar in animals depend quite heavily on being able to tell exactly where a sound came from. For bats and some birds, it’s the pair of ears on their head. For whales, they use their lower jaw and ears. These mechanisms allow them to precisely figure out direction as well as distance. A cell phone has a very limited ability to detect direction, not enough to give the detailed pictures in the movie. The microphones on cell phones are designed to be very responsive in a narrow band in front of it and almost deaf to anything outside of that band. So at best you’ve got a spotlight, but all you get back are a bunch of echoes that tell you nothing of the shape or direction. The microphone still can’t tell what angle the echo came in at.
The one part about that plot device that came close to reality was monitoring all that cell phone traffic for someone’s voice: there are rumors that the NSA’s ECHELON program can pick out voices of “parties of interest” from thousands of calls. Of course, I’m sure that’s just tin-foil hat thinking. :-)