In The Dark Knight a city-wide surveillance system resembling sonar is portrayed. What is the in-universe explanation of how Gotham's phone's speakers and microphones are able to be used to generate and interpret the waves used to operate Batman's surveillance system?

  • 1
    Some blind people use echolocation to navigate, using their tongue and palate to emit clicks and divining information about their surroundings from the return times, directions and attenuation of the echoes. A clever programmer could do similar work with a phone mic and speaker, but would be hampered somewhat by the lack of stereo reception.
    – Kyle Jones
    Mar 23, 2015 at 6:13
  • I always thought that (one of) Bruce Wayne's business's was the mobile phone industry. en.wikipedia.org/wiki/Wayne_Enterprises
    – JMP
    Mar 23, 2015 at 13:33
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    Completely plausible but technically unwieldy.
    – user16696
    Mar 23, 2015 at 16:45
  • Erm, this is explained in the film. In vivid detail.
    – Valorum
    Mar 23, 2015 at 22:18

2 Answers 2


Because the absurd and unrealistic technology is not based in the phones, but in the computer that registers the sonar.

Once a mobile phone has been hacked into (which is reasonably simple, particularly for someone like Lucius Fox or Bruce Wayne), it is simple enough to program the phone's speaker to emit a specific noise with a specific frequency, then have the microphone pick up that noise as it is pinged back (echolocation).

The real problem comes from trying to analyze the data from thousands of phones simultaneously. Processing a single phone's echolocation in real-time would be hard enough, you would need a massive processing power in order to do it quickly.

You would need to register the sound returning from every direction, along with the respective times it takes for them to return. Then you would need to return this data to the computer to analyze, and print a 3D graphic onto a monitor. All within less than quarter of a second for real-time purposes (like in the fight with the Joker).

And then you need to do it a few thousand times.

As we see in the film, a massive computer is used. It is not unreasonable to believe that Bruce Wayne could buy or build a computer that size in order to be able to do it.

I realize that cellphone technology doesn't necessarily have the level of technology needed for this to work (3 different microphones that are more sensitive etc.).

However, this isn't an absurd or unrealistic technology, they could theoretically be created now. The general population having them, therefore, would fall into the realms of suspension of disbelief.

  • The bit that I struggle with is the fact that Lucius' phone can do this to a full skyscraper on its own from the bottom floor. Although it's possible that Fox could have put in a ridiculously powerful speaker and a ridiculously sensitive microphone beforehand. Mar 23, 2015 at 10:58
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    It's been a while, but I never considered the single "skyscraper scan" to use the same technology. I always though they've used the phone as an entry point to hijack WiFi/security/sensors already in the building. To locate something in 3D space, you'll need at least 3 points from which you're able to measure a distance. With one location your target is on a sphere, with two locations your target is on a circle and only with three or more locations you're able to pinpoint something down. As such the single phone approach won't work, even if there'd be stereo microphones.
    – Mario
    Mar 23, 2015 at 13:31
  • Hacking phone is NOT simple. It's actually very hard. It's a bit easier if you have physical access to the phone but still very hard for most devices. The microphones and capturing hardware don't have enough quality for this either - definitely not in ultrasound/infrasound frequencies. Then, you would have to filter out the background noise. If you use the same frequency on two devices close to each other, you won't get anything because they won't register the echo, they will register the other device. Not even speaking about the need to know the exact position of all devices.
    – Sulthan
    Mar 23, 2015 at 16:15
  • @Sulthan If the phone's owner has left the wi-fi or bluetooth on, it's actually ridiculously easily to hack it remotely. Even if 10% of people leave it on, that's enough phones to get radar signals from for a decent image. Also, the devices can emit sounds with a tiny difference in frequencies. Then the computer can identify only the frequencies that were emitted from that particular phone. Even if there are only a limited amount of frequencies to emit, a simple algorithm could be designed to only emit from a single phone within a certain distance of another making the same noise. Mar 23, 2015 at 16:28
  • listening to phone communication - yes if you have the right (expensive) gear. hacking in the meaning of taking control of the device? no, impossible. Your ideas are crazy. A tiny difference in frequency would work if every1 had the exact same type of phone. Masuring distance between phones is impossible if you have positions with error of several tens od meters.
    – Sulthan
    Mar 23, 2015 at 17:00

Per the screenplay

FOX : I had R and D work it up – it sends out high frequencies and records the response time to map an environment.

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