Can you crack a safe by just listening?

The most surreptitious way of cracking a safe is to manipulate the lock in order to obtain the combination required to open the safe without actually damaging the safe.

Some rotary combination locks can be manipulated by feel or sound in order to determine the combination required to open the safe. More sophisticated locks use wheels made from lightweight and soft materials such as nylon, which reduces this vulnerability. Another anti-manipulation mechanism is serrated wheels (false tumbler notches) that make tactile techniques much more difficult. Another defense is a clutch-type driver wheel that prevents contact of the fence to the tumblers except in one position. These locks can be identified by a “click-click” feeling in the dial or by a dial that is pushed in and turned. Manipulation is the locksmith’s preferred choice in lost-combination lockouts, since it requires no repairs or damage, but can be extremely time consuming due to lock improvements over the years, and is also a difficult art to master.

In the absence of any other information regarding the safe’s combination, a combination lock may be opened by dialing every possible combination. Richard Feynman[1] discovered that many combination locks allow some “slop” in the settings of the dial, so that for a given safe it may be necessary only to try a subset of the combinations. This drastically reduces the time required to exhaust the number of meaningful combinations. A further reduction in solving time is obtained by trying all possible settings for the last wheel for a given setting of the first wheels before nudging the next-to-last wheel to its next meaningful setting, instead of zeroing the lock each time with a number of turns in one direction.