Answer: Your smartphone.
This is just one more reason to be concerned that our phones are always listening.
A research team at Texas’ Southern Methodist University has discovered that a smartphone’s built-in microphone can be used to figure out what someone nearby is typing on a keyboard. Even in a loud place where there might be a lot of overlapping talking and typing, like a coffee shop, the team was able to use a simple smartphone to detect the unique soundwaves given off by each key pressed on a nearby keyboard.
They were then able to analyze these sounds to determine which keys were pressed and in what order, at an accuracy rate of 41 percent. That means that almost half the time, their smartphone was accurately telling them what was being typed. Furthermore, they believe that that number could be improved if they were to narrow their focus to the 10 most commonly typed words.