You’re on your cell phone, talking about something or another. Suddenly, you realize that the other person is strangely quiet — too quiet. “Still there?,” you ask the void, hopelessly, as you already know the answer. The other person didn’t just stop talking. Your call has been disconnected.

Again, you knew that already. But how did you know that?

The answer? Your cell phone provider tricks you. (In a good way, though.)

When we have real, in-person conversations, it is rarely, if ever, in a place of true silence. Perhaps an air conditioner is running, or birds are chirping, or someone is rustling papers quietly at his or her desk. These tiny background sounds aren’t interrupting your conversations, though. Subconsciously, we anticipate them (this is often, in aggregate, called “white noise”), and they act as a signal to our brains that all’s normal. But communications which aren’t in person — cell phones and radio, especially — don’t have this white noise. Sure, there are the background sounds in the area we are in, but as the speaker is in another area, that doesn’t do us much good.

So we fake it. Or, rather, the cell phone companies do, by adding what is called “comfort noise.” Wikipedia defines it as “synthetic background noise used in radio and wireless communications to fill the artificial silence,” and it’s not the easiest thing to provide. All the sounds we hear on our cell phones are just data bits being translated into sound, and transmitting that data takes up bandwidth. This is as true for real conversations and comfort noise alike, so many companies are in the business of optimizing this useful static.

And no, eliminating comfort noise isn’t an option; that would lead to a surprisingly number of “hello?”s and “you there?”s. According to one provider of comfort noise, “most conversations include about 50% silence.”

Bonus fact : Radio was an incredibly important form of mass communication during World War II. For example, before and after air raids, the city of Leningrad broadcast instructions to take cover and later, issued the all-clear over a municipal radio service. That radio system reached most of the city; it had about half a million households and businesses with speakers, and another 10 to 20, 000 loudspeakers on the street. But what if the Germans bombed the transmitters? How would the people of Leningrad know? To account for this, the city radio employed an early version of comfort noise, softly broadcasting the sound of a metronome when no announcements were being made.

From the Archives: The Sound of Silence: What does it sound like when there is no sound?

Related: A selection of white noise machines.