The Source
The Source
[Cueball is standing in an empty room looking in the direction of the next frame.]
[Cueball turns his head and looks the other way.]
[Three smaller panels with the same total height as the first two frames follows. In the first frame Cueball walks on a grey surface.]
[In the next Cueball is standing between two doors, looking over his shoulder towards the one to the left, but choosing the one to the right behind which a stair is. He is waking towards this door with his hand out towards the knob.]
[In the last of these smaller panels Cueball has just walked down to the bottom of the stairs.]
[Cueball walks towards a machine that is standing near a wall connected to a socket in the wall. On the machine it says:] High pitched hum generator
[Cueball kneels behind the machine and unplugs it from the socket in the wall.] High pitched hum generator
[Cueball walks away from the machine, the plug now lying on the floor between the wall and the machine.] High pitched hum generator
'Empty room hum' is also referred to in other waves in 273: Electromagnetic Spectrum and the title text in comic 597: Addiction .

This comic is about experiencing a high pitched hum in an empty room. An "empty-room hum" is a high pitched buzzing noise, often caused by tinnitus , which is a medical condition causing high-pitched noise when there is no other noise around. Tinnitus is normally a hearing condition, not a disease. It may result from the brain increasing its sensitivity to noises.
Sometimes not everyone can hear "empty-room hum"; however, those who can hear it usually find it immensely annoying. If you do hear the noise, you would like to locate The Source – hence the title of the comic. Hopefully when you find the source, you can do something about it. Or if you don't find it, you can at least be at ease knowing that others experience the empty-room hum, it having been referenced in two xkcd comics now and elsewhere on the internet.
This comic alludes to the perspective of an outside observer who doesn't hear the hum but is watching someone who can hear it: because the sound isn't written out in text, the comic reader at first is confused by Cueball's inexplicable searching.
In the first two frames of the comic we see Cueball trying to locate the direction of the sound, by standing in the middle of the room, turning his head from one to the other side. Finally he walks down a flight of stairs (probably to the basement) and here he locates the source: A machine whose only function is to generate a high pitched hum. The title text asks why on Earth they had such a machine in the first place, which is somewhat difficult to explain and likely the crux of the title text's joke.
Luckily it was thus easy for Cueball to get rid of this sound at the source. But in real life most electronics generate hums and cannot reasonably be turned off without losing functionality. For instance fluorescent lights, phone chargers and computer modems are common culprits, refrigerators and washing machines less commonly. It could also come from outside the house, in which case it will be much harder either to locate the source or to do anything about it. Power lines and transformers are common outside sources.
There do, however, exist devices that are meant to create a high pitched hum, that people might wish to install in their house. These will be humming in the ultrasonic regions, although cheap versions can often be heard by young people. They are typically used for electronic pest control , while slightly lower frequencies which can typically be heard only by young people are sometimes used to repel children . It is possible that someone tried to get rid of Cueball.
There do exist white noise generators (which make equal volume noise on every frequency) and pink noise generators (which make noise that sounds equally loud to the human ear at every frequency) which are used to test recording studios to see if they have good sound quality. It seems unlikely that the device is one of these, as it seems to be designed to generate a high-pitched hum: pink/white noises are categorically and perceptually different from a hum.
The sound wave spectrum in 273: Electromagnetic Spectrum also contains a line for "that high-pitched noise in empty rooms".
The empty white room also could be a reference to a scene from The Matrix Reloaded in which Neo searches for "The Source," though this is likely just a coincidence.
There is a story by A.E. Van Vogt in "The War Against the Rull" where an all-pervasive vibration leads to a coming of age for the youthful protagonist.
[Cueball is standing in an empty room looking in the direction of the next frame.]
[Cueball turns his head and looks the other way.]
[Three smaller panels with the same total height as the first two frames follows. In the first frame Cueball walks on a grey surface.]
[In the next Cueball is standing between two doors, looking over his shoulder towards the one to the left, but choosing the one to the right behind which a stair is. He is waking towards this door with his hand out towards the knob.]
[In the last of these smaller panels Cueball has just walked down to the bottom of the stairs.]
[Cueball walks towards a machine that is standing near a wall connected to a socket in the wall. On the machine it says:] High pitched hum generator
[Cueball kneels behind the machine and unplugs it from the socket in the wall.] High pitched hum generator
[Cueball walks away from the machine, the plug now lying on the floor between the wall and the machine.] High pitched hum generator
'Empty room hum' is also referred to in other waves in 273: Electromagnetic Spectrum and the title text in comic 597: Addiction .
