Binaural beats. A pattern of wah-wah type of noises that your brain generates when playing unmatched frequencies in your ears. Depending on the speed of the rhythm, your brain will assume different brain wave patterns, like alpha, theta, beta, and others. You can use this to force your brain into deep thought, relaxation, or whatever you would like.
If you’ve played an instrument before, and have tuned it by matching a note to another instrument, you may have heard the wah-wah sound occurring. This is believed to be due to the sound waves matching up and separating repeatedly. As the notes become closer to in-tune, the wah-wah noise slows and then stops.
When playing different notes through your headphones, the sound waves don’t actually interfere with one another, but your brain creates the same result within itself, so as to make you “hear” a wah-wah noise, even though it is not really occurring. This alters your brain waves as it occurs, making it useful for meditation or concentrating, especially when talking or other noises distract you.
If this is too freaky or new-agish to you, realize that it is completely physical and not spiritual or voo-doo. Your brain functions in waves and this just makes it easier to change your current pace. Also, there is no known side-effect to this. Finally, if you don’t want to try it, you don’t have to.
Here are some examples from GrooveShark: