There’s a science focused on affecting our brain-waves through sound, called brain-wave entrainment or brain-wave synchronization.
The thinking is, since we know that certain brain-wave frequencies pertain to certain brain states (light sleep, REM sleep, Deep Sleep, meditative states, etc), if we stimulate the brain with the appropriate tones or frequencies, we can “force” the brain to synchronize with these tones and establish the state we’re targeting.
There are two primary ways of generating these tones (they’re so low in frequency that they fall outside your typical stereo or radio):
Binaural Beats are generated by generating two tones, a few Hz apart in frequency. Each tone is “introduced” into the brain using headphones, and a “beat” tone is produce. For example, if you injected 505Hz into the left ear, and 495Hz into the right, your brain would recognize and synchronize to the “beat” between these two frequencies – 10Hz – an Alpha wave (505Hz – 495Hz = 10Hz).
Isochronic Tones are a more modern invention, where the use of headphones is not required. In this case, we generate a steady tone, and turn it on and off at a prescribed rate. Your brain picks up on the rate of this “on-off” phenomenon – maybe 10 times per second – and gives the same (most say stronger) 10Hz perception to your brain. The same Alpha wave is produced as with the Binaural Beat.
With both Binaural Beats and Isochronic Tones, the common practise is to embed the actual effect into a soundtrack that’s pleasant to listen to. There are hundreds of tracks you can download, and a great many of them are free on the Internet. Some of the more popular Binaural/Isochronic apps are available on most Smart-phones through their on-line stores (I use ones from Pzizz, Sleeptracks and Pure Sleep).