Delta waves (1-4 Hz) are associated with improved sleep onset and maintenance, theta rhythms (4-8 Hz) with subtle sleep or meditation states, alpha waves (8-13 Hz) with recall of existing information, beta waves (15-20 Hz) with focus for sustained thought or new information incorporation, and gamma waves (32-100 Hz) with learning and problem-solving.

“Delta waves would be big, slow waves, so low frequency. And indeed there is quality evidence from peer-reviewed studies that tell us that delta waves like 1 to 4 hertz, so very low frequency sounds, can help in the transition to sleep and for staying asleep, and that theta rhythms, which are more like 4 to 8 hertz, can bring the brain into a state of subtle sleep or meditation, so deeply relaxed but not fully asleep. And you'll find evidence that alpha waves, 8 to 13 hertz, can increase alertness to a moderate level-- that's a great state for the brain to be in for recall of existing information-- and that beta waves, 15 to 20 hertz, are great for bringing the brain into focus states for sustained thought or for incorporating new information. And especially gamma waves, the highest frequency, the most frequent ripples of sound, so to speak, 32 to 100 hertz, for learning and problem-solving.”