Delta waves (0.5–4 Hz) — what they are and what the research actually shows
What the band is
Delta is the slowest of the conventional EEG bands, roughly 0.5 to 4 Hz. On a scalp recording it appears as large, slow, high-amplitude waves. In healthy adults it dominates during the deepest stage of non-REM sleep, which is why that stage is also called slow-wave sleep. Delta activity in a waking adult recording is unusual and is one of the things a clinician looks for.
What research associates with it
Delta is the defining signature of slow-wave sleep, and slow-wave sleep is the stage most associated with physical restoration and with the consolidation of newly learned material.
Slow oscillations in this range have been proposed as a coordinating rhythm that groups faster activity during sleep, rather than as a passive by-product of an idling brain.
Claims that listening to a delta-frequency audio track produces delta-dominant EEG in a waking listener are a separate question from what delta does during sleep, and the evidence for the first is much thinner than for the second.
What it is not
Delta is not something you can decide to enter while awake and alert. It is the electrical signature of a state the body goes into, not a lever the state responds to. A tone at a delta frequency in your headphones does not put the brain into slow-wave sleep, and no audio track is a substitute for sleep itself.
It is also not a treatment for insomnia. If falling or staying asleep is a persistent problem, that is a conversation with a doctor, not a frequency to listen to.
What listeners report
Individual response varies, and some people notice nothing. Most commonly reported: