What are 2.5 Hz delta waves? Delta waves are very slow patterns of electrical activity measured in the brain by an EEG.

They are commonly defined as roughly 0.5–4 Hz and become especially prominent during deep, non-REM sleep. A frequency of 2.5 Hz means 2.5 cycles per second, placing it within the delta range.

2.5 Hz is probably not an audible tone on its own—it is below the normal range of human hearing. Instead, the recording may create a 2.5 Hz rhythmic pulse using:

• Binaural beats: slightly different audible tones are sent to each ear, and the brain perceives the difference as a beat. Headphones are required.
• Monaural or isochronic pulses: the sound's volume rises and falls 2.5 times per second. These can work t ...