Why 40 Hz is special
Gamma waves - brain oscillations between roughly 30 and 100 Hz, centered near 40 Hz - accompany attention, sensory binding, and memory processes. In several neurodegenerative conditions, gamma activity weakens. That observation led MIT researchers to a striking experiment: exposing mice to 40 Hz flickering light and 40 Hz sound restored gamma rhythms and reduced amyloid plaques, a hallmark of Alzheimer's pathology.
The follow-up matters as much as the headline: those results are in mice. Human trials of 40 Hz light-and-sound stimulation (a field now called gamma sensory stimulation, with dedicated clinical programs) have shown the stimulation is safe, entrains gamma rhythms in human EEG, and early trials report slowed decline on some measures - but this research is ongoing and no audio tone is an approved treatment for anything. This page exists because the science is genuinely fascinating, not because listening treats disease.
What you are actually hearing
As sound, 40 Hz is a very low hum - just below E1 (41.2 Hz), the lowest string of a bass guitar. Small speakers cannot reproduce it at all; you need headphones, decent speakers, or a subwoofer to hear the fundamental rather than just its harmonics. If you hear a thin buzz instead of a deep hum, your playback system is showing you overtones only - our subwoofer test will confirm what your system can reach.
Note the distinction between hearing a 40 Hz tone and gamma entrainment protocols: research stimulation typically uses sound modulated at 40 Hz (clicks or amplitude-modulated tones, usually combined with 40 Hz flickering light), not a plain sine wave. A related approach with headphones: our binaural beats generator can produce a 40 Hz beat frequency from two higher tones - a common consumer approximation of gamma-band stimulation.
Practical uses today
Beyond the research interest, 40 Hz earns its keep as a test signal: it sits in the heart of the sub-bass band, making it a quick check of subwoofer output, room boom, and port noise. Producers also use it as a reference when checking how much true sub-bass energy a mix carries.
If you want to explore gamma-adjacent listening, keep expectations calibrated: enjoyable, harmless, scientifically interesting - and download the WAV if you want to loop it or inspect it in a DAW.