20 Hz: the edge of hearing and the beta beat

What does 20 Hz mean?

20 Hz means twenty cycles a second. The hertz counts how many times something repeats each second, so one cycle of a 20 Hz wave lasts 50 milliseconds. The number says nothing about what repeats: a sound, a rhythm in the brain and a beat between two tones can all run at 20 Hz, and each behaves differently.

People who search for 20 Hz are usually asking about one of three. As a sound, 20 Hz is the number quoted for the bottom of human hearing. In the brain, it sits inside the beta band. And a 20 Hz binaural beat is a pair of tones you can hear that beat at that rate. This guide takes each in turn, and then shows, from Harmonance's own measurements, where the 20 Hz in a 20 Hz recording really is.

Can you hear a 20 Hz sound?

Yes, if it is loud enough, but only just as a tone. The range of human hearing is usually given as 20 Hz to 20,000 Hz, and a review of hearing at low and infrasonic frequencies notes that the lower limit is given as 16 or 20 Hz. It is not a wall. The ear grows steadily less sensitive as a tone goes lower, so a 20 Hz tone has to be far louder than a mid-range one to be heard at all.

What changes at 20 Hz is the character of the sound. The same review summarises it: pure tones become gradually less continuous, the sense of a tone stops around 20 Hz, and below 10 Hz the single cycles can be perceived (Møller and Pedersen). Earlier listeners described tones above 20 Hz as smooth and tonal. Sound below 20 Hz is called infrasound, but the authors stress that there is no sharp change in hearing at 20 Hz, so the line is a convention. The guide to 10 Hz follows hearing further down.

Where else does 20 Hz turn up?

Low sound near 20 Hz is more common than most people think. The review notes that the largest organ pipes are tuned to about 17 Hz, and that a car at motorway speed with a window open exposes its passengers to audible sound at 10 to 20 Hz. In the sea, fin whales call in pulses at about 20 Hz: one study measured 1,241 of those calls with eight seismometers on the floor of the north-east Pacific (Weirathmueller and colleagues).

The Earth rings near 20 Hz too, though not with sound. Lightning keeps the space between the ground and the ionosphere ringing with very slow radio waves, and NASA scientists give the average measured frequencies of the five lowest resonances as about 7.8, 14.3, 20.8, 27.3 and 33.8 Hz. The third sits close to 20 Hz. The guide to the Schumann resonance explains the first, 7.8 Hz, and why it is electromagnetic, not acoustic.

20 Hz in four places

Where	What runs at about 20 Hz	What it is
Hearing	The usual lower limit of the hearing range	A sound wave in the air
The sea	Fin whale calls	A sound wave in water
The Earth	The third Schumann resonance, about 20.8 Hz	A radio wave between ground and sky
The brain	Beta activity over the movement areas	An electrical rhythm

What does 20 Hz mean in the brain?

In the brain, 20 Hz sits near the middle of the beta band. The clinical glossary of the International Federation of Clinical Neurophysiology, in its corrected form, sets beta at above 13 to 30 Hz, a single wave lasting 33 to 76 milliseconds (Kane and colleagues). A 20 Hz wave, at 50 milliseconds, falls inside it.

The best known 20 Hz rhythm comes from movement. After a brisk finger movement, beta over the brain's movement area recovers and briefly rises above its resting level within a second: Pfurtscheller and colleagues described this rebound at around 20 Hz and read it as the movement area going idle. The guide to beta waves measures that dip and rebound in more than a hundred people's open recordings.

Brain activity at 20 Hz and above is also hard to measure cleanly from the scalp. The muscles of the face, jaw and scalp make electrical activity at the same rates. In two people recorded with and without full muscle paralysis, power above 20 Hz differed 10- to 200-fold, and the authors concluded that most of the scalp EEG above 20 Hz comes from muscle (Whitham and colleagues). So a headband that reports a lot of activity above 20 Hz may be reporting a tense jaw.

How does a 20 Hz binaural beat work?

A 20 Hz binaural beat plays one steady tone in each ear, 20 Hz apart, and your hearing turns the difference into a beat. Harmonance's plays 108 Hz in the left ear and 128 Hz in the right, two low hums that are easy to hear. The beat is in neither ear alone.

The beat is made in the brainstem. Neurons of the superior olivary complex, which respond to timing differences between the two ears, fire at the rate of the difference and create the percept, first reported by H. W. Dove in 1839; Oster found binaural beats were heard only with tones below about 1,000 Hz (Chaieb and colleagues' review).

That puts 20 Hz on a second border. A 2023 systematic review sums up the classic listening studies: below a difference of 3 Hz the sound seems to turn in the head, up to 20 Hz the beat is heard as a tone fluctuating in loudness, above 20 Hz as a rough sound, and beyond a difference of about 30 Hz the two tones are heard apart, with no beat at all. The review stresses that these borders are approximate and vary from person to person. Its own example of a 20 Hz beat is 400 Hz in one ear and 420 Hz in the other.

The arithmetic is easy to redo. 128 minus 108 is 20, so every 50 milliseconds the right ear's tone gains exactly one wave on the left's. Each beat spans 5.4 waves of the left ear's tone, and nine hours hold 648,000 beats. Played through a speaker, both tones reach both ears and mix in the air instead; binaural beats and isochronic tones explains the difference.

Where is the 20 Hz in a 20 Hz recording?

Not in the sound. Harmonance measured one steady minute of the approved master of its 20 Hz recording, the 24-bit audio the nine-hour version is made from, and looked for energy at every frequency from 1 to 200 Hz in each ear, and in the two ears added together as a single loudspeaker would play them.

One steady minute of the master, measured

Signal	What it holds	At 20 Hz
Left ear	108.00 Hz, and nothing else within 150 dB	Nothing
Right ear	128.00 Hz, and nothing else within 150 dB	Nothing
Both ears added	108.00 and 128.00 Hz	Nothing
Their loudness	A rise and fall every 50.00 ms	Its strongest rate, 20.00 Hz

Download the three spectra (CSV)

Each ear carries a single pure tone and nothing else: everything else in either channel sits at least 150 dB below its tone, far below anything a listener could hear, and at 20 Hz the measurement finds nothing at all. Even added together, the sound still holds only 108 and 128 Hz. What moves 20 times a second is their combined loudness: the two tones fall into step and out again, so the sum swells and then falls almost to silence, more than 120 dB down, once every 50 milliseconds, 20 times a second.

So the 20 Hz in a 20 Hz binaural beat is a rate, not a sound. Through headphones, no air carries it at all: your brainstem makes it from the timing of the two ears. Through a speaker, the air makes it, as a rise and fall in loudness. Either way, the lowest sound reaching your ears is 108 Hz, comfortably above the edge where pitch fades out. That is why a beat at 20 Hz is easy to hear while a 20 Hz tone is not.

What is in Harmonance's 20 Hz recording?

Two pure tones and nothing else, for nine hours: 108 Hz in the left ear and 128 Hz in the right, from the first second to the last. The 20 Hz beta binaural beat plays it free, with no ads, and it is part of the Binaural Beats collection.

The uploaded nine-hour master, measured

Moment	Left and right ear	Beat
First second	108.00 and 128.00 Hz	20.00 Hz
Halfway, 4 h 30 min	108.00 and 128.00 Hz	20.00 Hz
Last second	108.00 and 128.00 Hz	20.00 Hz

The uploaded file was measured too, not only the recipe. Its loudness reads -23 LUFS, and its peaks stay more than 21 dB below the loudest a recording can go. In each channel the other ear's tone sits at least 90 dB down, so the two tones meet only in your hearing. The steady rate is a listening choice: no study has played a 20 Hz beat for nine hours.

Does a 20 Hz binaural beat change your brainwaves?

The studies that looked have not found it. The 2023 review examined 14 EEG studies of binaural beats: five fitted entrainment, the idea that brain rhythms fall into step with the beat, eight did not and one was mixed. It found entrainment for theta, alpha and gamma beats in some studies, and none of the studies using beats in the beta range found it.

EEG studies with a 20 Hz beat

Study	What people heard	What was found
Vernon and colleagues	22 people, a 10 Hz or a 20 Hz beat in ten one-minute blocks	The EEG did not follow either beat
Gao and colleagues	13 people, five-minute beats at 1, 5, 10 and 20 Hz	During the 20 Hz beat the share of theta fell; no clear entrainment at any rate

Both studies were small and short, minutes rather than hours, and they recorded the scalp, where activity at 20 Hz mixes with muscle. A 20 Hz beat is real to hear; whether it moves the brain's own beta rhythm has not been shown.

What are the benefits of 20 Hz binaural beats?

None is established for 20 Hz itself. A 2019 meta-analysis pooled 22 studies of binaural beats at many rates and found a medium average effect on memory, attention and other measures, varying with the beat and how long people listened, so it cannot say what a 20 Hz beat does on its own. Beta is often linked with alert, active thinking, and that link is the reason many people choose a beta beat for work. Listen because you like how it sounds, and notice for yourself.

How do you listen to a 20 Hz binaural beat?

• Use headphones. The binaural beat forms only when each ear gets its own tone. Turn off spatial audio or head tracking, and check that left and right are the right way round.
• Keep it quiet. The World Health Organization's safe listening advice gives 80 dB as safe for up to 40 hours a week and suggests keeping a device below 60 per cent of its maximum. Nine hours a day for a week is 63 hours, so for a session this long, keep the level well below that.
• Listen for the border. At 20 Hz some people hear a fast flutter and others a slight roughness. Play the 20 Hz beat for a few minutes and notice which you hear.
• Try it through one block of work. Play it quietly through one block, work the next in silence, and compare. The Need to focus page gathers tones people reach for at the start of focused work.
• Keep a practice. In Studio you can set a timer and save the mixes you come back to.
• Stop when you want to. Do not listen while driving or doing anything that needs your full attention, and take the headphones off if anything feels uncomfortable.

If you prefer a beat that moves, the Rising Beta Session climbs from 16 to 24 Hz over nine hours and crosses 20 Hz halfway. The guide to binaural beats covers what the reviews found across every rate, and the guide to brainwave frequencies sets beta beside the other bands.

Sources and limits

The chart measures one minute of the recording's approved master; the recording holds the same two tones throughout, which the measurement of the uploaded nine hours confirms. The listening borders at 20 and 30 Hz come from classic laboratory studies and vary between people. The studies of 20 Hz binaural beats are few and small, used short listening times, and none tested Harmonance's nine-hour recording.

1. Møller and Pedersen, hearing at low and infrasonic frequencies: where the sense of a tone stops.
2. The same review, full text: the 16 or 20 Hz limit, organ pipes and the open car window.
3. Weirathmueller and colleagues, fin whale 20 Hz pulses: 1,241 calls on seafloor seismometers.
4. Simões and colleagues, Schumann resonances, NASA: the five lowest resonances.
5. Kane and colleagues, correction to the IFCN glossary: the beta band.
6. Pfurtscheller, Stancák and Neuper, beta after movement: the rebound around 20 Hz.
7. Whitham and colleagues, scalp recording during paralysis: muscle activity above 20 Hz.
8. Chaieb and colleagues, auditory beat stimulation: how a binaural beat is made.
9. Ingendoh and colleagues, binaural beats and brain activity, a systematic review: how a beat sounds at each rate, and the 14 EEG studies.
10. Vernon and colleagues, alpha and beta binaural beats in the EEG: beats at 10 and 20 Hz.
11. Gao and colleagues, EEG and binaural beats at four rates: beats at 1, 5, 10 and 20 Hz.
12. Garcia-Argibay and colleagues, a meta-analysis of binaural beats: the average effect across 22 studies.
13. World Health Organization, safe listening: safe levels and times.