2 Hz frequency: sound, sleep and the brain

2 Hz means two cycles a second, one every half second. As sound, it is too slow to hear as a tone: the sense of pitch stops around 20 Hz. In the brain, 2 Hz sits in the delta band and marks the top of the slow waves that define deep sleep. A 2 Hz binaural beat plays 108 Hz to one ear and 110 Hz to the other.

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Twelve long blue ribbons of light on a deep violet ground, rising in two gentle crests across the picture and never touching — Twelve ribbons in delta blue, drawn in code: one slow wave with two crests across the picture, one for each beat in a second of the 2 Hz recording.

What does 2 Hz mean?

2 Hz means two cycles a second, so one cycle lasts half a second, 500 milliseconds. The hertz only counts repeats. It says nothing about what is repeating: a sound wave, a flashing light and an electrical rhythm in the brain can all run at 2 Hz, and each behaves differently.

You already know the pace. Over ten hours of ordinary daily life, the head movements of 20 people showed a sharply tuned walking rhythm at 2 Hz, about 120 steps a minute, with no link to age, sex, height or weight (MacDougall and Moore). An easy walk is a fair feel for 2 Hz: one step on every cycle.

Most people who search for 2 Hz are asking about one of two things. One is the brain's slowest rhythms, where 2 Hz marks a line that sleep science uses every night. The other is the 2 Hz binaural beat, a pair of tones you can hear that make a beat at that rate. This guide takes each in turn, with Harmonance's own measurements of both.

Can you hear a 2 Hz sound?

Not as a tone. A 2 Hz sound wave takes half a second to rise and fall, far slower than anything the ear turns into pitch. A review of hearing at low and infrasonic frequencies traces what happens as a tone gets lower: it becomes less continuous, the sense of a tone stops around 20 Hz, and below 10 Hz the single cycles of the sound can be perceived. Sound below 20 Hz is called infrasound, and it is perceived only when it is loud enough, often as a feeling of pressure at the eardrums.

So a 2 Hz binaural beat contains no 2 Hz sound at all. It plays two tones you hear as a low, even hum, 108 Hz and 110 Hz, and the rate of two a second comes from the difference between them.

Where does 2 Hz sit in the brain's rhythms?

In the delta band, the slowest of the named brain rhythms, and right on a line that matters in sleep science. The clinical glossary of the International Federation of Clinical Neurophysiology puts delta below 4 Hz, starting for practical purposes at 0.5 Hz. It defines deep sleep, stage N3, by slow delta waves of 0.5 to 2 Hz, more than 75 microvolts from peak to trough and measured over the front of the head, that fill at least 20 per cent of the recording (IFCN glossary). So 2 Hz is the fastest a wave can run and still count toward deep sleep: every wave that counts lasts from half a second to two seconds.

Sleep is scored in 30-second pieces called epochs. When the American Academy of Sleep Medicine rewrote the scoring rules, it kept the 30-second epoch, found slow waves largest over the front of the head, and joined the old stages 3 and 4 into one stage of slow wave sleep (Silber and colleagues).

2 Hz also marks a change in how slow activity behaves through a night. In all-night recordings of eight healthy young men, sleep's slow activity peaked at 0.7 to 0.8 Hz, and the usual fall in delta from the first to the second spell of non-REM sleep did not happen below 2 Hz (Achermann and Borbély). The guide to delta waves follows the band through a whole night, and the guide to brainwave frequencies sets it beside the faster bands.

What does 2 Hz look like in sleeping brains?

We measured it from open recordings. The Sleep-EDF Database on PhysioNet, shared by Kemp and colleagues, holds whole nights from a 1987 to 1991 study of healthy people aged 25 to 101 taking no sleep medication. Each 30 seconds was scored by a trained technician under the 1968 Rechtschaffen and Kales rules, and the brain was recorded 100 times a second. We took each of its 78 volunteers once, on their first night, 40 of them women, and read the electrode pair that runs from the forehead to the crown, over the front of the head where the scoring rule measures slow waves.

For every volunteer we measured how much power the recording carried at each frequency from 0.5 to 8 Hz, averaged over each stage of sleep, then took the middle value across volunteers. Of the 78, 18 had less than five minutes of deep sleep that night, 17 of them aged 51 or over, so the deep sleep line rests on the other 60.

The power of sleeping brains from 0.5 to 8 Hz: deep sleep towers over light and REM sleep below 2 Hz, and all three lines fall away steeply — Deep sleep carries far more slow power than light or REM sleep, and most of it sits below 2 Hz. At 1 Hz the deep sleep line is 11.8 times the REM line; by 2 Hz the gap has narrowed to 6.2 times, and deep sleep's own power has fallen to 27 per cent of its level at 1 Hz. Medians over the Sleep-EDF volunteers, electrode Fpz-Cz.

Median power, in microvolts squared per hertz
Stage	At 1 Hz	At 2 Hz
Deep sleep, stages 3 and 4, 60 volunteers	312	83
Light sleep, stage 2, 78 volunteers	78	33
REM sleep, 78 volunteers	26	13

Two things stand out. In every stage, most of the power under 4 Hz sits at or below 2 Hz: 90 per cent in deep sleep, 84 in light sleep and 83 in REM sleep, as medians across volunteers. What sets deep sleep apart is the amount. Below 2 Hz its line towers over the others, and above 2 Hz the three draw together. So 2 Hz is not where slow activity stops. It sits on the slope where deep sleep's lead is falling away: 11.8 times REM sleep's power at 1 Hz, 6.2 times at 2 Hz and 3.5 times at 4 Hz.

How does a 2 Hz binaural beat work?

A 2 Hz binaural beat plays one steady tone in each ear, 2 Hz apart, and your hearing turns the difference into a beat. Harmonance's plays 108 Hz in the left ear and 110 Hz in the right. Neither ear hears a beat on its own; it forms only where the signals from the two ears meet.

That meeting happens in the brainstem. Neurons of the superior olivary complex, which respond to differences in timing between the two ears, fire at the rate of the difference and create the beat, and Oster found the beats were heard only with tones below about 1,000 Hz (Chaieb and colleagues' review). The effect was first described by H. W. Dove in 1839, and a 2023 systematic review sums up how it sounds: below a difference of 3 Hz the beat is heard as a rotating tone, a sound that seems to move in the head from ear to ear. The review adds that these limits are approximate and vary from person to person (Ingendoh and colleagues).

So a 2 Hz beat sits in the range people describe as movement rather than a pulse in loudness. Played through a speaker, both tones reach both ears and mix in the air instead; binaural beats and isochronic tones explains the difference.

Half a second of 108 Hz in the left ear and 110 Hz in the right, in step, opposite and in step again, then the brainstem, what you hear and slow waves — In 500 milliseconds the left ear hears 54 waves of 108 Hz and the right ear 55 waves of 110 Hz, so the right gains one whole wave and the two fall in and out of step twice a second. The brainstem makes the beat from that difference; whether the brain's slow waves follow it is the open question.

The arithmetic is easy to check. 110 minus 108 is 2, so every half second the right ear's tone gains exactly one wave on the left's: crest meets crest at the start, crest meets trough at 250 milliseconds, and crest meets crest again at 500. Nine hours hold 64,800 beats.

What is in Harmonance's 2 Hz recording?

Two pure tones, 108 Hz on the left and 110 Hz on the right, held for nine hours with nothing added. The 2 Hz delta binaural beat plays it free, with no ads, and it is one of the pieces in the Binaural Beats collection.

The uploaded nine-hour master, measured
Moment	Left and right ear	Beat
First second	108.00 and 110.00 Hz	2.00 Hz
Halfway, 4 h 30 min	108.00 and 110.00 Hz	2.00 Hz
Last second	108.00 and 110.00 Hz	2.00 Hz

Read from the decoded audio of the uploaded master, 6,481 readings in all. The largest difference from the planned tones was 0.00040 Hz.

We checked the uploaded file itself, not only the recipe that made it. 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 153 dB down, so the two tones never mix in the file: the beat forms only in your hearing. The steady rate is a listening choice. No study has played a 2 Hz beat for nine hours.

Does a 2 Hz binaural beat change your brainwaves?

The evidence is thin and points different ways. The 2023 review found 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. Only three used beats in the delta band, and one of those reported more alpha power, a faster rhythm, in response to a delta beat (Ingendoh and colleagues).

Studies of slow binaural beats, oldest first
Study	What people heard	What was found
Lane and colleagues	29 people on a 30-minute watching task, beats at 1.5 and 4 Hz or at 16 and 24 Hz in pink noise	Fewer correct detections and more false alarms with the slow beats
Karino and colleagues	9 people, slow beats at 4.00 or 6.66 Hz, recorded as magnetic fields from the head	The cortex responded at the rate of the beat
Gao and colleagues	13 people, five-minute beats at 1, 5, 10 and 20 Hz	During the 1 Hz beat the share of theta and alpha rose and beta fell; no clear sign the brain followed the beat
Ioannou and colleagues	Musicians and non-musicians, one-minute beats from 1 to 48 Hz, 2 Hz among them	Alpha beats drew the strongest steady responses; slow beats, analysed as groups, changed network patterns in the alpha band
Jirakittayakorn and Wongsawat	24 young adults over three nights, a 3 Hz beat from the start of light sleep to the start of deep sleep, or a silent sham	Deep sleep, N3, lasted longer and light sleep, N2, shorter with the beat
Shumov and colleagues	221 people at home, a bedtime sound with binaural beats, monaural beats or no beats	The time taken to fall asleep did not depend on the sound

Six studies that used slow beats, most of them beside faster ones. Most are small, and none reports a 2 Hz beat on its own: Ioannou and colleagues played one for a minute, but analysed it with the other delta beats.

The one sleep study that found a change was small: 15 people analysed against 8, a 3 Hz beat rather than a 2 Hz one, played only from the first 30 seconds of light sleep to the first 30 of deep sleep. The larger study at home found no difference in how long people took to fall asleep. The guide to delta waves sets out the bedtime studies in more detail.

What are the benefits of 2 Hz binaural beats?

None is established. The widest pooled result, a 2019 meta-analysis of 22 studies, found a medium average effect across many beat rates and many kinds of task, so it cannot say what a 2 Hz beat does on its own. The studies of slow beats above are few and small, and they disagree. What a 2 Hz beat is, for certain, is a slow, steady sound, the same from the first minute to the last. Listen because you like how it sounds, and notice for yourself.

Is it safe to listen to a 2 Hz binaural beat?

Two cautions come from the research. First, attention: in Lane and colleagues' study, people watching for targets made fewer correct detections and more false alarms with beats at 1.5 and 4 Hz than with faster beats, so never listen while driving or doing anything that needs your full attention. Second, volume: 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 night for a week is 63 hours, so for a session this long, keep the level well below that.

How do you listen to a 2 Hz binaural beat?

Use headphones. The 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 low. Set the volume just loud enough to hear the hum clearly, well inside the safe listening limits above.
Lie back and listen for the movement. Start the 2 Hz beat, close your eyes for ten minutes and notice whether the tone seems to move between your ears. Then rest the same way in silence, and notice what you notice.
Keep a practice. In Studio you can set a timer and save the mixes you come back to.
Stop when you want to. Take the headphones off if anything feels uncomfortable.

If you prefer a beat that moves, the Delta Brainwave Session falls from 3.5 to 1.5 Hz over nine hours. Just below 2 Hz, the 1.618 Hz beat takes its rate from the golden ratio, and the 4 Hz theta beat sits where delta ends and theta begins. The guide to binaural beats covers what the reviews found across every rate, the guide to theta waves follows the band above delta, and the guide to music and sleep looks at sound at bedtime more widely.

Sources and limits

The chart shows the median of 78 volunteers' first nights from one study recorded between 1987 and 1991, at one electrode pair, with a recorder that filtered out activity below 0.5 Hz, so the chart starts there. Its stages follow the 1968 rules, whose stages 3 and 4 are today's N3. It shows where sleep's slow activity sits, not what any sound does to it. The studies of slow binaural beats are few and small, used different designs and listening times, and none tested Harmonance's nine-hour recording.