666 Hz benefits: meaning, sound and evidence

What are the claimed benefits of 666 Hz?

The benefits claimed for 666 Hz come from its number, not from a test of the tone. Harmonance's own page calls it Grounded Balance and offers it as a prompt for balance, responsibility and steady reflection: an intention to listen with, never a promise.

None of these descriptions comes from a study of 666 Hz. What can be checked is below: where the number's meaning comes from, what kind of note 666 Hz is, why it sits so easily beside the other angel tones, what the 666 Hz page really plays, and what people said when researchers played them pure tones on either side of it.

Why is 666 one of the angel numbers?

Because angel numbers are runs of repeated digits, and the set runs from 111 to 999. Dictionary.com defines an angel number as "a number sequence that contains a repeated number series" and notes that most sources credit the name to Doreen Virtue, whose book Angel Numbers came out from Hay House in 2005, as the Internet Archive's catalogue record shows. The same entry adds a line worth keeping in mind: "The significance of angel numbers varies depending on who is discussing them."

666 arrives with a much older story. In the King James Bible, Revelation 13:18 reads:

Let him that hath understanding count the number of the beast: for it is the number of a man; and his number is Six hundred threescore and six.

The guide to angel numbers and their frequencies tells the rest of that story, including the early copies of Revelation that read 616. No dictionary or primary text gives an angel number reading of 666 that could be traced, so this guide offers none as tradition. Grounded Balance is Harmonance's own name for the tone: a choice to listen for steadiness, never a forecast of what is to come.

What makes 666 an unusual number?

Its arithmetic, which anyone can redo. 666 is the sum of every whole number from 1 to 36: 1 + 2 + 3 + … + 36 = 36 × 37 ÷ 2 = 666. That makes it a triangular number, the count of dots in a triangle 36 rows tall. It is also the sum of the squares of the first seven prime numbers: 4 + 9 + 25 + 49 + 121 + 169 + 289 = 666.

And it is 6 × 111. That last line is the one that matters for the sound. Every angel tone is a whole multiple of 111 Hz, and 666 Hz is the sixth. Six divides by two and by three, and that is why 666 Hz fits beside so many of the others.

What note is 666 Hz?

A bright note just above an E. With the A above middle C tuned to 440 Hz, the usual convention, the E an octave above middle C is seven equal semitones higher: 440 × 2^7/12 = 659.26 Hz. 666 Hz sits 17.6 cents above it, where a cent is a hundredth of a semitone. Played on its own, it sounds as an E, a shade sharp.

In air, one wave of 666 Hz is about 52 centimetres long. Sound travels about 344 metres a second at 20 °C, so 344 ÷ 666 = 0.52 m. Its lower octaves, the same note further down, are 333, 166.5 and 83.25 Hz, and you will meet all three again on the 666 Hz page.

Why does 666 Hz sit so easily beside the other angel tones?

Because six divides so many ways. In the words of UNSW's music acoustics pages, "the nth harmonic is the one with frequency nf", so 666 Hz is 111 Hz times six, just as 333 Hz is 111 Hz times three and 444 Hz is 111 Hz times four. Set against any other angel tone, 666 Hz makes a ratio of small whole numbers, and each of those ratios has a name in the Huygens-Fokker Foundation's list of intervals.

Against 333 Hz it makes the octave, two waves for one. Against 444 Hz it makes three waves for every two, the perfect fifth: 702 cents, all but the 700 of a piano's fifth. Against 888 Hz it makes the perfect fourth, four waves for every three of 666 Hz, at 498 cents against the piano's 500, and against 999 Hz the fifth again. These pairs line up again every 3.0 or 4.5 ms.

The pairs with 555 and 777 Hz, like the pair with 111 Hz, take longest to line up: 9.0 ms. 666 and 555 Hz make six waves for every five, the minor third, at 316 cents, 16 wider than a piano's 300. 777 and 666 Hz make seven for every six, the septimal minor third, at 267 cents: 33 cents narrower than a piano's minor third, too far for any key to stand in for it.

So 666 Hz meets four other angel tones in an octave, a fifth or a fourth, the intervals a piano can play almost exactly.

What does the 666 Hz page play?

It plays the tone in two ways, and Harmonance measured both from the page's own audio files.

The 666 Hz page, measured

Octave band	Tone alone	Strings
2,664 Hz, two octaves up	0%	0%
1,332 Hz, one octave up	0.5%	1.5%
666 Hz, the tone itself	99.5%	14.0%
333 Hz, one octave down	0%	51.6%
166.5 Hz, two octaves down	0%	23.3%
83.25 Hz, three octaves down	0%	9.6%

Download the measurements (CSV)

With Strings off, the page plays the tone on its own, and 99.5% of the sound sits in the band around 666 Hz. It is not a perfectly pure sine: two faint overtones ride above it, at 1,332 Hz, 23.1 dB quieter, and at 1,998 Hz, 34.1 dB quieter, which the site's tone carries by design. This is the bright pitch the number names.

With Strings on, the sound moves down. 51.6% of it lies one octave below, in the band around 333 Hz, 23.3% two octaves below and 9.6% three octaves below; 14.0% stays in the band around 666 Hz, and 1.5% sits an octave above. The strings play a soft chord around the note's lower octaves. Their strongest line is 373.9 Hz, a whole tone above 333 Hz, with 333.1 Hz itself only 1.3 dB behind. The exact 666 Hz tone still plays underneath, 35.0 dB below the music.

So the two modes suit different moments. The tone alone keeps the number's bright, high pitch; the strings carry the same note an octave lower, with more room around it.

What does research say about 666 Hz?

No study has tested 666 Hz itself: PubMed, the main index of medical research, lists no listening study of the tone. One study comes close. In an online test, 37 adults in Canberra rated twelve pure tones, from 110 to 963 Hz, for how happy, sad, calm and angry each one sounded (Kumar, Abhayapala and Samarasinghe, 2022). Two of the twelve, 639 and 741 Hz, sit either side of 666 Hz.

Ratings of three pure tones

Tone	Calm	Angry
110 Hz, the lowest	8.3	3.1
639 Hz, just below 666	2.5	7.4
741 Hz, just above 666	2.2	8.1

Both neighbours of 666 Hz were heard as among the least calm of the twelve tones and among the angriest. The authors describe the trend above 440 Hz plainly:

This clearly reflects the dominance of anger emotion class in the higher frequency range.

The volunteers judged how each tone sounded, not how they felt. Each tone played for 20 seconds, always in rising order, and was rated against its neighbours, so the result says as much about high against low as about any one pitch. What the test does suggest is that a bright pure tone in this range can sound edgy rather than soothing. If the tone alone feels sharp to you, the Strings mode puts most of the same note an octave lower.

The wider field is not much further along. The US National Center for Complementary and Integrative Health sums up research on music in general:

However, because much of the research on music-based interventions is preliminary, few definite conclusions about their effects have been reached.

For listening, take 666 Hz for what it is: a bright, steady tone with a famous number behind it. If you sit with it for a few minutes, notice what it does for you, and keep that as your own experience rather than a measured effect.

How long should you listen to 666 Hz, and how loud?

No study sets a length for 666 Hz. Harmonance's page suggests five to fifteen minutes to begin with, seated or lying down, somewhere you will not be interrupted, and you can stop whenever it no longer feels comfortable.

Keep the volume low, low enough to talk over. Loud listening is the one risk that is well established. The US National Center for Complementary and Integrative Health notes that "listening to music at too high a volume can contribute to noise-induced hearing loss", and the World Health Organization's safe listening advice is that "you can safely listen to a sound level of 80dB for up to 40 hours a week". Set the volume once at a comfortable level and leave it there.

How do you listen to 666 Hz on Harmonance?

Press play on the 666 Hz page. With Strings off it plays the tone alone; switch Strings on beside the player and the strings carry the same note in its lower octaves. Nothing plays until you press play, and no account is needed.

In Harmonance Studio you can set 666 Hz beside other tones and shape your own mix. Try it over 444 Hz to hear the perfect fifth, over 333 Hz for the octave, or beside 777 Hz for the septimal minor third. The root of the family is 111 Hz, and the angel number collection plays all nine.

Two Solfeggio tones sit either side of 666 Hz: the 639 Hz guide and the 741 Hz guide tell their stories, and the 111 Hz guide and the 888 Hz guide follow two more angel tones. For the wider field, what the science of sound healing shows sets out what research on sound and music has and has not found.

Sources and limits

The history of the number rests on the text of Revelation, a dictionary's entry for angel numbers and the catalogue record of the book that named them; no traceable source gives an angel number reading of 666, so none is given. The arithmetic of the number, the note and the intervals is shown in full, from published conventions, so you can redo it. The research on listeners is one online rating test of twelve pure tones, two of them either side of 666 Hz, with its ratings read from the paper's graph. The measurements of the 666 Hz page are Harmonance's own, made from the page's audio files.

1. Revelation 13:18, King James Version, eBible.org: the number of the beast.
2. Dictionary.com, angel number, Pop Culture Dictionary: what an angel number is, and who named it.
3. Internet Archive, catalogue record of Doreen Virtue (2005), Angel Numbers, Hay House: the book that named angel numbers.
4. UNSW Music Acoustics, note names and frequencies: A at 440 Hz, twelve equal semitones, and how to count cents.
5. UNSW Music Acoustics, harmonics: the nth harmonic is n times the frequency.
6. Huygens-Fokker Foundation, list of intervals: the octave, the perfect fifth and fourth, and the two minor thirds.
7. Hass, an acoustics primer: the speed of sound, Indiana University: 344 metres a second at 20 °C.
8. Kumar, Abhayapala and Samarasinghe (2022), frequency cues for human emotions, in Acoustics: 37 adults rating twelve pure tones.
9. National Center for Complementary and Integrative Health, music and health: the state of music research, and loud listening.
10. World Health Organization, safe listening: weekly limits.