999 Hz frequency meaning: completion, in sound

What does 999 Hz mean?

The meaning of 999 Hz comes from numerology, not from a test of the tone. Numerology reads 999 as an angel number for completion, and Harmonance's page names the tone Completion and Integration: an intention to listen with as something in your life comes to an end. Videos elsewhere promise far more, and none of it rests on a study of a 999 Hz tone.

PubMed, the main index of medical research, lists no listening study of 999 Hz. What can be checked is below: where the number comes from, which note it lands on, why it sits so close to the tone that loudness itself is measured against, what listeners made of the nearest tone ever tested, and what the 999 Hz page actually plays, measured from its own audio.

Where does the angel number 999 come from?

The idea of angel numbers is recent. In numerology, as Dictionary.com explains, an angel number is "a number with repeating digits, which is considered to have special importance", and most sources credit the idea to the author Doreen Virtue and her book Angel Numbers, published by Hay House in 2005, as the Internet Archive's catalogue records it.

999 is read through the number nine. Nine is the last of the single digits, so numerology takes it as the end of a cycle. As one numerologist told Bustle, nine is "the last single digit number, representing a full cycle of experience that teaches us to go beyond ourselves", and 999 is read as an ending that makes room for a new beginning. Numerology also adds a number's digits until one is left, and nine always comes back to itself: 9 + 9 + 9 = 27, and 2 + 7 = 9. The arithmetic behind that is real. The digits of any multiple of nine add up to a multiple of nine, which is the old schoolroom test for dividing by nine.

In Britain, 999 means something else first. It has been the emergency number since 1937, when it opened in London as the world's oldest emergency call service, and its digits were chosen because the coin payphones of the day could be adapted to dial 9 free. So the number many British readers know best was picked for how telephones worked, not for what nine stands for.

As a tone, 999 Hz is the number read as a frequency: 999 waves of pressure a second. Hertz is a modern unit, so nobody in the tradition chose this pitch; the number came first. Every angel tone is made the same way, so the nine of them are 111 Hz and its whole-number multiples, and 999 Hz is exactly nine times 111 Hz, the last and highest of the set. The guide to angel numbers and their frequencies follows the whole family, and the 111 Hz guide starts at its root.

What note is 999 Hz?

999 Hz is a slightly sharp B. With the A above middle C tuned to 440 Hz, the usual convention, each semitone is a twelfth of an octave, so the B fourteen semitones above that A is 440 × 2^14/12 = 987.77 Hz. 999 Hz sits 1,200 × log₂(999 ÷ 987.77) = 19.6 cents above it, where a cent is a hundredth of a semitone. That is about a fifth of a semitone, enough that the two played together beat 11.2 times a second, since 999 minus 987.77 is 11.23 Hz.

That B is B5, a little under two octaves above middle C. Halve 999 Hz and you reach its lower octaves, 499.5, 249.75 and 124.88 Hz, each the same fifth of a semitone above a lower B. Sound travels about 344 metres a second in air at 20 °C, so one wave of 999 Hz is 344 ÷ 999 = 0.344 metres long, a little longer than a 30 cm school ruler.

Being nine times 111 Hz also fixes its place among the other angel tones. 999 ÷ 888 = 9 ÷ 8, the ratio of a pure whole tone, 203.9 cents, so 999 Hz sits one whole step above the 888 Hz tone. From 111 Hz to 999 Hz is 1,200 × log₂(9) = 3,803.9 cents: three octaves and that same whole tone. The highest Solfeggio tone, 963 Hz, sits 36 Hz lower, 63.5 cents below, about two thirds of a semitone.

Why does 999 Hz sit so close to 1,000 Hz?

999 Hz is 1 Hz below the tone that loudness is measured against. The ear does not hear every pitch as equally loud, so acoustics compares sounds in phons, and the phon is defined on a 1,000 Hz tone. In the words of the encyclopedia entry on the phon, "the phon matches the sound pressure level in decibels (dB SPL) of a similarly perceived 1 kHz pure tone". At 1,000 Hz the two scales are the same by definition, and at 999 Hz they all but agree.

Harmonance measured what that means on the page. At the level Harmonance measures its sound against, the 999 Hz tone plays at 64.0 decibels and comes out, by the standard equal-loudness model, at 64.0 phon. A 111 Hz tone at the same level comes out at 41.9 phon. Turned into the ear's own scale of loudness, that is 5.29 sones against 1.14, so the 999 Hz tone sounds about four and a half times as loud as 111 Hz for the same setting on your device. The ear's most sensitive range lies higher still, between 2 and 5 kHz, so 999 Hz is not the loudest tone a meter can show, only one the ear takes almost at face value.

Being 1 Hz apart has a simple consequence you can hear for yourself. Play 999 Hz beside a 1,000 Hz tone and the two beat exactly once a second, a slow swell that comes from nothing but the difference between the numbers.

What happened when researchers played tones near 999 Hz?

No study has played 999 Hz itself and asked what it does. The nearest is an online rating test at the Australian National University, in which 37 adults heard twelve pure tones for 20 seconds each and rated how happy, sad, calm and angry each one sounded (Kumar, Abhayapala and Samarasinghe, 2022). The tones were the nine Solfeggio numbers with 110, 220 and 440 Hz. The highest was 963 Hz, the closest any study has come to 999 Hz.

The higher the tone, the less calm it sounded, and 963 Hz sat at the far end. Read from the paper's graph of average ratings, it scored about 1.9 out of 10 for calm and 9.1 for anger, the lowest calm and the highest anger of all twelve, against 8.3 for calm at 110 Hz. For the tones from 440 Hz up, the authors write, the ratings for calm, happy and sad fell while anger rose:

the emotion rating tends to dampen continuously for three primary emotions happy, sad, and calm

The volunteers judged how each tone sounded, not how they felt. The tones always came in rising order and each was rated against its neighbours, so the highest tone sat at the angry end of every comparison, and most volunteers were young, with a median age of 27. 999 Hz was not among the tones. What the test does suggest is that a pure tone this high is heard as bright and sharp more readily than as calm, which is worth knowing when you choose how to play it.

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.

What does the 999 Hz page play?

The 999 Hz page plays the tone in two ways, and Harmonance measured both from the page's own audio files: the pure tone the player loops, and the Strings arrangement at the page's own settings.

The numbers behind the chart

Octave band	Tone alone	Strings
3,996 Hz, two octaves up	0%	0%
1,998 Hz, one octave up	0.5%	0.2%
999 Hz, the tone itself	99.5%	3.3%
499.5 Hz, one octave down	0%	14.9%
249.75 Hz, two octaves down	0%	53.7%
124.88 Hz, three octaves down	0%	26.5%

Download the measurements (CSV)

With Strings off, the page plays the tone on its own, and 99.5% of the sound sits in the octave band around 999 Hz. It is not a perfectly pure sine: two faint overtones sit above it, at 1,998 Hz, 23.1 dB quieter, and at 2,997 Hz, 34.0 dB quieter, which the site's tone carries by design.

With Strings on, the note stays but the sound moves a long way down. 53.7% of the energy lies in the band around 249.75 Hz, two octaves below, and 26.5% around 124.88 Hz, three octaves below; 14.9% sits an octave below and only 3.3% stays near 999 Hz. The strings play a chord on B. Their strongest line is 187.9 Hz, the F sharp a fifth above B2, and six notes of the B major scale, B, C sharp, D sharp, E, F sharp and G sharp, carry 92.3% of their energy. They are tuned to the tone, not to the piano, so like 999 Hz itself they sit about a fifth of a semitone sharp. The exact 999 Hz tone still plays underneath, 34.9 dB below the music.

The same six-step shape turns up on the 555 Hz page, starting there on C sharp, as the 555 Hz guide shows, so it belongs to the strings' score, which the site pitches to each page's tone, and was not chosen for 999 Hz. That move downwards meets the rating test halfway. The tones its listeners heard as calmest were the low ones, and the strings keep 999 Hz's note while putting most of its sound two or three octaves lower. The test played pure tones, not strings, so this is a reason to try both versions and notice the difference, not a finding.

How long should you listen to 999 Hz, and how loud?

No study sets a length for 999 Hz; the rating test played each tone for 20 seconds. Harmonance's page suggests five to fifteen minutes to begin with, seated or lying down, and stopping whenever the tone stops feeling comfortable. One way in is to play it at a close, the end of a day, a project or a chapter, which is what its number stands for.

Set the volume low before you press play, especially if you have just come from a low tone. As the measurements above show, 999 Hz plays about four and a half times as loud to the ear as 111 Hz at the same setting, so a level that felt gentle on a low tone can feel sharp here.

Keep it 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", with your device's volume at no more than 60% of maximum.

How do you listen to 999 Hz on Harmonance?

Press play on the 999 Hz page. With Strings off it plays the tone alone; switch Strings on beside the player and the strings carry the same note as a chord, mostly two or three octaves lower. Nothing plays until you press play, and no account is needed.

In Harmonance Studio you can set 999 Hz beside other tones and shape your own mix. Its neighbour in the family is 888 Hz, a whole tone below, the nearest Solfeggio tone is 963 Hz, and the angel number collection plays all nine 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 meaning of the number rests on a numerologist's reading reported by a magazine, a dictionary's account of angel numbers and the catalogue record of the book that named them; the book's own words for 999 could not be read. The history of 999 as an emergency number rests on an encyclopedia entry. The pitch, the beats, the wavelength and the whole tone are arithmetic from the standard tuning convention and the speed of sound, worked in full above. The research on listeners is one online rating test of twelve pure tones whose highest, 963 Hz, is the nearest any study has come to 999 Hz. The measurements of the 999 Hz page are Harmonance's own, made from the page's audio files, and the loudness figures come from the standard equal-loudness model, not from listeners. No listener's words are quoted here.

1. Bustle, the meaning of angel number 999, according to a numerologist: nine as completion, and 999 as an ending that opens a new beginning.
2. Dictionary.com, 111 angel number: what an angel number is, and where the idea comes from.
3. Internet Archive, catalogue record of Doreen Virtue (2005), Angel Numbers, Hay House: the book that named angel numbers.
4. Wikipedia, 999 emergency telephone number: the British emergency number since 1937, and why its digits were chosen.
5. UNSW Music Acoustics, note names and frequencies: A at 440 Hz, equal semitones, and how to count cents.
6. Hass, an acoustics primer: the speed of sound, Indiana University: 344 metres a second at 20 °C.
7. Wikipedia, phon: loudness level, defined on a 1,000 Hz tone.
8. Kumar, Abhayapala and Samarasinghe (2022), frequency cues for human emotions, in Acoustics: 37 adults rating twelve pure tones up to 963 Hz.
9. Wikipedia, equal-loudness contour: how the ear's sensitivity changes with pitch.
10. National Center for Complementary and Integrative Health, music and health: the state of music research, and loud listening.
11. World Health Organization, safe listening: weekly limits and device volume.