What is a frequency in sound?

Frequency is the number of pressure cycles a sound wave completes per second, measured in hertz (Hz). Higher frequencies are heard as higher pitches. Typical adult hearing spans roughly 20 Hz to 20,000 Hz, with the upper limit falling with age.

A harmonic is a component of a sound at a whole-number multiple of the fundamental frequency. A 220 Hz fundamental has harmonics at 440 Hz, 660 Hz, 880 Hz and so on. Most natural sounds contain many harmonics at once.

Why do two instruments playing the same note sound different?

Because their harmonics have different relative levels and different attack and decay behaviour. That combination is called timbre, and it is why a violin and a flute at the same pitch are easy to tell apart.

Does listening to a specific frequency produce a health effect?

This tool makes no such claim. It demonstrates the physics and psychoacoustics of tones and harmonics. Claims that particular tones heal or tune the body are not supported by reliable evidence; see our frequency myths page for a claim-by-claim review.

Is it safe to listen to test tones?

Keep the volume low and sessions short. Sustained pure tones can be fatiguing and can mask how loud you are actually listening, especially on headphones. Stop if you notice discomfort, ringing or dulled hearing.

Interactive Frequency & Harmonics Explorer

A single frequency is the simplest sound that exists: one steady rate of pressure change, heard as one pitch. Almost nothing in the real world is that simple. Voices, strings, drums and rooms all produce many frequencies at once, and the pattern of those frequencies is what your auditory system uses to tell one sound from another.

The tool below lets you build a sound from the ground up. Set a fundamental, switch harmonics on and off, and adjust how loud each one is. The drawing updates whether or not audio is playing, so you can explore the shapes silently.

Starts silent. Audio only plays when you press play.

Two cycles of the combined waveform. Adding harmonics changes the shape of the wave, not the pitch you hear.

Fundamental frequency220 Hz · A3

Frequency is how many times per second the air pressure cycles. Double the number and you hear the same note one octave higher.

Volume30%

Keep test tones quiet. Sustained pure tones can be fatiguing, and loudness judgement is unreliable with headphones on.

Harmonic series

Each harmonic is a whole-number multiple of the fundamental. Their mix is what makes a violin and a flute sound different at the same pitch.

220 Hz
440 Hz
660 Hz
880 Hz
1100 Hz
1320 Hz
1540 Hz
1760 Hz

Try a shape

These approximate classic waveforms with the first eight harmonics only, so they are close to - not identical with - a true sawtooth or square wave.

How to read what you are seeing

The curve shows two cycles of the combined wave. With only the fundamental on, it is a smooth sine. As you add harmonics the curve develops corners and ripples, but the repeat rate stays the same - which is why the perceived pitch does not move. Pitch tracks the repetition rate of the whole pattern; timbre tracks the mix of parts inside it.

Doubling the frequency raises the pitch by an octave, and the note name repeats. That is why harmonics 1, 2, 4 and 8 all sound like the same note in different registers, while harmonics 3, 5 and 7 introduce new pitch classes - the raw material of musical intervals.

What this page does not claim

Interactive tone generators are frequently packaged with claims about healing frequencies, cellular repair or chakra tuning. There is no reliable evidence for those claims, and nothing on this page should be read as supporting them. Music-based interventions do have documented effects in some clinical contexts - anxiety before surgery, for example - but those findings concern music, not isolated sine tones, and they are covered separately with their evidence ratings attached.

Frequently asked questions

What is a frequency in sound?: Frequency is the number of pressure cycles a sound wave completes per second, measured in hertz (Hz). Higher frequencies are heard as higher pitches. Typical adult hearing spans roughly 20 Hz to 20,000 Hz, with the upper limit falling with age.
What is a harmonic?: A harmonic is a component of a sound at a whole-number multiple of the fundamental frequency. A 220 Hz fundamental has harmonics at 440 Hz, 660 Hz, 880 Hz and so on. Most natural sounds contain many harmonics at once.
Why do two instruments playing the same note sound different?: Because their harmonics have different relative levels and different attack and decay behaviour. That combination is called timbre, and it is why a violin and a flute at the same pitch are easy to tell apart.
Does listening to a specific frequency produce a health effect?: This tool makes no such claim. It demonstrates the physics and psychoacoustics of tones and harmonics. Claims that particular tones heal or tune the body are not supported by reliable evidence; see our frequency myths page for a claim-by-claim review.
Is it safe to listen to test tones?: Keep the volume low and sessions short. Sustained pure tones can be fatiguing and can mask how loud you are actually listening, especially on headphones. Stop if you notice discomfort, ringing or dulled hearing.

References & further reading

Moore BCJ (2012). An Introduction to the Psychology of Hearing (6th ed.). Brill Source
Oxenham AJ (2012). Pitch perception. Journal of Neuroscience 32(39):13335-13338 DOI: 10.1523/JNEUROSCI.3815-12.2012 Source
McDermott JH, Simoncelli EP (2011). Sound texture perception via statistics of the auditory periphery. Neuron 71(5):926-940 DOI: 10.1016/j.neuron.2011.06.032 Source
World Health Organization (2022). Make Listening Safe: global standard for safe listening. WHO, World Health Organization Source

Back to Frequency & the Human Experience.

This article is an educational summary of publicly available research and is not medical advice. It does not diagnose, treat, or cure any medical or psychiatric condition. Where evidence is emerging or mixed, we say so. Consult a qualified professional for personal guidance.