Resonance and Formants
An instrument has favourite frequencies. Its body amplifies certain regions no matter which note you play, and those fixed regions are what make a violin recognisable across its whole range. Move the pitch, keep the peaks, and the identity stays.
- fixed peaks
- moving pitch
- the body decides
Where you run into this
- Instrument building
- Singing
- Synthesis
- Mixing
In short
Any hollow object resonates at frequencies set by its size and shape. For an instrument that means some harmonics get a boost and others do not, and crucially the boosted regions stay put when the note changes. Those fixed regions are called formants. The same mechanism works in your throat: vowels are formant patterns, which is why you can sing ah on any pitch and it stays ah.
Hearing pitch slide while the formants hold still is odd at first, because the sound seems to be two things at once. That is exactly what it is: a source and a filter.
How it works
Source and filter is one of the most useful ideas in this pillar. The source makes the pitch, the filter shapes the identity, and almost every synthesiser is built on that division.
Where you run into this
- A violin that sounds like a violin from its lowest note to its highest
- Vowels, which are formant patterns rather than pitches
- A talkbox or vocoder, which imposes one thing as a filter onto another as a source
Our own examples
Two demos on the same source: a harmonic-rich tone with two formant peaks around seven hundred and eleven hundred hertz, which reads roughly as an ah vowel, and the same tone without them. Slide the pitch and the peaks do not move.
Keep going
Everything in sound and hearing
- Sound and Vibration
- The Harmonic Series
- Waveforms and Timbre
- Resonance and Formants
- Tuning and Temperament