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Acoustic resonance is an enhanced response when an acoustic system is driven near one of its natural oscillation frequencies. Reflections and boundary conditions select standing waves, while damping limits the steady response and broadens its frequency peak. Resonance concerns the response to a drive; a large sound at one frequency alone does not identify the mechanism.

A classic measurement holds a vibrating tuning fork above an air column whose lower end is closed by water. Varying the water level changes the column length. In the ideal narrow-tube approximation, pressure has an antinode at the closed end and a node at the open end, givingwhere $c$ is speed of sound and $L_{\mathrm{eff}}$ includes an open-end correction. Successive resonant lengths at fixed frequency differ by approximately half a wavelength. If a 500 Hz fork gives successive peaks 0.34 m apart, $c\simeq 2f\Delta L=340m/s$. Using the difference reduces a common end-correction error.^[ref]

A vowel is a speech sound produced with a relatively open vocal tract, allowing its configuration to shape the sound without the complete closure or strong constriction characteristic of many consonants. Vowel height, vowel backness and lip rounding describe major articulatory differences. The vocal tract's acoustic resonances help determine the resulting formants.^[ref]

For example, [i] is a close front unrounded vowel, whereas [u] is close, back and rounded. Moving between them changes the tongue and lips, even if the fundamental frequency of vocal fold vibration is kept approximately constant. This separates vowel quality from pitch.

Many vowels occupy a syllable nucleus, but articulatory and syllabic descriptions answer different questions: a syllabic consonant can occupy that position too. Written vowel letters are also distinct from vowel sounds; the same spelling can correspond to different pronunciations.