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Acoustic phonetics studies speech as a physical sound wave. A microphone records a signal related to changing sound pressure; analysis asks how its amplitude, timing and frequency content vary. A waveform displays variation over time, whereas a spectrogram displays the distribution of signal energy across both time and frequency.

For an original comparison, record a sustained vowel followed by a sustained hiss. Inspect each waveform at the same time scale and compare their spectrograms. Approximately repeated vocal cycles can give a harmonic structure, while the hiss provides an example of a noisier excitation. Broad spectral concentrations associated with the vocal tract are formants; they should be distinguished from the fundamental frequency of a repeating source. The spectrogram's analysis-window duration affects which temporal and spectral details are visible.^[ref]

These are measurements of the signal, rather than direct readings of a listener's perceived pitch or a speaker's tongue position. Relating the signal to perception and production requires auditory phonetics and articulatory phonetics.

Articulatory phonetics studies how the moving structures of the vocal tract produce speech. Airflow supplies energy; vocal fold vibration can provide a repeating source; the tongue, lips and other articulators shape or interrupt the flow. Producing voice and shaping recognizable speech are related but distinguishable processes.^[ref]

A simple observation compares a sustained “mmm” with a sustained “sss”. Lightly touching the outside of the throat can help detect the vibration accompanying voicing, while a mirror shows the different lip configurations. This is a qualitative demonstration: the external sensation alone does not measure the detailed motion of the vocal folds.

Descriptions of consonants commonly distinguish place of articulation, where a constriction is formed, from manner of articulation, how the airflow is modified. These descriptions explain why changing the tongue or lips can change the recorded sound without changing the source of airflow. Acoustic phonetics measures the resulting signal, while articulation describes its physical production.

A formant is a resonant spectral feature associated with the filtering of speech by the vocal tract. Its centre frequency and bandwidth describe different aspects of the feature; the first few formants are conventionally labeled $F_1,F_2,\dots$. Praat represents estimated formant structure as time-sampled centre frequencies and bandwidths.^[ref]

A simple physical model treats the vocal tract as a uniform tube, closed at one end and open at the other. Under those ideal boundary conditions its resonance frequencies arewhere $c$ is the speed of sound and $L$ the tube length. For $L=0.17\mathrm{m}$ and $c=340\mathrm{m}{\mathrm{s}}^{-1}$, the first three model resonances are 500, 1500 and 2500 hertz. A real vocal tract has a varying cross-section, losses and moving articulators, so this illustrates the mechanism rather than predicting every vowel.

These resonances are distinct from the fundamental frequency of vocal fold vibration. The source-filter model separates the repeating excitation from the filter's spectral envelope, explaining how a speaker can change vowel quality without making the same change in pitch.

A fricative is a consonant produced with a narrow constriction that generates sustained turbulent airflow and its associated noise. The articulators approach closely without creating the complete oral closure of a plosive. Place of articulation and voicing further distinguish different fricatives.^[ref]

Sustain [f] and then [v]. Both use the lower lip and upper teeth, but the second normally adds vocal fold vibration. A spectrogram can display a noise component and, where present, periodic structure. The physical source of the noise is the constricted airflow, while the vocal tract shapes its spectrum. Merely calling a sound unvoiced does not identify it as a fricative.

Gunnar Fant developed acoustic models connecting the physical production of speech to its measurable signal. His Acoustic Theory of Speech Production by Gunnar Fant relates vocal-tract geometry to acoustic behavior, including calculations based on X-ray studies of articulations. The original book dates to 1960; the inspected publisher preview is its 1970 second printing.^[ref]

His source-filter model of speech production separates excitation from the filtering of the vocal tract. Changing a filter's resonances can move formants without requiring the same change in the fundamental frequency of the source. This makes speech synthesis an experimental tool: a researcher can vary model parameters and compare the generated signal with recorded speech. KTH's account explicitly describes his synthesis work as a means of testing production hypotheses.^[ref]

KTH's curriculum vitae records 1919–2009 and his leadership of the Speech Transmission Laboratory from 1951 to 1966.^[ref]

A lip is a movable fold at the entrance to the mouth, with skin externally and oral mucosa internally. The lips contain the orbicularis oris muscle, whose contraction helps close and shape the oral opening. Their sensory supply helps guide contact with food and objects.^[ref]

The oral seal has several functions. During eating it helps retain food; during speech, closure and release can interrupt airflow, while rounding changes the geometry of the vocal tract. A bilabial stop such as /p/ uses contact between both lips, whereas a rounded vowel uses an open but reshaped outlet. The visible fold and the muscle controlling it are related structures, rather than identical anatomical objects.

Lip rounding is the narrowing and reshaping of the lip opening, often with protrusion, used in producing some vowels. Rounded and unrounded configurations alter the acoustic filtering of the vocal tract. Lip rounding is distinct from vowel height and vowel backness, even though particular languages commonly associate these properties.^[ref]

Compare [i] with [y]: both are close front vowels, while [y] is rounded. A mirror makes the lip difference visible; a recording permits their formants to be compared. Keeping the intended tongue position and pitch similar helps separate the manipulation from simultaneous changes. The unrounded/rounded label is a concise category, while actual movements and the opening's geometry vary continuously.

A monophthong is a vowel with relatively stable quality through its central portion, in contrast with a diphthong's substantial movement between qualities. Relatively stable does not mean physically unchanging: neighboring sounds, vocal tract movement and the analysis interval can all affect a recording.^[ref]

For a stipulated acoustic illustration, hold the first two formants at $F_1=300$ and $F_2=2200$ hertz, while varying fundamental frequency from 100 to 150 hertz. The model changes voice pitch without requiring the same change in the formant pattern. Conversely, changing formants can change vowel quality while fundamental frequency stays fixed.

An experimental analysis can record a sustained vowel and compare several points within a consistently chosen interval. Changes concentrated near consonant boundaries need not make the whole segment a diphthong. The distinction therefore combines a phonetic trajectory with an analysis of the language's sound units.

The mouth is the entrance to the digestive tract and a movable part of the vocal tract. Its cavity is bounded by the lips, cheeks, palate and floor of the mouth, with the tongue occupying much of the space. The teeth break food into smaller pieces while the tongue and cheeks position it for chewing and swallowing.^[ref]

Mechanical processing and chemical processing cooperate: mixing food with saliva makes a movable bolus and exposes it to salivary components. The same cavity also changes shape during speech. Opening the jaw or repositioning the tongue changes the passage through which air and sound travel, so digestive anatomy is also an articulatory instrument.

A nasal consonant combines a constriction or closure in the mouth with an open airflow route through the nose. Lowering the soft palate opens that route; the mouth can remain closed at the consonant's place of articulation. A sustained [m] therefore produces sound despite closed lips, whereas closing both routes stops sustained outward flow.^[ref]

Compare [m], [n] and [ŋ]. They share a nasal manner of articulation, while their oral closures occupy different places. Comparing [m] with [b] instead holds the bilabial closure approximately fixed while changing the available airflow route. Voicing and the resonant vocal tract shape the recorded signal; the written letter alone does not specify those physical properties.

Production of voiced sound waves through oscillation of the vocal folds, driven by air from the lungs. Resonances of the vocal tract then modify that source.^[ref]

A phonetician studies the physical production, transmission or perception of speech. Articulatory phonetics investigates the moving vocal tract, acoustic phonetics measures recorded sound, and auditory phonetics studies listeners' responses. One research question can require evidence from more than one of these stages.

For example, a phonetician investigating a vowel contrast can record repeated tokens, estimate formants and test whether listeners distinguish the recordings. Speaker variation and analysis settings must be controlled or measured before differences are attributed to the contrast. Praat provides tools for waveform, spectrogram, pitch contour and formant analysis, making several of these observations reproducible.^[ref] The profession therefore connects language questions to instruments, explicit annotation rules and testable measurements.

Phonetics studies the physical production, transmission and perception of speech. Articulatory phonetics examines the moving structures that shape the airflow; acoustic phonetics examines the resulting sound waves; auditory phonetics examines how a listener perceives them. These are different measurements along the same communication chain.

During voiced speech, airflow from the lungs drives vocal fold vibration. Movements of the tongue, lips and other parts of the vocal tract shape the emerging sound. This separates the production of a vibrating source from the articulation of recognizable speech, and both from the language expressed by it.^[ref]

A simple acoustic comparison records the same spoken phrase twice, once as a statement and once as a question. The sound pressure time series and its spectrogram allow changes in fundamental frequency, segment duration and pauses to be compared. The comparison investigates physical differences in prosody; the written words alone do not specify those measurements.

A pitch contour describes how perceived pitch varies over an utterance. For speech with voicing, a common measurable correlate is the fundamental frequency $F_0(t)$ of vocal fold vibration. A rising estimated $F_0(t)$ indicates increasing repetition rate; stretches without sufficiently periodic vibration need not have a meaningful estimate.^[ref]

For an ideal periodic segment whose repetitions are separated by $T=0.01\mathrm{s}$, $F_0=1/T=100\mathrm{Hz}$. Detecting repetition at $2T$ instead produces an erroneous 50 hertz estimate, illustrating why a visually smooth pitch contour can still contain an octave error. Noise, the analysis interval and the allowed frequency range affect the estimate. Current Praat documentation distinguishes methods intended for speech intonation from methods intended for other periodicity measurements.^[ref]

A pitch contour is not the same as a formant contour: changing the fundamental frequency changes the repetition rate, whereas formants describe the resonant spectral structure shaped by the vocal tract.

Praat is software for analyzing, synthesizing, annotating and manipulating speech, created by Paul Boersma and David Weenink. It makes the connection between a recorded acoustic signal and measurable speech properties explicit: a waveform, a spectrogram, a pitch contour and estimated formants describe different aspects of the same recording.^[ref]

For a sustained vowel, one can compare the repeating structure of the waveform with the estimated fundamental frequency, then examine how the frequency spectrum is shaped by the vocal tract. A change in vowel quality need not be a change in pitch. Annotations can align intervals with phonemes or syllables without treating every acoustic boundary as an unambiguous linguistic boundary.

Measurements depend on the analysis settings and the signal. In particular, a displayed pitch contour can contain octave errors, while estimated formants depend on the analysis model. Inspecting the recorded sound and analysis assumptions is therefore part of the experiment, rather than accepting every displayed trajectory as ground truth.

The source-filter model of speech production represents a speech sound as an acoustic source shaped by a filter describing the vocal tract. During voiced speech, vibration of the vocal folds supplies a roughly periodic source; constrictions can also generate noise. The tract's resonances emphasize particular frequency regions called formants.^[ref]

Paul Boersma credits Gunnar Fant's 1960 account as the theoretical basis of this construction.^[ref]

In a simplified linear model, output spectrum $Y(f)$ is $S(f)H(f)$, where $S$ describes the source and $H$ the tract's transfer function. Keeping the source fixed while changing the filter can change vowel quality without changing the source's fundamental repetition rate. The Praat tutorial makes this test concrete by constructing a source signal and a formant filter separately, then combining them. The model's separation is an approximation; real sources and tracts can interact.^[ref]

Speech production coordinates linguistic planning with movements of the respiratory and vocal apparatus. The vocal folds supply a sound source during phonation, while the vocal tract shapes it. Planning an utterance and executing its movements contribute different sources of variation.

The vocal tract is the air passage above the sound-producing region of the larynx, including the pharyngeal, oral and, when acoustically coupled, nasal cavities. Movements of the tongue, lips, jaw and soft palate change its shape. These changes help turn a laryngeal sound source or a turbulent air stream into distinguishable speech sounds.^[ref]

A simple acoustic model treats an unbranched tract of length $L$ as a uniform tube closed at one end and open at the other. A pressure maximum at the closed end and pressure minimum at the open end allow odd quarter wavelengths, givingWith $L=0.17$ m and sound speed $c=340$ m/s, these resonances are $500$, $1500$ and $2500$ Hz. A real vocal tract is neither uniform nor permanently closed at the glottis; its varying cross-section and branching shift the resonances. These spectral resonances, associated with formants, differ from the repetition frequency of vocal-fold vibration. This is why changing articulation can change a vowel without requiring the same change in voice pitch.

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.

Vowel backness describes how far forward or backward the principal tongue constriction lies during a vowel. Front, central and back are broad regions of this articulatory continuum. It is a separate descriptive dimension from vowel height and lip rounding.^[ref]

The close vowels [i] and [u] give a familiar front/back contrast, but they also differ in ordinary lip rounding. Comparing them alone consequently does not isolate the effect of vowel backness. Holding another articulatory dimension approximately fixed, or measuring it separately, makes a stronger experiment. A vowel chart summarizes configurations; it is not a literal scale drawing of every speaker's vocal tract.