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Acoustic phonetics studies the physical properties of speech sounds as sound waves. Vibration of the vocal folds produces a source sound with a fundamental frequency, heard as pitch, plus higher harmonics; the shape of the vocal tract then acts as a filter that strengthens some frequencies, called formants. Different vowels have different formant patterns, mainly in the first two formants, F1 and F2. Spectrograms display these patterns over time, letting phoneticians measure vowels, consonant cues such as voice onset time, and intonation.
Record your own voice in free software such as Praat and look at the waveform and spectrogram of simple words. Say "heed," "had," "hod," and "who'd" and watch the dark formant bands move. Compare "pin" and "bin" to see the delay before voicing in the voiceless stop. Link each acoustic pattern back to the articulatory gesture that causes it, so the two descriptions reinforce each other.
Articulatory phonetics describes how we make speech sounds. Acoustic phonetics describes what those sounds are physically: patterns of air pressure that travel as waves to a listener's ear. The two descriptions connect closely, and the acoustic one can be measured with a microphone and a computer.
Start with the source. When you speak a vowel, your vocal folds open and close rapidly. The rate of that cycle is the fundamental frequency (F0), measured in hertz, and listeners hear it as pitch. The folds produce a buzz made of the fundamental and many harmonics at multiples of it.
Next comes the filter. The buzz passes through the throat, mouth, and sometimes the nose. Like a tube, this space resonates at certain frequencies and boosts the harmonics near them. These resonance peaks are formants. Moving the tongue and lips changes the shape of the tube, so it changes the formants. This explanation is called the source-filter model, developed in detail by the Swedish scientist Gunnar Fant.
The first two formants carry most vowel information:
For a typical adult male speaker of American English, /i/ has an F1 near 300 Hz and an F2 above 2,000 Hz, while /u/ has a similar F1 but an F2 below 1,000 Hz. Exact values differ by speaker, since longer vocal tracts produce lower formants.
The main tool for seeing all this is the spectrogram. Time runs left to right, frequency runs bottom to top, and darkness shows intensity. Vowels appear as dark horizontal bands, the formants. Fricatives such as /s/ appear as fuzzy, noisy energy, high in frequency for /s/. Stops such as /t/ appear as a short gap of silence (the closure) followed by a burst.
Spectrograms also reveal voice onset time. In English "pin," voicing begins well after the /p/ is released, with a puff of aspiration in between. In "bin," voicing starts at or soon after release. Listeners use this timing difference to hear the contrast.
Acoustic phonetics is the basis for speech technology, forensic voice comparison, and the study of intonation and tone, all of which depend on measuring sound rather than describing it by ear.