Measuring Voice Onset Time (VOT) in English Stops with Praat

Plosives, occlusives, and stops are synonyms for the same class of consonants where airflow is completely blocked for a moment before release. In the IPA chart, voiced variants are placed next to their voiceless counterparts. The fundamental difference between voiceless (like English /p/) and voiced (like English /b/) consonants is whether the vocal cords vibrate during production. This voicing contrast is essential for distinguishing words in many languages, though some languages like modern Icelandic and Chinese contrast aspiration instead of voicing.

Plosive consonants are speech sounds produced by completely blocking airflow in the mouth, followed by a sudden release; they are divided into voiced (with vocal cord vibration) and voiceless (without vocal cord vibration) types, with examples including /p/ and /b/ (bilabial), /t/ and /d/ (alveolar), and /k/ and /g/ (velar).

The /p/ sound is voiceless, meaning no vibration occurs in the larynx when producing it, while the /b/ sound is voiced with vibration. Despite being produced with the same lip position, the voicing difference creates two distinct sounds. Longer words with these sounds are generally harder to pronounce correctly, requiring attention to stress patterns and individual sound segments.

English [p] and [b] differ fundamentally from Russian 'п' and 'б'. English [p] is aspirated at the beginning of stressed syllables, while Russian [p] is not. English [b] at word beginning is often devoiced, sounding between Russian [b] and [p]. The contrast relies on Voice Onset Time (VOT) rather than voicing. In [sp] clusters, aspiration does not occur even in stressed syllables. In [pr], [pl], [pɹ̥], [pj] clusters, aspiration 'eats' voicing of second consonants - produced whisper-like. Final [p] and [b] distinction relies on vowel length, not voicing. Devoicing [p] before voiced consonants creates strong foreign accent. Replacing [b] with [p] at word boundaries is extremely unnatural.

English contains 24 consonant sounds divided into voiced and voiceless categories. Voiced consonants (12: /b/, /d/, /g/, /v/, /z/, /ʒ/, /m/, /n/, /ŋ/, /l/, /r/, /w/, /j/, /ð/, /dʒ/) are produced with vocal cord vibration. Voiceless consonants (12: /p/, /t/, /k/, /f/, /s/, /ʃ/, /h/, /θ/, /tʃ/) are produced without vocal cord vibration. Consonants involve airflow obstruction through the mouth using tongue, lips, and teeth. The instructor demonstrates pronunciation through example words and emphasizes understanding the distinction between voiced and voiceless sounds for proper English pronunciation.

This section explains the spectrogram as a 2D visualization where the x-axis represents time or sample number, the y-axis represents frequency, and amplitude/power is represented by color intensity (black = high power, white = no power). The instructor demonstrates how to identify different sound types: fricative sounds show power distribution across all frequencies appearing as noise-like patterns, silence shows no power at all, and voiced sounds show periodic patterns indicating vocal cord vibration. The section covers how spectrograms reveal the frequency content of speech signals over time, enabling identification of manner of articulation without direct observation of articulator movements.

Periodic sounds (vowels and voiced consonants) have repeating wave patterns allowing frequency measurement, while aperiodic sounds (voiceless consonants and fricatives) have irregular, non-repeating waveforms. This distinction is visible in oscillograms - periodic sounds show regular repeating waveforms, aperiodic sounds show irregular, chaotic waveforms. Vowels produce periodic waves with varying amplitude - 'a' typically has the largest amplitude, 'i' has smaller amplitude, and nasal vowels have even smaller amplitude due to sound energy escaping through the nose. Consonants have different waveform characteristics: stops (p, t, k) show silence followed by sudden energy bursts (explosions), fricatives (s, f) show irregular aperiodic waveforms from turbulent airflow, and nasal consonants show reduced amplitude. Glottal stops produce aperiodic sounds because vocal cords close completely, blocking airflow and creating turbulence without regular periodicity. These acoustic characteristics allow phonetic analysis of speech sounds through waveform examination.

A spectrogram is a three-dimensional visualization tool that displays speech signals with time on the x-axis, frequency on the y-axis, and darkness (or color) representing amplitude on a dB scale; periodic sounds like voiced speech appear as vertical striations, noise sounds appear as white or light gray areas, and transients appear as brief vertical stripes, with the trade-off between time and frequency resolution determined by the window size used in spectrogram creation.

This comprehensive section covers the complete workflow for working with existing sound files in Praat. Users learn to navigate the Objects window as the primary workspace for file management, access the Open menu to select 'Read from file' for loading audio files, and use the file selection dialog. The section teaches how to use the View and Edit button to visualize acoustic features including the oscilograma (waveform), spectrogram, and pitch contour. Users learn to zoom into specific sections using the Zoom to selection feature, select portions of audio, and use playback controls to listen to entire files, visible portions, or selected segments. This analysis capability is essential for examining acoustic features of speech sounds.

Praat is a widely-used software for speech and sound acoustics analysis across Linguistics, Phonetics, Hearing Sciences, Music Science, Psychology, and Computer Science. Students must download it from pr.org, selecting their operating system. Audio files for exercises are available in the course Canvas portal under the 'Pro' folder, and should be downloaded simultaneously using Shift-click selection. Sound files open through drag-and-drop or Control+O navigation. The Object List displays opened sounds as temporary copies. The View and Edit button reveals the waveform (raw pressure deviations over time) and spectrogram (frequency content over time). Time information appears with three decimal places for millisecond precision. Clicking and dragging creates selection boxes displaying start time, end time, and duration. Periodic signals like vowels show equally spaced vertical stripes in the waveform, representing repeated glottal cycles. The fundamental frequency equals the reciprocal of the period duration—for a 7.5ms period, this is approximately 133 Hz. Pitch represents the rate of repetition in periodic signals, displayed as a contour showing fundamental frequency over time. Pitch range settings should match the speaker's actual range (typically 70-200 Hz for adult males) for optimal visualization.

Praat is a free speech analysis software downloadable from Google, available for Windows and Mac. After installation, users access two main windows: a display window (which can be closed) and a 'Praat objects' window for file management. Files are opened via 'Open' > 'Read from file'. Spectrograms are viewed by selecting sounds and choosing 'View & Edit', displaying time horizontally and frequency vertically with intensity shown through color gradients. Zooming is achieved by selecting regions and pressing Ctrl+N. Plosive releases appear as vertical bars in spectrograms, with white spaces indicating complete closures before release.

This section covers the Analysis Window controls and display functions. The Analysis Window shows audio files with waveform and spectrogram visualizations. Below the display, three gray lines indicate duration: Total Duration (entire signal), Visible Part (currently visible section), and marked section duration when you select an area. Clicking any gray line plays that section or the entire signal. Zoom controls in the lower left corner include: All (reset zoom to show entire signal), In (zoom into signal), Out (zoom out), Select (enlarge marked sections), and Back (reverse edits). To close Praat, click the X in the upper right corner, but remember to close each window individually and save any sound objects in the Object Window to prevent data loss.