Domain: Humanities and Social Sciences — Linguistics
Target concept: An experimental comparison of spectral characteristics between lateral consonants (e.g., /l/) and resonants (e.g., /r/)

Video generation prompt: A speaker wearing a headset microphone first recorded a sustained /l/ sound, followed by a /r/ sound. The audio was displayed as a spectrogram on the computer screen. The left side of the computer screen showed the spectrogram of /l/, and the right side showed the spectrogram of /r/.

Evaluation guide:
Target concept: Acoustic distinction between two sonorant consonants (the lateral approximant /l/ and the rhotic approximant /r/) grounded in spectrographic evidence: /l/ shows a damped/weakened frequency band (a lateral antiresonance produced by the side-branch oral cavity) alongside otherwise clear, continuous formants, while /r/ shows a markedly lowered third formant (F3) that is pulled close to F2. Neither sound produces frication-type turbulent noise: both are voiced approximants with continuous formant structure; broadband turbulent noise is instead the signature of lateral fricatives (e.g., Welsh /ɬ/), a different consonant class not depicted here. Minimum mechanistic assumptions: (i) Voiced /l/ retains continuous formant bands (F1, F2, F3) but shows a damped/weakened band from its lateral antiresonance, with no broadband noise; (ii) Voiced /r/ reshapes the vocal tract (via tongue-tip retroflexion or tongue-body bunching) to markedly lower and narrow the F2-F3 gap, again without turbulence; (iii) Real-time spectrogram rendering faithfully maps these articulatory-acoustic relationships. Expected phased storyline: (1) Speaker wears headset mic; (2) Sustains /l/; (3) Left spectrogram fills with continuous formant bands and a visibly damped/weakened band; (4) Brief pause; (5) Sustains /r/; (6) Right spectrogram fills with F2 and F3 drawn close together and no antiresonance damping. Visibility constraints: Must show speaker's head/mic *and* full computer screen simultaneously in frontal view; spectrograms must be large enough to resolve formant bands; no occlusion; labels must be readable without zoom.

Manual:
I. Basic Demonstration Information
Demonstration Title: Spectral Comparison of Lateral /l/ vs. Resonant /r/ Sounds
Subject: Linguistics - Phonetics - Acoustic Phonology
Demonstration Purpose: To visually compare the spectrographic signatures of the lateral approximant /l/ and the rhotic approximant /r/, highlighting their distinct spectral patterns (a lateral antiresonance/damped band for /l/ versus a lowered third formant for /r/) and supporting phonetic classification based on acoustic evidence.

II. Demonstration Carrier/Equipment
Core Carrier: Native English-speaking adult (male or female, age 20-40, no speech disorder)
Auxiliary Equipment: Headset microphone (boom-style, visible earpiece + directional mic), laptop computer (screen resolution ≥1366x768), real-time spectrogram software (e.g., Praat, Audacity with spectrogram view enabled), neutral background (matte gray), front-facing camera (fixed position, eye-level).

III. Demonstration Content Description
Layout: Speaker seated center-frame, headset properly worn; laptop placed directly in front, screen fully visible and unobstructed. Screen split into two equal vertical panels: left labeled '/l/', right labeled '/r/' in 18pt bold sans-serif font.
State Control: Speaker produces sustained, steady-state /l/ for ~1.0 second (tongue tip alveolar, airflow around the sides of the tongue), then pauses briefly (~0.2 s), then sustains /r/ for ~1.0 second (tongue bunched/retroflex, vocal tract reshaped). Audio is captured and rendered live as a spectrogram.
Comparison Baseline: Identical spectrogram settings used for both panels: window length = 25 ms, dynamic range = 50 dB, frequency range = 0-4000 Hz, grayscale colormap (darker = higher energy).

IV. Observation of Core Phenomena
/l/ Spectral Characteristics (Left Panel): Continuous voiced formant bands with a clear F1 (~450 Hz) and F2 (~1500 Hz); a visibly weaker or damped band (a lateral antiresonance created by the side-branch oral cavity) appears in the pattern; no broadband turbulent noise anywhere in the panel.
/r/ Spectral Characteristics (Right Panel): F2 and F3 pulled close together (~1800-2400 Hz), F3 visibly lower than in a typical vowel (~2300 Hz or below); continuous, periodic formant structure throughout, with no antiresonance damping and no broadband noise.
Characteristic Stability: Both spectrograms remain stable during sustained articulation; no flickering, dropout, or software UI interference; labels remain fixed and legible throughout.
Auxiliary Comparison: Side-by-side layout enables direct contrast — /l/ shows a damped/weakened band from its lateral antiresonance alongside otherwise clear formants; /r/ shows F2 and F3 drawn close together with a low F3 and no such damping — confirming the two consonants' distinct articulatory-acoustic classes.

V. Summary of Key Knowledge Points and Principles
Core Concept Definition: Lateral approximants (e.g., /l/) involve continuous voiced airflow escaping around the sides of the tongue; the oral cavity forms a side-branch resonator that produces a damped, weakened frequency band (a lateral antiresonance) alongside otherwise clear formants — not broadband turbulent noise, which instead characterizes lateral fricatives (e.g., Welsh /ɬ/), a separate consonant class. Rhotic approximants (e.g., /r/) reshape the vocal tract without a full obstruction, most notably lowering and narrowing the gap to F3, alongside continuous periodic voicing.
Acoustic-Phonetic Significance:
- Diagnostic Feature for /l/: A damped/weakened band (lateral antiresonance) alongside otherwise continuous F1/F2/F3 formant structure, with no broadband noise.
- Diagnostic Feature for /r/: F3 lowered and pulled close to F2, with continuous periodic voicing and no antiresonance damping.
- Functional Implication: These spectral fingerprints enable automatic speech recognition and clinical phonetic assessment — distinguishing articulatory strategies acoustically.

VI. Demonstration Precautions
Articulation Control: Speaker must avoid glides or diphthongs (e.g., no /l/->/ə/ or /r/->/ə/); sustain pure targets using phonetic training. Use visual feedback (mirror) during prep to ensure tongue posture.
Software Calibration: Spectrogram settings must be identical for both panels; disable smoothing filters that obscure fine formant and antiresonance detail; ensure real-time rendering (not post-processed playback).
Labeling Integrity: Labels '/l/' and '/r/' must be permanently affixed to screen (not overlaid later), matching IPA orthography, centered top of each panel.
Duration Constraint: Total demonstration ≤ 3 seconds of acoustic signal (1.0s /l/ + 0.2s pause + 1.0s /r/) to fit 10s video limit — no extended silence or explanation.

Prompt Grounding rubric:
level_1: No headset microphone is visible anywhere in frame; no speaker is present; no computer screen is shown; nothing resembling a spectrogram appears; there is no left/right split, or the split exists but nothing is drawn on either side.
level_2: The speaker is present and wearing a headset microphone, but the computer screen is blank, distorted, or shows something other than a spectrogram (e.g., a media player, a text document); if spectrograms do appear, there is no clear left-side/right-side split, or both spectrograms sit on the same side, overlap, or stack vertically instead of appearing side by side.
level_3: The speaker wears the headset microphone and appears to produce sustained sounds; the screen shows two spectrogram-like panels side by side, but they are unlabeled, mislabeled (e.g., '/l/' printed on the right, '/r/' on the left), or the text is too small or blurry to read; the panels lack visible time or frequency axis marks; there is no visual cue indicating which sound came first.
level_4: The speaker wears the headset microphone and is clearly shown producing sound; the screen shows two labeled panels, left marked '/l/', right marked '/r/', each rendered as a proper spectrogram (a banded pattern across a time-frequency grid, not a plain color block); the layout is cleanly split left/right with readable labels; only minor cosmetic issues remain, such as a slightly off-center label or a small stray UI icon.
level_5: Matches the setup precisely: (i) the speaker is clearly wearing a headset microphone with a visible earpiece and boom mic; (ii) the speaker's mouth is visibly open in a sustained-articulation pose while the recording appears to be happening; (iii) the screen shows two full, clean spectrogram panels side by side, left labeled '/l/' and right labeled '/r/', each rendered as an actual time-frequency image rather than placeholder graphics; (iv) there is no stray UI clutter, and the font and color are consistent for both labels; (v) some visible cue reinforces that /l/ was recorded before /r/ (e.g., a small on-screen timestamp, or a highlight that moves from the left panel to the right panel).

Scientific and Causal Correctness rubric:
level_1: The two spectrograms look essentially identical, or either one is a flat line, a pure tone, or plain broadband hiss with no formant structure at all; the speaker appears silent, or is clearly producing a different sound altogether (e.g., a vowel or /s/) rather than a sustained /l/ or /r/.
level_2: The two spectrograms differ, but in a way that contradicts what /l/ and /r/ should look like: for example, one panel shows dense frication-like noise spread across the whole frequency range (broadband turbulent hiss is not a property of either sound), or the /r/ panel shows sharp, abrupt vertical stripes like a stop consonant rather than continuous bands; the speaker's visible mouth or tongue posture contradicts the sound supposedly being produced (e.g., no tongue-tip-up gesture during /l/, no lip or tongue-body change into /r/).
level_3: The left spectrogram shows continuous formant bands (roughly an F1 near 400-500 Hz and an F2 near 1200-1800 Hz) with a visibly weaker or damped band somewhere in the pattern (the lateral antiresonance), and no dense broadband noise; the right spectrogram shows an F3 noticeably lower than a typical vowel's F3 and closer to F2 than in the left panel, but the contrast is only loosely distinguishable, e.g. the F3 lowering on the right is slight, or the damped band on the left is faint.
level_4: The left spectrogram (/l/) clearly shows continuous, smoothly varying formant bands with one visibly damped or weakened region and no dense broadband noise anywhere in the panel; the right spectrogram (/r/) clearly shows F3 pulled down close to F2, with a visibly narrower F2-F3 gap than in the left panel; the speaker's mouth is visibly doing something different for each sound (tongue-tip contact position and jaw posture change) consistent with switching from /l/ to /r/.
level_5: The acoustic contrast is fully clear: (i) the /l/ spectrogram shows continuous voiced formant bands with one clearly visible damped or weakened band (the lateral antiresonance) and no broadband noise anywhere in the panel; (ii) the /r/ spectrogram shows F3 pulled down so close to F2 that the two bands nearly merge, with no such merging visible in the /l/ panel; (iii) the speaker's articulatory posture visibly differs between the two sounds (e.g., tongue tip raised for /l/ versus tongue drawn back or lips slightly rounded for /r/) and lines up with which spectrogram is currently filling in; (iv) the contrast between the two panels is obvious at a glance, with no need to guess which panel is which from the pattern alone.

Spatiotemporal Consistency rubric:
level_1: The speaker's head blocks the view of the screen; the microphone appears to float apart from the speaker's head; a spectrogram appears on screen before the speaker has started making any sound; the screen is rotated 90 degrees from upright; or the left/right labels are swapped while the speaker is also shown facing an implausible direction (e.g., a mirror-flipped setup that makes no sense).
level_2: The /r/ spectrogram appears before the /l/ spectrogram; the speaker stops moving their mouth well before the corresponding spectrogram panel begins filling in; one panel is still actively drawing while the other has already gone fully static, well out of step with the speaker's actions; the headset microphone appears positioned behind the speaker's head rather than near the mouth, or its cable appears disconnected partway through; the two panels sit at different heights with no shared baseline.
level_3: Small timing mismatches are visible: the speaker's mouth closes noticeably before the /l/ panel finishes filling in, or the /r/ panel starts filling in a beat after the speaker has clearly begun the /r/ sound; the screen and speaker are both in frame but look flat or pasted together rather than occupying a coherent shared space; the two spectrogram panels are drawn at different sizes or use different time/frequency ranges, making them hard to compare directly.
level_4: The speaker's mouth movement for /l/ and the left panel filling in happen at essentially the same time, and the same is true for /r/ and the right panel, with only a brief, natural pause between the two sounds; the headset microphone stays in place and looks naturally worn throughout; the screen stays frontal, centered, and framed the same way from start to finish; both spectrogram panels use the same time span and frequency range so they can be compared directly at a glance; the brightness and color of the lighting on the speaker's face and the screen stay constant across the whole clip.
level_5: The whole clip reads as one continuous, well-timed sequence: the recording begins, the /l/ panel fills in while the speaker sustains /l/, there is a short pause, then the /r/ panel fills in while the speaker sustains /r/, with no visible gap between what the speaker is doing and what appears on screen; the speaker's visible mouth and tongue posture changes match which sound is being produced at each moment; the camera stays locked in the same fixed frontal position for the entire clip with no shift in perspective; there is no visible stutter, freeze-frame, or juddering as the spectrogram panels update or as the clip moves from one phase to the next.