# The Acoustics of Speech

Languages I · English Phonetics and Phonology · https://tryals.app/learn/languages-i/the-acoustics-of-speech

## Source and Filter

The standard acoustic model separates speech into two parts:

* The **source** is the buzz from vibrating vocal folds, rich in harmonics.
* The **filter** is the vocal tract, which amplifies specific frequencies.

Changing the source alters pitch. Changing the filter changes the vowel.

### Reading a Spectrogram

A **spectrogram** plots time horizontally, frequency vertically, and intensity as darkness:

* **Fundamental frequency (F0)**: the rate of vocal fold vibration, heard as pitch.
* **Formants**: vocal tract resonances, seen as dark horizontal bands.
* **Voicing**: shown by a voice bar along the bottom.
* **Silence then a burst**: marks a plosive.
* **Random energy at high frequencies**: marks a fricative.

### Identifying Vowels

Two formants identify a vowel:

* **F1 varies inversely with vowel height.** High vowels have a low F1, around 300 Hz.
* **F2 varies with frontness.** Front vowels have a high F2.

For a neutral schwa, the values sit at roughly **500 Hz and 1,500 Hz**.

### Voice Onset Time

**VOT** is the duration between stop release and the start of voicing. In English, it distinguishes /b/ from /p/.

| Category | VOT | English example |
|---|---|---|
| Voicing lead | negative | rare in English |
| Short lag | 0 to 20 ms | /b d g/ |
| Long lag | 40 to 80 ms | /p t k/ |

Perception is **categorical**. Listeners hear continuous variation as Two distinct categories with a sharp boundary near 30 ms.

> **Common pitfall:** confusing F0 with F1. F0 is the source, heard as pitch. F1 is the first filter resonance, heard as vowel height.

## Practice questions

7 of this lesson's 13 practice questions, with answers. The full set is in the app.

### 1. A speaker raises their pitch while continuing to say the same vowel. Why do the formants stay roughly where they were?

A. F0 rises while the formants stay roughly where they were
B. F1 rises alongside pitch whilst F2 stays stationary
C. Both F1 and F2 shift upward in step with the harmonics
D. The vocal tract filter changes shape as the source shifts

**Answer:** A. F0 rises while the formants stay roughly where they were

**Why:** **F0 rises and the formants do not follow.** The source and filter are independent, which is the whole content of the model, and the everyday proof is that you can sing a scale on a single vowel without the vowel changing.

Page: https://tryals.app/practice/languages-i/the-acoustics-of-speech/a-speaker-raises-their-pitch-while-continuing-to-say-the-same-vowel

### 2. F1 varies inversely with vowel height: a high vowel has a low first formant.

**Answer:** True

**Why:** **True**, and the inversion is what trips people up. Higher tongue position, lower F1, the vowel of *see* sits near 300 Hz and the vowel of *father* near 700 Hz. Plotting F1 downwards on a chart is a convention adopted precisely so the acoustic plot resembles the articulatory quadrilateral.

Page: https://tryals.app/practice/languages-i/the-acoustics-of-speech/f1-varies-inversely-with-vowel-height-a-high-vowel-has-a-low-first

### 3. Approximately what is the perceptual boundary in voice onset time between English /b/ and /p/, in milliseconds?

**Answer:** 30 (within ±10)

**Why:** **About 30 ms.** The boundary is categorical: a continuum of stimuli is heard as two categories with a sharp switch rather than as a gradual change. Where the boundary sits differs by language, which is why the same continuum divides differently for a Thai listener.

Page: https://tryals.app/practice/languages-i/the-acoustics-of-speech/approximately-what-is-the-perceptual-boundary-in-voice-onset-time

### 4. The source-filter theory models the vocal folds as the generator and the supraglottal tract as a resonator. What follows from this independence when a speaker produces a whisper?

A. The resonator shapes silent turbulence in place of periodic buzz
B. The fundamental frequency drops below the audible speech threshold
C. The filter ceases resonance because vocal fold vibrations vanish
D. The absence of glottal pulses eliminates the vocal tract formants

**Answer:** A. The resonator shapes silent turbulence in place of periodic buzz

**Why:** Whispering removes the periodic glottal source, substituting unvoiced airflow while leaving the vocal tract filter active to shape formants. Conflating filter resonance with source activity ignores that resonances simply modify whatever acoustic energy enters the tract.

Page: https://tryals.app/practice/languages-i/the-acoustics-of-speech/the-source-filter-theory-models-the-vocal-folds-as-the-generator-and

### 5. Sort each property by whether it belongs to the SOURCE or to the FILTER.

**Answer:**

- Source: Rate of vocal fold vibration, Heard as pitch, Changes when the folds are tensed
- Filter: Resonances determined by tract shape, Heard as vowel quality, Changes when the tongue moves

**Why:** Keeping the two columns apart resolves most confusions in this lesson, F0 against F1 in particular. They are produced by different structures, vary independently, and are heard as different things.

Page: https://tryals.app/practice/languages-i/the-acoustics-of-speech/sort-each-property-by-whether-it-belongs-to-the-source-or-to-the

### 6. Complete the account of speech acoustics.

**Answer:** The buzz produced at the larynx is the **source**, and the vocal tract acts as a **filter**. Resonances of the tract are called **formants**, and the first of them varies inversely with vowel **height**.

**Why:** The final blank is the one to get right: F1 tracks height inversely and F2 tracks frontness. Swapping them makes every vowel chart unreadable, and it is the single most common error in a first phonetics exam.

Page: https://tryals.app/practice/languages-i/the-acoustics-of-speech/complete-the-account-of-speech-acoustics

### 7. The perception of voice onset time is continuous: listeners hear a gradual change across the VOT range.

**Answer:** False

**Why:** **False.** Perception is **categorical**: a continuum of VOT values is heard as two categories with an abrupt boundary, and listeners discriminate pairs that straddle the boundary far better than equally spaced pairs within a category. The effect is one of the foundational results of speech perception research.

Page: https://tryals.app/practice/languages-i/the-acoustics-of-speech/the-perception-of-voice-onset-time-is-continuous-listeners-hear-a