Audiology Module 1 – Basics of Sound (Comprehensive Notes)

What Is Sound?

Sound = result of vibratory energy transmitted through a medium - Requires matter (gas, liquid, or solid) to propagate; cannot travel through a vacuum.

Mechanical (NOT electromagnetic) wave created by the back-and-forth vibration of medium particles.

Wave classification - Longitudinal: particle displacement is parallel to energy transport direction.

Creates alternating regions of:

Compressions (high pressure)

Rarefactions (low pressure)

Hearing = perception/interpretation of those pressure variations by the auditory system.

Key Resources (Optional Enrichment)

Written primer: Physics Classroom – “Sound is a Pressure Wave” (link provided).

Video: “What is Sound? The Fundamental Science Behind Sound” (YouTube).

Sound Properties & Their Perceptual Counterparts

All audible sounds possess two foundational physical descriptors, each with a psychological correlate:1. Frequency

→ perceived as Pitch

Intensity

→ perceived as Loudness

Pause-Point prompt (lecture slide): - "Name something else that vibrates and creates a sound" (e.g., tuning fork, vocal folds, guitar string, loudspeaker diaphragm).

"Do all sound vibrations sound the same?" → No; differences arise from each object’s unique frequency spectrum, intensity envelope, and mode of vibration.

Frequency (Pitch)

Definition: Rate of particle vibration per unit time.

Mathematical expression: $f = \frac{1}{T}$ where $f$ = frequency (Hz) and $T$ = period (seconds).

Units: Hertz (Hz) = cycles/second.

Human auditory range: $20\,\text{Hz} \;\text{to}\; 20{,}000\,\text{Hz}$ (≈10 octaves).

Crucial for speech intelligibility: - Vowel formants, consonant place cues, intonation patterns, etc.

Link to detailed reading on intensity also included on slide (though that URL actually points to intensity content).

Pause-Point Video: Human Audio Spectrum

YouTube link demonstrates a sweep through $20\,\text{Hz} \rightarrow 20{,}000\,\text{Hz}$ both aurally and visually.

Caution: volume becomes very loud near mid-spectrum frequencies; lower playback level advised.

Reflection prompt: "Why can’t we hear the very beginning/end?" - Ear’s sensitivity rapidly decreases below $\approx 20\,\text{Hz}$ (infrasound) and above $\approx 20\,\text{kHz}$ (ultrasound).

Biomechanical limits of basilar-membrane mechanics and hair-cell transduction.

Intensity (Loudness)

Definition: Magnitude of sound energy

→ quantified by particle displacement distance / sound-pressure variation.

Physical quantity: Sound Intensity (I) = power per unit area $\big(\text{W}\,\text{m}^{-2}\big)$.

Logarithmic measure: Decibel (dB) scale. - General formula: $\text{dB} = 10 \log{10}\left( \frac{I}{I{0}} \right)$

$I*{0} = 1\times10^{-12}\,\text{W}\,\text{m}^{-2}$ (standard threshold of hearing at $\approx 1\,\text{kHz}$).

Psychological correlate = loudness, which grows non-lin_early with physical intensity.

Multiple reference-dependent dB scales: - dB SPL (Sound Pressure Level): compares pressure to 20 µPa reference.

dB HL (Hearing Level): normalized threshold for average young listeners across frequencies; used in audiograms.

dB SL (Sensation Level): level above an individual’s own threshold.

Typical conversational speech intensities: $45\text{–}50\;\text{dB HL}$ or $55\text{–}60\;\text{dB SPL}$.

Intensity of Various Everyday Sounds (Lecture Table)

Source

Level (dB)

$I\,(\text{W m}^{-2})$

Jet plane at $30\,\text{m}$

$140$

$100$

Threshold of pain

$120$

$1$

Loud indoor rock concert

$120$

$1$

Siren at $30\,\text{m}$

$100$

$1\times10^{-2}$

Automobile $90\,\text{km h}^{-1}$

$75$

$3\times10^{-5}$

Busy street traffic

$70$

$1\times10^{-5}$

Ordinary conversation (50 cm)

$65$

$3\times10^{-6}$

Quiet radio

$40$

$1\times10^{-8}$

Whisper

$20$

$1\times10^{-10}$

Rustle of leaves

$10$

$1\times10^{-11}$

Absolute threshold of hearing

$0$

$1\times10^{-12}$

Observations / Implications of Table

Each $+10$ dB step ≈ 10× increase in intensity; perceived loudness doubles roughly every $+10$ dB.

Threshold of pain ($120\,\text{dB SPL}$) coincides with $I \approx 1\,\text{W m}^{-2}$; risk of immediate damage.

Conversational levels sit about 65 dB SPL, $\;\approx$ $3\times10^{-6}\,\text{W m}^{-2}$.

Pause-Point Video: “What’s the Loudest Possible Sound?”

Explores physical limit: $194\,\text{dB SPL}$ in air at sea level (pressure variation equals atmospheric pressure).

New/clarified takeaways expected from students, e.g.: - Shock waves vs. normal sound waves; beyond $194\,\text{dB}$ regular wave description breaks down.

Astrophysical or underwater scenarios can exceed that limit via different reference conditions.

Synthesis & Upcoming Questions

Essential take-home points - Sound ← vibration of matter.

Frequency (Hz) determines pitch perception.

Intensity (dB) determines loudness perception.

Forward-looking module themes: - How the auditory system encodes frequency & intensity information (place theory, temporal coding, neural firing patterns).

Use of sound stimuli to evaluate auditory-system function (pure-tone audiometry, immittance, otoacoustic emissions).

Sound-based rehabilitation (hearing aids, cochlear implants, auditory training).

Ethical / practical considerations: safe listening levels, noise-induced hearing loss prevention, accessibility of assistive tech.

Mnemonic