How Do We Hear? — Understanding the Auditory System
Flocolor Hearing Health Series — Article 1 of 7
Table of Contents
1. Introduction
Imagine this: you're sitting in a bustling café, surrounded by the hum of espresso machines, the clink of cutlery, and dozens of overlapping conversations — yet when your friend across the table speaks, you can still understand every word clearly. What seems effortless actually relies on one of the human body's most sophisticated sensory systems: the auditory system.
Every time you hear a sound, it has traveled a remarkable journey — from a physical vibration to a neural signal. Understanding this journey is the first step toward understanding hearing loss.
2. The Auditory Pathway: From Sound to Brain
The path of sound can be divided into five stages, each one indispensable.
The Outer Ear: Capturing and Channeling
The pinna (also called the auricle) is shaped like a funnel that collects sound waves from all directions and funnels them into the ear canal. About 2.5 cm long, the ear canal guides the sound waves to its end, where they reach the tympanic membrane — commonly known as the eardrum.
The Middle Ear: Amplifying and Transmitting
When sound waves reach the eardrum, it begins to vibrate. These vibrations are then passed along to the three smallest bones in the human body, collectively known as the ossicles:
- • Malleus (Hammer) — attached to the inner side of the eardrum
- • Incus (Anvil) — connects the malleus to the stapes
- • Stapes (Stirrup) — sits against the oval window of the inner ear
These three bones form a precise lever system that amplifies the eardrum's vibrations by approximately 22 times, ensuring that sound energy is efficiently transferred from air into the fluid-filled inner ear.
The Inner Ear: From Mechanical Vibration to Electrical Signals
The amplified vibrations travel through the stapes into the cochlea — a spiral-shaped organ resembling a snail shell. The cochlea is filled with fluid (lymphatic fluid), and the vibrations create ripples that cause the basilar membrane to move.
Lining the basilar membrane are tens of thousands of hair cells. When the basilar membrane vibrates, the tiny hair-like projections (stereocilia) on these cells bend, and the hair cells convert this mechanical energy into electrical signals. This is the most critical "transduction" step in the entire auditory pathway.
Key Fact: Once damaged, hair cells do not regenerate. This is why hearing loss caused by noise exposure or presbycusis (age-related hearing loss) is typically permanent.
The Auditory Nerve: A High-Speed Signal Pathway
The electrical signals released by the hair cells are captured by the auditory nerve (Cranial Nerve VIII) and rapidly transmitted to the brainstem, then relayed upward to the thalamus.
The Brain: Interpreting the Meaning of Sound
Finally, the signals arrive at the auditory cortex on both sides of the brain, where the brain completes its analysis of the sound: distinguishing speech from music, locating the direction of the sound source, and recognizing emotional tone in voices. From eardrum vibration to brain interpretation, the entire process takes only a few milliseconds.
3. Fundamental Parameters of Sound
To understand hearing, we need to know two fundamental parameters that describe sound.
Frequency (Hz) — The Pitch of Sound
Frequency refers to the number of sound wave vibrations per second, measured in Hertz (Hz). Higher frequencies are perceived as higher-pitched sounds.
- • Human hearing range：20 Hz – 20,000 Hz
- • Human speech frequency range：250 Hz – 4,000 Hz
Source: ASHA; HearingPartners 2025 (frequency range compiled from multiple sources)
This means that even if hearing loss only affects frequencies above 4,000 Hz, a person may still feel they can "hear" — but understanding speech becomes difficult, because the energy of many consonant sounds (such as s, f, th) is concentrated in this frequency range.
Loudness (dB SPL) — The Intensity of Sound
Loudness is measured in decibels (dB SPL). The decibel is a logarithmic unit — every 10 dB increase represents approximately a tenfold increase in sound intensity.
- • Daily conversation：55–70 dB SPL
- • Normal hearing threshold：0–20 dB HL (IEC 60645-1:2017 audiometer reference zero standard)
Source: ASHA 1998; compiled from multiple sources
4. When Transmission Is Disrupted: Types of Hearing Loss
When any component of the auditory pathway is disrupted, hearing decline results. Based on the location of the damage, hearing loss falls into three main categories.
Conductive Hearing Loss
The problem lies in the outer or middle ear — the sound conduction chain is blocked, preventing sound waves from reaching the inner ear effectively.
| Cause | Description |
|---|---|
| Earwax impaction | Accumulated cerumen blocks the ear canal |
| Tympanic membrane perforation | The eardrum ruptures due to infection or trauma |
| Otosclerosis | Abnormal fixation of the stapes, preventing normal vibration |
Conductive hearing loss is treatable in many cases, for example through earwax removal or surgical repair.
Sensorineural Hearing Loss
The problem lies in the inner ear or auditory nerve — hair cells are damaged or the auditory nerve degenerates, making it impossible to convert mechanical vibrations into electrical signals.
| Cause | Description |
|---|---|
| Noise exposure | Prolonged or intense noise damages hair cells |
| Presbycusis | Age-related hair cell degeneration, basilar membrane stiffening, vascular degeneration |
| Vestibular schwannoma | Auditory nerve tumor, often accompanied by hearing loss and dizziness |
This is the most common type of hearing loss and, because hair cells cannot regenerate, it is typically irreversible.
According to the WHO World Report on Hearing (2021), approximately 1.5 billion people worldwide have hearing loss, of whom 430 million experience disabling hearing loss.
Mixed Hearing Loss
Both conductive and sensorineural factors coexist — meaning there is damage in both the outer/middle ear and the inner ear/auditory nerve.
Common causes: Chronic otitis media (long-standing conductive issue) combined with presbycusis (sensorineural degeneration), or residual sensorineural damage following otosclerosis surgery.
5. The Silent Threat of Hearing Loss
Many people believe hearing loss happens "suddenly," but the opposite is true. The vast majority of hearing loss is progressive — it worsens so gradually that you may not notice for years, until people around you start commenting that "the TV is too loud" or "you keep asking people to repeat themselves."
"Despite its high prevalence, ARHL remains underdiagnosed and undertreated, partly due to its gradual onset, stigma, and lack of standardized screening and management protocols."
Hearing Loss Severity Classification
Based on the grading criteria consistently adopted by AAO-HNS and WHO:
| Severity | Threshold | Everyday Impact |
|---|---|---|
| Normal | 0–25 dB HL | No noticeable difficulty |
| Mild | 26–40 dB HL | Difficulty hearing whispers |
| Moderate | 41–55 dB HL | Everyday conversation becomes challenging |
| Moderately Severe | 56–70 dB HL | Hearing aids needed for conversation |
| Severe | 71–90 dB HL | Must speak loudly to be heard |
| Profound | ≥91 dB HL | Nearly impossible to communicate by hearing alone |
Source: AAO-HNS / WHO (2021) Classification
If you find yourself struggling more and more to follow conversations in noisy environments, this may be an early sign of hearing loss — in the next chapter, we will discuss in detail how to assess your own hearing status.
References
- World Health Organization. World Report on Hearing. Geneva: WHO; 2021.
- World Health Organization. "Grades of Hearing Impairment." World Report on Hearing, 2021.
- American Speech-Language-Hearing Association (ASHA). "The Prevalence and Incidence of Hearing Loss in Adults." ASHA, 1998.
- American Academy of Otolaryngology–Head and Neck Surgery (AAO-HNS). "Hearing Loss Classification." AAO-HNS Clinical Indicators, 2021.
- International Electrotechnical Commission. IEC 60645-1:2017 — Electroacoustics — Audiological equipment — Part 1: Pure-tone audiometers.
- Hughes SE, et al. "Age-Related Hearing Loss: Underdiagnosis and Undertreatment." ORL Head and Neck Nursing, 2025.
- HearingPartners. "Understanding Hearing Ranges." 2025.
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