Every fact in the three ear lessons is listed here with where it comes from. The numbers in [brackets] on the lesson pages link to this list.
How we rate sources
High: space agencies (NASA), peer-reviewed open textbooks (OpenStax, Rice University) and professional scientific societies (American Physical Society).
Medium-high: reputable science news reporting directly on a primary document.
Medium: single-author essays and popular history sites. We only use these for history details, and never on their own: each one is paired with a second source.
Wikipedia was not used as a source. Numbers are rounded (for example 9.8 m/s² for Earth's gravity). The text was drafted with help from AI (Claude) and then checked against these sources. Corrections: hello@echoesof.science.
Where the facts are simplified or less certain: the numbers of hair cells (about 3,500 inner and 12,000 outer) are rounded textbook values [2]. The interaural time difference of about 0.68 ms is a worked example with an assumed head width of 23 cm; the source notes that real maximum differences are a little over half a millisecond [4]. The damage times for 80–85 dB and 95 dB are guide values from CDC [5]; real risk depends on the person and the sound. Phone sound-meter apps are not calibrated instruments, so treat their readings as estimates. These lessons are general education, not medical advice: anyone worried about their hearing should talk to a doctor, the school nurse or a pharmacist.
All sources
[1] How Do We Hear?
National Institute on Deafness and Other Communication Disorders (NIDCD, NIH) · Reliability: High
Type: National health research institute
Step by step: the outer ear channels sound down the ear canal to the eardrum; the eardrum vibrates; three middle-ear bones (malleus, incus, stapes) amplify the vibrations; the fluid-filled, snail-shaped cochlea ripples; a travelling wave on the basilar membrane moves the hair cells; hair cells near the wide end detect higher-pitched sounds and those near the centre lower-pitched sounds; bending stereocilia open channels and create an electrical signal that the auditory nerve carries to the brain.
[2] Biology 2e, 36.4 Hearing and Vestibular Sensation
OpenStax, Rice University · Reliability: High
Type: Peer-reviewed open textbook
Sound is a mechanical pressure wave needing a medium; frequency is heard as pitch and amplitude as loudness; humans hear up to about 20,000 Hz (higher is ultrasound). The ossicles collect force and amplify sound; the stapes presses on the oval window. The basilar membrane is stiffer and narrower at the base (high frequencies) and floppier towards the apex (low frequencies). About 3,500 inner hair cells are the main receptors and about 12,000 outer hair cells fine-tune; loudness depends on how many hair cells are stimulated; signals reach the auditory cortex in the temporal lobe. The vestibular system (utricle, saccule, three semicircular canals) senses balance, gravity and head rotation.
Binaural cues (interaural level difference, because the head weakens the sound at the far ear, and interaural timing difference) locate sounds left-right; the pinna gives monaural cues for up-down and front-back. Conductive hearing loss (blocked ear canal, hole in the eardrum, ossicle problems, fluid behind the eardrum) is often helped by hearing aids; sensorineural loss (most common; causes include ageing, noise, some medications, tumours) is not; cochlear implants stimulate the auditory nerve directly. Many Deaf people use sign language.
[4] Sensation and Perception: Interaural Time Difference
University of Minnesota (open course text, PSY 3031) · Reliability: Medium-high
Type: University open course text
A sound on the left reaches the left ear slightly before the right ear. Worked example: an extra path of 0.23 m at 340 m/s gives about 0.68 ms (686 µs); real maximum differences are a little over half a millisecond, largest for sounds directly to the left or right.
Centers for Disease Control and Prevention (CDC) · Reliability: High
Type: National public health agency
Normal conversation is about 60 dB and typically causes no damage; noise above 70 dB over a long time may start to damage hearing; if you must raise your voice to be heard at arm's length the noise is probably above 85 dB; lawn mowers and leaf blowers (80–85 dB) can damage hearing after about 2 hours, a motorcycle (95 dB) after about 50 minutes; firecrackers (140–150 dB) cause pain and ear injury. The louder the sound, the shorter the time before damage. Advice: hearing protection, rest for the ears, keep listening devices away from maximum volume, sound-meter apps can measure noise.
Stanford Medicine, Ear Institute · Reliability: High
Type: University medical centre
There are inner and outer hair cells. Loud music or noise damages hair cells by bending the stereocilia too far, which can kill the cell; because cochlear hair cells cannot grow back, this causes permanent hearing loss.
World Health Organization (WHO) · Reliability: High
Type: International health agency
By 2050 nearly 2.5 billion people are projected to have some degree of hearing loss; more than 1 billion young adults are at risk of permanent, avoidable hearing loss from unsafe listening; hearing loss from loud sounds is preventable.
NHS (UK National Health Service) · Reliability: High
Type: National health service
Earwax normally falls out on its own. Do not use your fingers or objects like cotton buds to remove earwax. A pharmacist can help when it blocks your ears.