The ear canal acts as a quarter-wavelength resonator amplifying 3-4 kHz by 10 dB

Evidence

The ear canal (external auditory meatus) is a relatively hard-walled tube that is approximately 22.5 mm long and has a diameter of about 7.5 mm, so that its volume is about 1 cm3. Acoustically, it is a short transmission line where sound waves propagate from the external environment to the eardrum. Because one end is open (low acoustic impedance) and the impedance of the eardrum is higher than for the tube itself, the ear canal acts as a quarterwavelength resonator, emphasizing signals around frequencies of 3–4 kHz by about 10 dB, attenuating them at around 7–8 kHz, and showing the next (weak) resonance above 10 kHz. — Ville Pulkki, Matti Karjalainen, Communication Acoustics- An…, 728

Categorization

• DEFINE: Classic quarter-wavelength resonator example: open tube with higher impedance at the far end, ~22.5 mm tuning 3-4 kHz boosts.
• XREF: Connects to why hearing is most sensitive around 3-4 kHz and why it aligns with speech emphasis and hearing-aid design bands. — connects to: Outer ear canal resonance explains peak sensitivity near 4 kHz — Eric J. Heller, Why You Hear What You Hear- An…, 2005; A normal-sounding ear is internally a sound source audible in the closed canal; Helmholtz resonators serve as transducers, amplifiers, and sound controllers. (+6 more)
• corroborated: 2 independent passages support this claim — Ville Pulkki, Matti Karjalainen, Communication Acoustics- An…, 728

Broader Connections

• Hearing damage first appears at the ear canal’s resonance frequency near 4 kHz #expands (0.95) — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 677
• Outer and middle ear structures provide frequency-specific amplification of incoming sound #exemplifies (0.85) — Jennifer Lentz, Psychoacoustics- Perception of Normal…, 212
• The ear’s full dynamic range exists only near 4 kHz #expands (0.80) — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 612
• An open mouth amplifies faint sounds by serving as a resonant chamber #exemplifies (0.70) — Gordon Hempton, John Grossmann, One Square Inch of Silence- One Man’s…, 191