The ear canal’s quarter-wavelength resonance boosts speech-critical frequencies around 3-4 kHz.

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

Category: Causal Relationships

• XREF: This explains why human speech evolved to concentrate energy in the 3-4 kHz range — acoustic anatomy and vocal evolution are co-adapted. Connects to ethology / evolutionary acoustics reading.
• DEFINE: Introduces the quarter-wavelength resonator concept in a concrete anatomical context.

Related

• The ear canal acts as a quarter-wavelength resonator amplifying 3-4 kHz by 10 dB (similarity 0.90)
• Each new cochlear structure discovered was cast as the resonant sound receiver (similarity 0.86)
• The singer’s formant arises from a quarter-wave resonator in the larynx (similarity 0.81)
• Outer ear canal resonance explains peak sensitivity near 4 kHz (similarity 0.81)
• The outer ear both locates sound sources and boosts certain frequencies (similarity 0.80)
• A normal-sounding ear is internally a sound source audible in the closed canal (similarity 0.78)
• A quarter-wavelength resonator tube boosts a bar’s fundamental loudness (similarity 0.78)