Hearing begins when vibrating objects create pressure waves that travel through air and enter the ear. The human ear is organized into the outer ear, middle ear, and inner ear, each with a specialized job in collecting, amplifying, and translating sound. This system matters because it lets us communicate, detect danger, enjoy music, and understand our surroundings.
The same inner ear also helps the brain sense head motion and maintain balance.
The outer ear funnels sound waves to the eardrum, which vibrates back and forth. Tiny middle ear bones called ossicles amplify these vibrations and transfer them into fluid inside the cochlea. In the cochlea, hair cells bend and convert mechanical motion into electrical nerve signals that travel through the auditory nerve to the brain.
Balance is detected by fluid motion in the semicircular canals and other vestibular structures, which signal changes in head position and movement.
Key Facts
- The outer ear includes the pinna and ear canal, which collect sound and guide it to the eardrum.
- The middle ear contains three ossicles: malleus, incus, and stapes.
- Frequency is the number of vibrations per second, measured in hertz: 1 Hz = 1 cycle/s.
- Wave speed equation: v = fλ, where v is wave speed, f is frequency, and λ is wavelength.
- Sound intensity level is measured in decibels: β = 10 log10(I/I0).
- Hair cells in the cochlea convert vibration into nerve impulses sent through the auditory nerve.
Vocabulary
- Pinna
- The visible outer part of the ear that helps collect and direct sound waves into the ear canal.
- Eardrum
- A thin membrane, also called the tympanic membrane, that vibrates when sound waves strike it.
- Ossicles
- The three tiny bones of the middle ear that amplify vibrations from the eardrum.
- Cochlea
- A spiral-shaped inner ear organ where vibrations are converted into electrical signals by hair cells.
- Auditory nerve
- The nerve that carries hearing signals from the cochlea to the brain.
Common Mistakes to Avoid
- Thinking sound travels as particles moving from the source to the ear is wrong because sound is a wave of pressure changes passed through a medium.
- Saying the eardrum sends electrical signals directly to the brain is wrong because the eardrum produces mechanical vibrations that must pass through the ossicles and cochlea first.
- Confusing loudness with pitch is wrong because loudness depends mainly on wave amplitude, while pitch depends mainly on frequency.
- Ignoring the role of the inner ear in balance is wrong because the semicircular canals and vestibular organs detect head movement and position.
Practice Questions
- 1 A sound wave has a frequency of 500 Hz and travels through air at 340 m/s. What is its wavelength?
- 2 A person hears a tone with wavelength 0.68 m in air where sound travels at 340 m/s. What is the frequency of the tone?
- 3 Explain why damage to cochlear hair cells can cause hearing loss even if the outer ear, eardrum, and ossicles are working normally.