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Hearing
The properties of sound waves determine what we hear.
Wavelength - determines pitch (highness or lowness)
Amplitude - determines volume (loudness or quietness)
Purity - determines timbre. Timbre is what makes an oboe and a piano sound different when they play the same note.
Sound waves pass from the outer ear to the inner ear. As the sound travels, the form of the wave changes:
- Outer ear - movement of air
- Middle ear - movement of bone
- Inner ear - movement of fluid
The Cochlea
The cochlea in the inner ear is lined with hair cells.
When the fluid in the cochlea moves, the hair cells on the basilar membrane vibrate.
The vibration of the hair cells is transduced into a neural signal and sent to the brain.
Pitch Perception
Place Theory - the vibration of particular hair cells corresponds to particular pitches. Think of plucking a string on a guitar or harp .
Corresponds well to evidence from age-related hearing loss - we tend to lose high pitches before low pitches.
Frequency Theory - the whole basilar membrane vibrates at the rate of the pitch you are hearing. The firing of the auditory nerve corresponds to the rate of vibration. Think of a tympani.
- The auditory nerve cannot fire at the rate of the highest pitches we can hear
- Volley principle - different axons in the auditory nerve fire at different rates and it adds up to the right rate.
We use a combination of place theory and frequency theory to explain pitch perception - place for high sounds and frequency for low sounds.
Practice: Hearing
If the basilar membrane in the human ear were longer, place theory would predict that humans would be able to: