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Sound

Just like vision, our sense of hearing involves the transduction of environmental energy into electrical messages our brain can interpret. Sound waves are vibrations of air molecules caused by pulses of air pressure.

The amplitude, or height of the wave, reflects the volume of the sound and is measured in  decibels . High amplitude waves are louder compared to waves with smaller amplitude.

The pitch of the sound is determined by the frequency and is measure in cycles per second, or  hertz . The higher the wave frequency, the higher the pitch.

Humans have a spectrum of sensitivity for sound ranging from an absolute threshold of 20 Hz to a peak of about 20,000 Hz. A dog’s hearing ranges from 40 Hz up to a high of 60,000 Hz and bats upper register is a soaring 120,000 Hz.

Image credit: USGS - Serenia Project

The Florida manatee’s poor sensitivity to the low frequency sounds of slow moving boats is one of the reasons for this animal’s threatened population status.

The  timbre , or tone, of a sound is the quality that distinguishes the different voices of sound sources. For example, a middle C note on a piano will have a different timbre compared to the same note played on a guitar or woodwind instrument.

The trick to sensing sound is a complex system of structures that convert air vibrations into electrical impulses.

Ear Structures

Follow the pathway of sound waves as the move through the structures of the ear with these simulations. View the simulations on the mechanics of sound and ear structure functions. Remember the simulations enhance your understanding, so you can be successful on exams. Be sure to take the self-quiz at the end.

Outer Ear

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The outer ear is made up of the pinna, or what we think of as our ears. The job of the pinna is to funnel sound waves from the environment into the ear canal. The shape of our ears allows us to direct our attention either to louder sounds from the front of our bodies or quieter sounds from the rear. Humans have fixed pinna so we must move our heads to the direction of the sound, unlike the outer ear structures of a cat.

Middle Ear

Once the sound waves enter the ear canal, they vibrate the eardrum, or  tympanic membrane . Just like a big bass marching drum, the tympanic membrane resonates, or vibrates, with the frequency of the air vibrations. In turn, the vibrations are conducted to the  ossicles , or bones, of the inner ear. The hammer, anvil, and stirrup (or malleus, incus, and stapes if you prefer some Latin) are the smallest bones in the human body.

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Inner Ear

The stirrup bone transfers the vibrations to the oval window membrane of the  cochlea  of the inner ear. As this membrane vibrates, fluid inside the cochlea jostles tiny hair cells along the  basilar membrane , located inside the cochlea. The vibrations of the hair cells trigger signals to the attached neurons and send electrical impulses along the auditory nerve to the thalamus. From the thalamus, the signals are relayed to the temporal lobe’s auditory cortex for processing.

Pitch Theories

Place Theory

While the mechanics of audition are straight forward, the puzzle of how we perceive pitch differences remains. The research of Herman von Helmholtz (1863) and Georg von Békésy (1947) synthesized a place theory where vibrations along different locations of the basilar membrane (located inside the cochlea) match the perception of different pitches. This theory assumes that the hair cells of the basilar membrane function independently of one another and are capable of sending individual messages to the auditory cortex.

Frequency Theory

Ernest Rutherford proposed another theory in 1886 suggesting the entire basilar membrane vibrates in unison similar to the resonance of a drum head. The brain then interprets the pitch of the tone by measuring the specific frequency of the basilar membrane vibrations.

Volley Principle

Both place theory and frequency theory have merit, but do not completely explain pitch perception. Place theory is correct with the exception that hair cells do not function independently. They work together in unison, as suggested by frequency theory. However, individual neurons have a maximum firing rate of 1000 times per second. This cannot explain our perception of pitches above this register. The volley principle suggests that groups of neurons can work together to increase the rate of firing. It seems hearing, like with vision, is best explained by an eclectic theory.

Hearing Impairments

Hearing problems can be caused by diverse situations. Some careers carry risk of hearing loss. What about a helicopter pilot? Yes, flying the helicopter is challenging and rewarding, but the chance of losing hearing is very high because of its loud, high pitch sounds that continually envelop the pilot.

Conduction hearing loss  occurs when sound wave vibrations are not transferred between the different areas of the ear due to some mechanical damage to the ear drum, tympanic membrane, or other ear structures.

If the hair cells along the basilar membrane in the cochlea are injured, the result is  sensorineural hearing loss . This nerve damage is currently irreversible, although new breakthroughs in hearing aid design and cochlear implants are reversing the once irreversible.

Follow this link for an audio report about how cochlear implants work.