The Ear and Hearing
2/14/03
Waves travel through water, and something like an earthquake may cause waves to travel through solid ground. Sound waves are waves traveling through the air and are what we hear.
What Are The Different Parts Of The Ear And What Do They Do?
The outer part of the ear, the part you hang an earring on, is called the pinna. The plural is pinnae (yes, I looked it up, and it comes from the Latin word for “wing.”). The pinna functions to funnel the waves traveling through air into the auditory canal. This is where earwax builds up. Eventually those waves strike the eardrum (tympanic membrane). These structures, the pinna, auditory canal, and eardrum make up the outer ear.
Attached to the eardrum is a small bone called the malleus (hammer). The malleus is the first of three small bones referred to as ossicles. The second and third ossicles are the incus and the stapes. They are also known as the anvil and the stirrup. They are so-named because of their shapes and the area of the bones and surrounding structures is known as the middle ear.
Anyways, when the waves strike the eardrum, the eardrum vibrates in response. The waves are transmitted to the three bones, which vibrate in turn, conveying the wave along. Eventually the wave will reach the end of the stirrup, which is attached to a structure called the cochlea. The cochlea resembles a snail and is the organ where the sound waves are actually translated into nerve signals to be sent to the brain. The word comes from the Latin and Greek words for snail shell (kochlos and konche, where “conch” also comes from). At this point we’ve reached the inner ear.
Inside the cochlea is a structure called the basilar membrane. It runs from one end of the cochlea to the other. The basilar membrane is surrounded by fluid. When the waves reach the stirrup, they are then conveyed to the fluid within the cochlea. As the waves, which were originally sound waves traveling through air, move through the fluid within the cochlea, they cause the basilar membrane to bend in response.
Along the length of the basilar membrane are small “hair cells.” These are actually sensory neurons. When the basilar membrane bends in response to the wave traveling through the surrounding fluid, the hair cells “fire” (action potentials). These action potentials are then conveyed to the brain via the axons that make up the auditory nerve. Eventually they reach the auditory cortex of the temporal lobe or, if the waves are from someone speaking, they are initially processed in Wernicke’s area of the same lobe.
How Can We Tell The Difference Between Different Sounds?
There have been a few theories put forward to try to explain how we tell between different types of sounds. First, keep in mind that we can hear sounds from about 20 to 20,000 Hz. Hertz (Hz) is the measurement scale for sounds, from low to high. Some animals can hear sounds above 20,000 Hz (e.g., dogs hear dog whistles, while we don’t) and other animals probably detect sounds below 20 Hz.
{They absolutely do! Elephants communicate in this "infrasound" low frequency world. It is probable that animals that "predict" earth quakes are actually hearing the low frequency sound waves that precede the earth's motion. MVT}
One theory of hearing is called the place theory. The place theory, as its name implies, holds that we differentiate between different types of sounds due to the location/place on the basilar membrane that the waves vibrate. The theory says that very high sounds might vibrate one area of the basilar membrane and that very low sounds vibrate another area. The theory is true for sounds above 4,000 Hz. Thus, a 5,000 Hz sound causes one area of the basilar membrane to vibrate and a 15,000 Hz sound causes another area to vibrate.
The problem with this theory is that it cannot explain sounds below 4,000 Hz (low sounds). Why? Very low sounds cause the entire membrane to vibrate. An analogy to this might be the situation where you are in your car and sitting at a red light and the person in the car behind you has some kind of loud bass stereo system. Every second or so you hear a low “boooooom” and your entire car vibrates.
Thus, we have our second theory of hearing: the frequency theory. The frequency theory of hearing holds that the frequency with which the basilar membrane is vibrated tells us what particular sound we hear. There is a problem with this theory, however. It does a good job explaining how we tell the difference between sounds of less than 1000 Hz. However, neurons can only fire so fast (about 1000 times per second). Thus, the frequency theory of hearing cannot explain how we tell the difference between sounds that are greater than 1000 Hz.
{We didn't discuss the "Volley Theory" but Dr. Palmer does a nice job of presenting the idea. MVT}
So, two theories can explain how we hear sounds above 4000 Hz and below 1000 Hz. How do we tell the difference between different sounds ranging from 1000 Hz to 4000 Hz? Volley theory is an attempt to explain that. What is a volley? The word doesn’t mean much, aside from volleyball. The word originally meant the simultaneous firing of a lot of arrows and, later, bullets. In other words, when some army several hundred years ago shot their arrows at once (or fired their guns at the same time) it was known as a volley.
Thus, volley theory holds that the sensory neurons on the basilar membrane (the hair cells) fire in groups, as volleys. The particular pattern of firing allows us to tell the difference between, say, 2000 Hz sounds and 3000 Hz sounds.
This information copied from
http://www.psychology.eku.edu/Palmer/200/Hearing.htm
with comments by M. Tulloch, Ph.D. {MVT}.