Man,
ears are cool. Face it,
microphones are neat, but they
suck compared to ears. If you ask me, ears are even cooler than
eyes, even if they are less studied. Your ears have the job of taking
sound and turning it into neural impulses (
action potentials), so the brain can use the information. This isn't an easy job. Natural sounds are very
complex. Also, sounds aren't like the
visual field. In vision, inputs enter the
retina with spatial separation. Your ear doesn't have that advantage. Sound inputs all come into the ear piled up on top of one another. Nonetheless, you can distinguish individual sounds, even if they occur simultaneously. Start winamp. I'll wait. Play something. See, you can differentiate between the sounds of the individual
instruments. But they don't come into your ear seperately. Now play something in
stereo. You can tell what direction the sounds are coming from. Now, a lot of this has to do with neural processing, but the ear is the first step. It provides the information to the brain in a format that allows the brain to extract all this information from two inputs.
So, how do ears do what they do? Well,
form follows function, so, let's start with the form of the ear. We'll work our way in from the
outer ear, and I'll talk about form as we go. Okay, the outer ear is the part of the ear that you see. The major parts are the
pinna and the
tragus. The pinna is the big flesy part, shaped kinda like a
radar dish. One of it's major functions is sound collection. It basically serves as a reflector, either reflecting sound directly into the ear canal, or off of the tragus and into the ear canal. Notice that your pinna is much bigger than your ear canal (duh). This is important. The sound
energy from the pinna, a large area is reflected into a smaller area. Basically, this amounts to
amplification of sound, which is vital, because the ear has to take sound from a gaseous medium and transfer it to a liquid medium. This isn't easy. If you don't belive me,
go stick your head in the toilet. Make sure your ears are submerged, and try to hear winamp playing. You can't, because most of the sound that hits the water from the air is reflected off of it's surface. Before we move on, dry off your ears and notice the
complex shape of the pinna. This shape affects how different frequencies enter your ear. It changes the
frequency spectrum of sound as it moves from the outside world into your ear. This change is non-trivial (See
Auditory Localization for some details). Try doing someting to
malform your pinna. It should be something which is quickly
reversible, such as pushing them out from your head. You'll notice that things sound slightly different when you do this. Try it a couple of times. If you walk around like this for a while, it will probably fuck with your ability to localize sounds. I would expect you would eventually adapt though.
Plasticity rules.
Okay, now we move through the ear canal and into the
middle ear. At the end of the ear canal is the
tympanic membrane, commonly known as the
eardrum. When sound hits the eardrum, it vibrates. The eardrum is connected to a series of small bones, the
malleus,
incus, and
stapes. These bones are important in amplifying the force of the tympanic membrane. They work like levers and pistons. Enery is transferred from the eardrum to the malleus, to the incus, and lastly to the stapes. The output portion of the stapes is much smaller than the eardrum. Thus, we have amplification once again. The stapes then transfers the energy to the
oval window of the
cochlea. The cochlea is a fluid filled cavity in the
inner ear. It is the coolest part of the ear, so much so that it is worthy of it's own
node. Not only that, but the writups there are flat out wrong, so I've got to correct them.
Back to
How your brain works.
Onward, into the fluidy recesses of the
cochlea.