Click here to purchase the entire book in PDF format. How your ears work We have seen that sound is simply a change in air pressure over time, but we have not yet given much thought to how that sound is received and perceived by a human being. The changes in air pressure arrive at the side of your head at what we normally call your ``ear.'' That wrinkly flap of cartilage and skin on the side of your head is more precisely called your pinna (plural is pinnae). The pinna performs a very important role in that it causes the sound wave to reflect differently for sounds coming from different directions. We'll talk more about this later. The sound wave makes its way down the ear canal, a short tube that starts at the pinna and ends at your tympanic membrane, better known as your eardrum. The eardrum is a thin piece of tissue, about 10 mm in diameter, and is shaped a bit like a cone pointing towards inside your head. The eardrum moves back and forth depending on the relative pressures on either side of it. The pressure on the inside of the eardrum cannot change rapidly because your head is relatively sealed, somewhat like an omnidirectional microphone. So, if the pressure wave in the ear canal is high, then the eardrum is pushed into your head. If the pressure wave is low, then the eardrum is pulled out of your head. Figure 5.1: The structure of the outer ear. 1. Pinna, 2. Bone, 3, Tympanic membrane (eardrum) 4. Ear canal Just like in the case of an omnidirectional microphone, there has to be some way of equalizing the pressure on the inside of the eardrum so that large changes in pressure over time (caused by changes in weather or altitude) don't cause the it to get pushed too far in or out (this would hurt...). To equalize the pressure, you have to have a hole that connects the outside world to the inside of your head. This hole is a connection between the back of your mouth and the inside of your ear called the eustachian tube. When you undergo large changes in barometric pressure (like when you're sitting in an airplane that's taking off or landing) you open up your eustachian tube (by yawning) to relieve the pressure difference between the two sides of the eardrum. We said above that the eardrum is cone-shaped. This is because it's constantly being pulled inwards by the tensor tympani muscle which keeps it taut. The tension on the eardrum is regulated by that muscle - if a very loud sound hits the eardrum, the muscle tightens to make the eardrum more rigid, preventing it from moving as much, and therefore making the ear less sensitive to protect itself. This is also true when you shout. Just on the inside of the eardrum are three small bones called the ossicles. The first of these bones, called the malleus (sometimes called the hammer) is connected to the eardrum and is pushed back and forth when the eardrum moves. This, in turn, causes the second bone, the incus (or anvil) to move which, in turn pushes and pulls the third bone, the stapes (or stirrup). Figure 5.2: The structure of the middle ear. 1. Cochlea, 2. Eustachian Tube, 3. Bone, 4. Tympanic membrane (eardrum), 5. Malleus, 6. Incus, 7. Stapes, 8. Oval window, 9. Semi-circular canals The stapes is a piston that pushes and pulls on a small piece of tissue called the fenestra ovalis (or oval window) which is a membrane that separates the middle ear from something called the cochlea. Looking at Figure 5.2, you'll see that the cochlea looks a bit like a snail from the outside. On the inside, shown in Figure 5.3, it consists of three adjacent tubes called the perilymphatic ducts called the scala vestibuli, the scala media, and the scala tympani. Separating the scala tympani from the other two is a an important little piece of tissue called the basilar membrane. Figure 5.3: A cross section of the interior of the cochlea. 1. Basilar membrane, 2. Tectorial membrane, 3. Scala vestibuli, 4. Scala media, 5. Scala tympani. When the oval window vibrates back and forth, it causes a pressure wave to travel in the fluid inside the cochlea (called the perilymph in the scala vestibuli and the scala tympani and the endolymph in the scala media), and down the length of the basilar membrane. Sitting on the basilar membrane are something like 30,000 very short hairs. At the end of the basilar membrane near the oval window, the hairs are shorter and stiffer than they are at the opposite end. These hairs can be considered to be tuned resonators: a pressure wave inside the cochlea at given frequency will cause specific hairs on the basilar membrane to vibrate. Different frequencies cause different hairs to resonate. Just to give you an idea of how much these hairs are vibrating, if you're listening to a sine wave at 1 kHz at the threshold of hearing, $20x10^{-6}$ Pa, the excursion of the hairs as they vibrate back and forth is much less than the diameter of a hydrogen atom. This is not very far... Figure 5.4: A simplified model of the ear. 1. Ear canal, 2. Tympanic membrane (eardrum), 3. Malleus, 4. Incus, 5. Stapes, 6. Oval window, 7. Cochlea (uncoiled for simplification), 8. Basilar membrane, 9. Round window, 10. Eustacian Tube To prevent standing waves inside the fluid in the cochlea, there is a second tissue called the fenestra rotunda (or round window) at the end of the scala tympani which, like the oval window, separates the middle air from the cochlea. The round window dissipates excess energy, preventing things from getting too loud inside the cochlea. Figure 5.5: A cross section of the interior of the cochlea. 1. Inner hair cells, 2. Tectorial membrane, 3. Outer hair cells, 4. Basilar membrane, 5. Supporting cells. When the hair cells vibrate back and forth, they generate electrical impulses that are sent through the cochlear nerve to the brain. The brian decodes the pitch or frequency of the sound by determining which hair cells are moving at which location on the basilar membrane. The level of the sound is determined by how many hair cells are moving.