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C A S Q U A R T E R L Y F A L L 2 0 1 6 9 F or this installment of my scattered book report on the very dense and rich Auditory Neuroscience (Schnupp, Nelken & King, MIT Press), I'll be moving from the phenomenon of sound production— represented as masses and springs, resonances and creating air pressure differential—to the mechanism for perceiving these minute pressure changes as sound, the anatomy, and function of the ear. Now, most of this will be familiar to some readers, and some of it will be familiar to most, but there are surprises along the way that will help you understand acoustics, sound gen- eration, and your perception of received sounds. The pinna is the outermost part of the ear, the flap of cartilage and skin that we commonly point to when we talk about ears. The book doesn't even mention this basic structure, but we all know it helps to direct sound into the ear canal and add a front-facing, more-or-less, unidirectional pattern to our hearing. Next is the ear canal itself. You might be surprised to learn it's about an inch long, con- tains a bend (to prevent you from poking a stick directly into your eardrum), and the wax produced in the ear canal helps protect the skin of the ear canal and is somewhat antifungal and antibacterial. The ear canal is a tuned port that amplifies sound in the 2kHz-4kHz range. These are, coincidentally, critical frequency bands for hearing human speech. This is prob- ably why we hear these frequencies best. Our soundwave finally comes to the tympa- num, or eardrum, which is a thin membrane that acts just like a drumhead—or another mass-and-spring system if you want to reduce it to physics. This is the outer boundary of the middle ear. Attached to the eardrum is the first of three tiny bones which mechanically transfer the motion of the eardrum to the cochlea, or the inner ear, where all the real action takes place. "You might wonder, if the sound has already traveled a potentially quite large distance from a sound source to the eardrum, why would it need a chain of little bones to be transmitted to the cochlea? Could it not travel the last centimeter traveling through the air-filled space of the middle ear just as it has covered all the previous distance?" (page 51). I'm glad you asked! The three bones, the malleus, incus, and stapes, physi- cally connect the eardrum to the oval window in the cochlea. Okay, sure. Well, it turns out that the cochlea is filled with lymph-like fluid, which is much denser than air, so it has a different impedance than air. Those three little bones are a mechanical impedance-matching transformer, ensuring maximum power transfer from one medium to the other. It is also the mass of these tiny Part 2: Anatomy b y G . J o h n G a r r e t t C A S Exploring the book: Auditory Neuroscience: Making Sense of Sound