Issue link: http://digital.copcomm.com/i/757053
10 F A L L 2 0 1 6 C A S Q U A R T E R L Y bones that limits our upper frequency perception to a great extent. That's pretty amazing, isn't it? It gets better. There is a tiny muscle attached between the stapes and the middle ear cavity called the stapedius, which contracts with loud noises to protect your hearing from damage. That's right, after the input transformer, there's a slow-attack limiter in the signal chain. It won't protect you from high-level impulse noises, but it does help with continuous noise levels, and does begin working when you hear an impulse noise. I suspect this muscle figures into ear-fatigue issues, and may partly explain why DJs turn up the music as the night goes on. Also, not in the book, but I have been told that the arrangement of the bones is such that the ear is not very sensitive to vibrations from your jaw, otherwise, you could break your eardrums from chewing. The oval window is the boundary of the inner ear, which is comprised of the cochlea, basilar membrane, and the struc- tures that transduce the pressure changes into voltages for neural transmission and processing. The cochlea is a snail-shaped structure, filled with fluid, and encased in the hardest bone in your body. There is a membrane bisecting this closed column of fluid, the basilar membrane. It extends from the middle ear through nearly the entire length of the cochlea and carries the tiny hair cells and organ of corti along its surface. There are two "windows" in the cochlea: the oval window and the round window. When the stapes pushes against the oval window, there is a pres- sure increase pretty much instantaneously throughout the cochlea. This increase in pressure makes the round window bulge outward as the oval window moves inward. It's kind of like a passive radiator in a speaker enclosure. How this pres- sure wave gets to the round window is the fun part. It takes the path of least resistance. The basilar membrane is narrow and stiff at the basal end, and wide and thin at the far end. The shorter wavelengths pass through the membrane closer to the basal end, and longer waves pass through at the distal end. So there's an increasing gradient of fluid in the column, and a decreasing gradient of stiffness in the membrane. It's not exactly linear, but you can think of each fre- quency exciting a specific part of the basilar membrane and the hair cells specific to that region. It's really a large set of mechanical filters that respond to pitch in a particular way, almost like a Fourier Transform, but not quite. These filters are the best compromise between frequency and time- resolution requirements to deliver maximum information to the brain. Here's a link to check out: AuditoryNeuroscience .com/topics/basilar-membrane-motion-0-frequency -modulated-tone There's another membrane called the reissner's membrane in there. It doesn't carry any significant acoustical proper- ties but it forms an ionic barrier between the fluid in scala media and scala vestibuli. The tectorial membrane appears to be connected to the outer hair cells, which move in response to changing pressure, wiggling the hair cells. The inner hair cells are thought to wave along in the fluid, not connected to the tectorial membrane. Along one side, you will see a structure called the stria vascularis. This brings potassium ions into the scala media from the bloodstream. The fluid in the scala media becomes positively charged to about 80 mV. When the hair cells get deflected, potassium channels open up and current flows. The hair cells also have different lengths (15um-17um), and are connected by tiny protein motors called "tip links" so that they work together in being deflected or not. So each cycle of vibration in the basilar membrane causes a cycle of varying tension in the tip links. Since more tension opens more K+ channels—and because the K+ current is proportional to the number of open channels—the mechani- cal vibrations are translated into corresponding patterns of electrical energy. This is how our ears work as transducers and signal generators for our brains to make sense of the world. What an incredibly impressive system! D������, I��. 25209 Avenue Tibbitts Valencia, CA 91355 Phone (661) 607‐0206 Fax (661) 257‐2236 www.denecke.com Email: info@denecke.com D������, I��. Makers of the original SyncBox ® Introduces the SB‐4! More features and higher stability than before.