0:00:06.769,0:00:09.708 You hear the gentle lap of waves, 0:00:09.708,0:00:11.913 the distant cawing of a seagull. 0:00:11.913,0:00:15.820 But then an annoying whine[br]interrupts the peace, 0:00:15.820,0:00:19.459 getting closer, and closer, and closer. 0:00:19.459,0:00:21.568 Until...whack! 0:00:21.568,0:00:26.617 You dispatch the offending mosquito, [br]and calm is restored. 0:00:26.617,0:00:31.730 How did you detect that noise from afar[br]and target its maker with such precision? 0:00:31.730,0:00:35.466 The ability to recognize sounds [br]and identify their location 0:00:35.466,0:00:38.518 is possible thanks to the auditory system. 0:00:38.518,0:00:43.160 That’s comprised of two main parts: [br]the ear and the brain. 0:00:43.160,0:00:47.374 The ear’s task is to convert sound energy[br]into neural signals; 0:00:47.374,0:00:52.079 the brain’s is to receive and process [br]the information those signals contain. 0:00:52.079,0:00:53.898 To understand how that works, 0:00:53.898,0:00:57.547 we can follow a sound [br]on its journey into the ear. 0:00:57.547,0:00:59.621 The source of a sound creates vibrations 0:00:59.621,0:01:03.323 that travel as waves of pressure [br]through particles in air, 0:01:03.323,0:01:04.221 liquids, 0:01:04.221,0:01:05.725 or solids. 0:01:05.725,0:01:07.986 But our inner ear, called the cochlea, 0:01:07.986,0:01:11.966 is actually filled [br]with saltwater-like fluids. 0:01:11.966,0:01:15.852 So, the first problem to solve [br]is how to convert those sound waves, 0:01:15.852,0:01:17.532 wherever they’re coming from, 0:01:17.532,0:01:20.249 into waves in the fluid. 0:01:20.249,0:01:23.833 The solution is the eardrum, [br]or tympanic membrane, 0:01:23.833,0:01:27.230 and the tiny bones of the middle ear. 0:01:27.230,0:01:30.170 Those convert the large movements [br]of the eardrum 0:01:30.170,0:01:33.928 into pressure waves [br]in the fluid of the cochlea. 0:01:33.928,0:01:35.986 When sound enters the ear canal, 0:01:35.986,0:01:40.013 it hits the eardrum and makes it vibrate [br]like the head of a drum. 0:01:40.013,0:01:43.939 The vibrating eardrum jerks a bone [br]called the hammer, 0:01:43.939,0:01:48.677 which hits the anvil and[br]moves the third bone called the stapes. 0:01:48.677,0:01:53.042 Its motion pushes the fluid [br]within the long chambers of the cochlea. 0:01:53.042,0:01:54.389 Once there, 0:01:54.389,0:01:59.179 the sound vibrations have finally[br]been converted into vibrations of a fluid, 0:01:59.179,0:02:03.204 and they travel like a wave [br]from one end of the cochlea to the other. 0:02:03.204,0:02:07.793 A surface called the basilar membrane [br]runs the length of the cochlea. 0:02:07.793,0:02:11.803 It’s lined with hair cells that have [br]specialized components 0:02:11.803,0:02:13.536 called stereocilia, 0:02:13.536,0:02:17.936 which move with the vibrations of the[br]cochlear fluid and the basilar membrane. 0:02:17.936,0:02:22.265 This movement triggers a signal [br]that travels through the hair cell, 0:02:22.265,0:02:24.154 into the auditory nerve, 0:02:24.154,0:02:28.301 then onward to the brain, [br]which interprets it as a specific sound. 0:02:28.301,0:02:31.720 When a sound makes [br]the basilar membrane vibrate, 0:02:31.720,0:02:34.369 not every hair cell moves - 0:02:34.369,0:02:39.244 only selected ones, [br]depending on the frequency of the sound. 0:02:39.244,0:02:41.715 This comes down to some fine engineering. 0:02:41.715,0:02:45.438 At one end, [br]the basilar membrane is stiff, 0:02:45.438,0:02:50.926 vibrating only in response to short[br]wavelength, high-frequency sounds. 0:02:50.926,0:02:52.745 The other is more flexible, 0:02:52.745,0:02:57.513 vibrating only in the presence of longer[br]wavelength, low-frequency sounds. 0:02:57.513,0:03:00.461 So, the noises made by the seagull [br]and mosquito 0:03:00.461,0:03:03.537 vibrate different locations [br]on the basilar membrane, 0:03:03.537,0:03:06.751 like playing different keys on a piano. 0:03:06.751,0:03:08.663 But that’s not all that’s going on. 0:03:08.663,0:03:12.639 The brain still has another [br]important task to fulfill: 0:03:12.639,0:03:15.576 identifying where a sound is coming from. 0:03:15.576,0:03:19.613 For that, it compares the sounds [br]coming into the two ears 0:03:19.613,0:03:22.126 to locate the source in space. 0:03:22.126,0:03:26.950 A sound from directly in front of you will[br]reach both your ears at the same time. 0:03:26.950,0:03:30.744 You’ll also hear it at the same intensity [br]in each ear. 0:03:30.744,0:03:34.305 However, a low-frequency sound [br]coming from one side 0:03:34.305,0:03:38.847 will reach the near ear microseconds [br]before the far one. 0:03:38.847,0:03:42.775 And high-frequency sounds will sound[br]more intense to the near ear 0:03:42.775,0:03:46.010 because they’re blocked [br]from the far ear by your head. 0:03:46.010,0:03:49.765 These strands of information [br]reach special parts of the brainstem 0:03:49.765,0:03:54.124 that analyze time and[br]intensity differences between your ears. 0:03:54.124,0:03:58.747 They send the results of their [br]analysis up to the auditory cortex. 0:03:58.747,0:04:01.733 Now, the brain has [br]all the information it needs: 0:04:01.733,0:04:04.539 the patterns of activity [br]that tell us what the sound is, 0:04:04.539,0:04:08.433 and information about [br]where it is in space. 0:04:08.433,0:04:10.604 Not everyone has normal hearing. 0:04:10.604,0:04:15.049 Hearing loss is the third most common [br]chronic disease in the world. 0:04:15.049,0:04:19.115 Exposure to loud noises [br]and some drugs can kill hair cells, 0:04:19.115,0:04:23.012 preventing signals from traveling [br]from the ear to the brain. 0:04:23.012,0:04:27.671 Diseases like osteosclerosis freeze [br]the tiny bones in the ear 0:04:27.671,0:04:29.841 so they no longer vibrate. 0:04:29.841,0:04:31.305 And with tinnitus, 0:04:31.305,0:04:32.964 the brain does strange things 0:04:32.964,0:04:36.672 to make us think there’s a sound [br]when there isn’t one. 0:04:36.672,0:04:38.208 But when it does work, 0:04:38.208,0:04:40.970 our hearing is an incredible, [br]elegant system. 0:04:40.970,0:04:44.723 Our ears enclose a fine-tuned piece [br]of biological machinery 0:04:44.723,0:04:48.397 that converts the cacophony of vibrations [br]in the air around us 0:04:48.397,0:04:51.537 into precisely tuned electrical impulses 0:04:51.537,0:04:56.299 that distinguish claps, taps, [br]sighs, and flies.