ENGINEERING Improving Hearing Loss Humans Adapting Echolocation JenniFer Jaco ‘13 n estimated 245 million people world-wide are visu- transmitting sound waves from the tympanic membrane ally impaired, and 39 million more are completely to the oval window, the middle ear functions as an acous- Ablind. Sixty-five percent of these people are over the tic transformer, amplifying the sound waves before they age of fifty (1). Fifty is becoming the new thirty in many move into the inner ear. “The pressure of the sound waves countries, as people are starting to live much longer. Despite on the oval window is about twenty times greater than that advances in technology, after blindness has set in, there on the eardrum. This pressure increases due to the differ- is nothing that can be done to reverse the damage. At this ence in size between the relatively large surface of the ear- time, particularly in the fifty and older group, people often drum and the smaller surface of the oval window” (6). From begin to feel helpless and separated from society. They no the oval window, sounds are then transmitted to the inner longer feel “normal” without their innate ability to see. This ear, which contains the cochlea (7). The cochlea is coiled problem can theoretically be solved by echolocation, defined two and a half times around a hollow central pillar, which as the ability to “hear the locations and properties of silent contains the cochlear artery, vein, and nerve. Inside the spi- objects by noticing how sound reflects off them” (2). Bats, rals of the cochlea is the Organ of Corti, which contains a dolphins, and toothed whales all use echolocation to “see” fluid, called perilymph, as well as tiny hair cells that respond objects around them while they are in motion by using ex- to sound vibrations. There are about 24,000 of these hairs tremely high frequency sound waves outside of the human embedded in the cochlea, arranged in four long rows. The hearing range. With a device that allowed the human ear to movements in the perilymph cause different hair cells to be use the principals of echolocation, people who were visu- put into motion. When the hair cells move, they send electri- ally impaired would no longer need to use a cane to detect cal signals to the auditory nerve, which is connected to the objects around them, thus preventing injuries and hassle. temporal lobe of the brain. In the brain, the electrical im- More advanced ear technology that incorporates echolo- pulses are translated into sounds that we can comprehend cation would help improve the lives of millions of people (8). As such, these hair cells are essential to hearing ability. worldwide. Rapidly changing technology allows us the abil- Damage to the hair cells limits the capacity of the hu- ity to improve lives around the world—why not push it even man ear to detect sound. Higher frequencies are harder further to revolutionize the way we perceive our world? to hear as age increases as a result of an age-related dete- rioration process called presbycusis (9). Most human ears can hear frequencies between 20 Hz and 20 kHz (10). The Anatomy and Physiology of the Ear highest frequencies that a middle aged human can hear are The ear is divided into three portions that focus and around 12 kHz to 14 kHz, a threshold which only decreases process sound: the external ear, the middle ear, and the in- with age. Other limitations of the human ear include deaf- ner ear (3). Sound funnels through the cartilege-covered ex- ness or severe hearing impairment (11). There are four main ternal ear into the external auditory canal, a short tube that categories of hearing impairment: conductive hearing loss, ends at the tympanic membrane, commonly known as the sensorineural hearing loss, mixed hearing loss, and central ear drum (4). Encyclopedia Britannica explains that the hearing loss. Conductive hearing loss is usually caused by middle ear is “a narrow, air-filled space that resembles a rectangular room with four walls, a floor, and a ceiling, and is made up of three bones [called the ossicles]: the malleus, incus, and stapes. The lateral wall of the middle-ear space is formed by the tympanic membrane, the inferior wall is a thin bone separating the cavity from the jugular vein and carotid artery below, the anterior wall is the opening of the Eusta- chian tube, which equalizes pressure between the external and middle ear, and the medial wall is a part of the bony otic capsule of the inner ear” (5). The medial wall has two open- ings: the oval window, closed by the stapes, and the round window, which is covered by a thin membrane (5). When sound travels through the external au- ditory canal, it vibrates the tympanic membrane. “It is the pressure from sound waves that makes the ear- drum vibrate” (6). A vibration travels from the tympanic membrane through the malleus, is transferred through to Image retrieved from http://www.britannica.com/EBchecked/topic/175622/human-ear (Accessed 2 November 2011). the incus and stapes, and finally the oval window (6). By Fig. 1: Structure of the ear. 36 DARTMOUTH UNDERGRADUATE JOURNAL OF SCIENCE either a disease such as measles or an obstruction of the ex- quency tones, so the latter can be assumed to be nearby (15). ternal ear. In this case, hearing can either be repaired surgi- cally or it can be successfully compensated with a hearing aid. Sensorineural hearing loss, by comparison, is more pro- Echolocation in the Animal Kingdom found than conductive hearing loss; it is caused by damage In the animal kingdom, several animals use echolo- to the hair cells or nerves in the inner ear. Generally, only the cation to compensate for their poor vision. While some loss of certain frequencies occurs, meaning a person can still animals’ visions may be inferior to that of humans, they hear some sounds. Mixed hearing loss is a combination of have managed to overcome the sound localization and conductive hearing loss and sensorineural hearing loss. This frequency ranges inadequacies that humans possess. By means a person suffers from problems in both the external using sonar and echolocation, the animal kingdom has and middle ear, as well as the inner ear (12). Lastly, in central found a solution to blindness before the medical world has. hearing loss, nerves leading to the central nervous system are Bats have the ability to hear much higher frequencies damaged. Of the four, sensorineural hearing loss is the most than the human ear can hear. Compared to the human’s common form of permanent hearing loss. This setback can range of 20 Hz to 20 kHz, a bat can hear anywhere between be caused by head trauma, illnesses, or malformation of the 1 kHz to 150 kHz, a range nearly eight times as wide (16). inner ear, as well as more common events, such as repeated While bats can detect lower frequencies, they generally ig- exposure to blaringly loud music at concerts or consistent- nore them, as the lower frequency sounds are of no use to ly listening to an iPods with the earbuds on full blast (13). echolocation (17). In order for echolocation to work, bats Another shortcoming of the human ear is the lack of emit a high-pitched, rhythmic sound from their larynx, sound localization. If a sound arrives a few microseconds ear- which reflects off objects and returns to the bat’s ears. When lier in one ear than the other, the sound is recognized to be the sound wave returns, the bat can determine where the coming from the side that hears it first. In general, this prin- object is. A bat’s brain combines normal auditory functions, ciple works better for lower frequency sounds (14). Sound a stopwatch to determine how quickly the sound returns— localization works best when both ears are working optimal- which indicates how far away the object is—and a calcula- ly. With only one functioning ear, it is difficult to determine tor to quickly compute these figures (18). Humans are able the source of a sound. Human ears also have a problem de- to locate which side sounds are on based on which ear the termining how far away a sound source is. The only tool the sound waves hit first; this is similar to how bats can deter- human ear can depend on is loudness and relative estima- mine the location of an object. If an echo hits the bat’s right tion. Low-frequency tones propagate farther than high fre- ear before the left ear, the object must be on the right. How- ever, bats have special folds in their ears that allow them to determine an object’s vertical position. Echoes that reach a bat’s ear from below will hit the fold differently than if the echoes come from above, producing a different sound (18). A bat’s ear can also determine the size of an object by the intensity of the echo—a smaller object reflects a smaller sound wave. Also, like humans, bats can sense how close an object is by the echo’s pitch; a closer object’s echo will have a higher pitch than an object moving further away (18). Like bats, dolphins and toothed whales use echolocation to locate objects when vision is obscured, such as when they are swimming in deep or murky waters. These animals have a frequency range from about 50 kHz to 200 kHz, a maxi- mum even greater than that of bats.