The Middle Ear: the ear (auditory) canal has two main functions: it provides a passageway for sound traveling from the pinna to the eardrum, and it protects the ear from infection. On average, the canal is 3.5 cm long and .7 cm wide. In order to protect the ear, the auditory canal has several lines of defense. The part of the tube closest to the pinna is full of nerve endings and is covered with soft, sensitive skin. Old skin is pushed out when new skin grows, naturally cleaning the ear canal. Further in, the canal contains small hairs that filter out debris. The final line of defense protecting the ear is a group of wax fibers called cerumen strands. These strands are electrostatically charged, allowing them to catch small dust particles. The only thing that is able to successfully pass through the auditory canal is the sound itself, which then continues to the eardrum. The ossicles are three tiny bones (the smallest in the human body) called the malleus, incus, and stapes, or alternately the hammer, anvil, and stirrup due to their shapes. These bones transfer the vibrations of the eardrum to the inner ear. Malleus (Hammer): the malleus is attached to the eardrum, and is the first bone in the chain to transfer vibrations from the eardrum to the inner ear. Incus (Anvil): the incus transfers the vibrations of the malleus to the stapes, and is the second bone in the chain to transfer vibrations from the eardrum to the inner ear. Stapes (Stirrup): the stapes transfers the vibrations of the incus to the oval window, a portion of the inner ear to which it is connected. It is the final bone in the chain to transfer vibrations from the eardrum to the inner ear.

The Inner Ear: Semicircular Canals: the semicircular canals help maintain balance, but have no function in auditory perception. They contain fluid and specialized hair cells. When the fluid moves, the hair cells send a message to the brain that the head has changed position. Cochlea: the cochlea converts physical vibrations into electrical impulses. In its natural shape, the cochlea is coiled up, but when unraveled it measures 3.5 cm (about 2 inches). Inside the cochlea is the basilar membrane, upon which form standing waves caused by the vibration of the fluid inside the cochlea. Over two million tiny hairs pick up the movement of the basilar membrane and cochlear fluid and then convert the signals into electrical impulses. Auditory Nerve: the auditory nerve is a pathway that allows the neurel signals produced by the stereocilia in the cochlea to travel to the brain. Once in the brain the signals from both ears are combined, processed, and interpreted to produce the sensation that we call hearing.

The intensity level of sound is measured in decibels (dB). Decibels units are based in increments of 10. This means that a sound with an intensity of 20 dB is ten times as loud as one with an intensity of 10 dB, 30 dB is ten times as intense as 20 dB, and so on. So, while a 20 dB sound is ten times as intense as a 10 dB sound, we perceive it as only twice as loud. The hearing threshold (level at which humans begin to perceive sound) is 0 dB. When a sound reaches upwards of 120 dB, it is above the threshold of pain (point at which most people begin feeling pain). Everything in between can be heard by a human with normal hearing.

Musical instruments: are set into vibrational motion at their natural frequency when a person hits, strikes, strums, plucks or somehow disturbs the object. Each natural frequency of the object is associated with one of the many standing wave patterns by which that object could vibrate. The natural frequencies of a musical instruments are sometimes referred to as the harmonics of the instrument. An instrument can be forced into vibrating at one of its harmonics (with one of its standing wave patterns) if another interconnected object pushes it with one of those frequencies. This is known as resonance - when one object vibrating at the same natural frequency of a second object forces that second object into vibrational motion. The word resonance comes from Latin and means to "resound" - to sound out together with a loud sound. Resonance is a common cause of sound production in musical instruments. Musical instruments produce their selected sounds in the same manner. Brass instruments typically consist of a mouthpiece attached to a long tube filled with air. The tube is often curled in order to reduce the size of the instrument. The metal tube merely serves as a container for a column of air; it is the vibrations of this column which produces the sounds which we hear. The length of the vibrating air column inside the tube can be adjusted either by sliding the tube to increase and decrease its length or by opening and closing holes located along the tube in order to control where the air enters and exits the tube. Brass instruments involve the blowing of air into a mouthpiece. The vibrations of the lips against the mouthpiece produce a range of frequencies. One of the frequencies in the range of frequencies matches one of the natural frequencies of the air column inside of the brass instrument. This forces the air inside of the column into resonance vibrations. And always, the result of resonace is a big vibration - that is, a loud sound. Woodwind instruments operate in a similar manner. Only, the source of vibrations is not the lips of the musician against a mouthpiece, but rather the vibration of a reed or wooden strip.

Pitch and the Doppler Effect: how the brain interprets the frequency of an emitted sound is called the pitch. We already know that the number of sound waves passing a point per second is the frequency. The faster the vibrations the emitted sound makes (or the higher the frequency), the higher the pitch. Therefore, when the frequency is low, the sound is lower. When an ambulance speeds towards you, sirens blazing, the sound you hear is rather high in pitch. This is because the sound waves in front of the vehicle are being squashed together by the moving ambulance. This causes more vibrations to reach your ear per second. As you know, more vibrations per second results in a higher pitched sound. When the ambulance passes you, the sound becomes lower in pitch. Behind the ambulance there are fewer vibrations per second, and a lower sound is heard. This change in pitch is known as the Doppler Effect. Look at the diagram to the right and see if you can interpret the pitch of the sound, based upon wavelength and frequency. When a vehicle travels faster than the speed of sound, about 330 meters per second (750 mph), a sonic boom can be heard. As the vehicle overtakes its own sound, the sound waves spread out behind in a shockwave, or sonic boom.

Ultrasonic Waves: humans can normally hear sound frequencies between 20 and 20,000 Hz (20kHz). When a sound wave's frequency lies above 20 kHz, it is called an ultrasonic wave. While we cannot hear ultrasonic waves, we apply them in various technologies such as sonar systems, sonograms, surgical tools, and cleaning sytems. Some animals also use ultrasonic waves in a specialized technique called echolocation that alows them to pinpoint objects and other animals, even in the dark.

Sonar: sonar stands for SOund NAvigation Ranging. Sonar is used in navigation, forecasting weather, and for tracking aircraft, ships, submarines, and missiles. Sonar devices work by bouncing sound waves off objects to determine their location. A sonar unit consists of an ultrasonic transmitter and a receiver. On boats, the receiver is mounted on the bottom of the ship. To measure water depth, for instance, the transmitter sends out a short pulse of sound, and later, the receiver picks up the reflected sound. The water depth is determined from the time elapsed between the emission of the ultrasonic sound and the reception of its reflection off the sea-floor. In the diagram, a ship sends out ultrasonic waves (green) in order to detect schools of fish swimming beneath. The waves reflect off the fish (white), and return to the ship where they are detected and the depth of the fish is determined.

Echolocation: In 1944, Donald R. Griffin coined the term echolocation. Echolocation is the use of echoes of sound produced by certain animals to detect obstacles and food. Animals that live where lighting is unpredictable use echolocation. Some of these animals are bats, porpoises, some kinds of whales, several species of birds, and some shrews. The first step in echolocation is emitting a sound. High-frequency sounds provide better resolution of targets than lower-frequency sounds. Not every animal uses ultrasonic sounds in echolocation, but they are more effective. Still, sounds used in echolocation can be produced in the voice box, the mouth, or some other part of the head. Then, a highly refined auditory system detects the returning echoes (the sounds that bounced of the object). In order for echolocation to work, the outgoing pulses of sound need to register in the organism's brain, so it can be compared to its echo. Using echolocation, some animals can effectively catch prey and "see" in the dark.

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