The origin of sound is always some vibrating body. In some cases the vibrations of the source may be very small or very large that it may not be possible to detect them. This type of vibrations is produced by tuning fork, drum, bell, the string of a guitar etc. Human voice originates from the vibrations of the vocal chords and the sound from the musical instruments is due to the vibrations of the air columns. Sound travels in the form of longitudinal wave and it requires a material medium for its propagation.
Wave Interference
When two or more sound waves from different sources are present at the same time, they interact with each other to produce a new wave. The new wave is the sum of all the different waves. Wave interaction is calledinterference. If the compressions and the rarefactions of the two waves line up, they strengthen each other and create a wave with a higher intensity. This type of interference is known as constructive.
Dead spots
Waves can interfere so destructively with one another that they producedead spots, or places where no sound at all can be heard. Dead spots occur when the compressions of one wave line up with the rarefactions from another wave and cancel each other. Engineers who design theaters or auditoriums must take into account sound wave interference. The shape of the building or stage and the materials used to build it are chosen based on interference patterns. They want every member of the audience to hear loud, clear sounds.
Sound Traveling Between Materials
Remember that sound travels faster in some materials than others. Sound waves travel outward in straight lines from their source until something interferes with their path. When sound changes mediums, or enters a different material, it is bent from its original direction. This change in angle of direction is called refraction. Refraction is caused by sound entering the new medium at an angle. Because of the angle, part of the wave enters the new medium first and changes speed. The difference in speeds causes the wave to bend.
Critical Angle
The angle of refraction depends on the angle that the waves has when it enters the new medium. As the angle from the wave to the barrier between the two mediums gets smaller, the angle of refraction also gets closer to the barrier. When the wave’s entering angle reaches a certain point, called thecritical angle, the refraction is parallel to the dividing line between the mediums. The critical angle depends on the two mediums the sound is coming from and going to. The speed of sound is different in every medium. Because of this, even if the sound hits at the same angle, the angle of refraction will vary for different mediums. The greater the difference in speed between the two mediums, the greater the critical angle will be.
If sound hits the new medium with any angle smaller than the critical angle, it will not be able to enter. Instead it will bounce off, or be reflected, from the dividing line. When a wave is reflected, it returns with an angle equal to the one with which it hit. Whenever sound hits a new medium, part of it is reflected back. The rest enters the new medium and is refracted. Imagine sound is traveling through the air and hits the wall of a brick building. Some of the wave is reflected, but much of it enters the brick. The part of the wave going through the brick is now going faster than the part in the air. This is because brick is a solid whose molecules are closer together and can transmit sound more quickly. This difference in speeds caused the wave to bend, or be refracted. Suppose that the wave hits the building with an angle that is smaller than its critical angle. This time, the wave cannot enter the brick and all of it is reflected. If the wave struck the wall with an angle of 15 degrees, it would reflect back with the same angle from the other side. Since there are 180 degrees total, the reflected angle would be 165 degrees, 15 degrees measured from the other direction.
Sound and speed
If you have ever been to a baseball game or sat far away from the stage during a concert, you may have noticed something odd. You saw the batter hit the ball, but did not hear the crack of the impact until a few seconds later. Or, you saw the drummer strike the drum, but it took an extra moment before you heard it. This is because the speed of sound is slower than the speed of light, which we are used to seeing. The same thing is at work during a thunderstorm. Lightning and thunder both happen at the same time. We see the lightning almost instantaneously, but it takes longer to hear the thunder. Based on how much longer it takes to hear thunder tells us how far away the storm is. The longer it takes to hear the thunder, the farther the distance its sound had to travel and the farther away the storm is.
The sound barrier
The speed of sound through warm air at sea level has been measured at 346 meters per second or 0.346 km per second. That is the same as a car traveling about 780 miles per hour! Even most jet airplanes do not travel that fast. When a plane does go faster than speed of sound, it is said to break the sound barrier and a sonic boom is produced. On October 14, 1947, Chuck Yeager did just that. In a small plane called the X-1, he was the first person to fly faster than the speed of sound and the listeners on the ground were the first to hear the loud shock wave of a sonic boom.
Why do we see lightning before the thunder?
The flash of light from lightning travels at about 300,000 kilometers per second or 186,000 miles per second. This is why we see it so much sooner than we hear the thunder. If lightning occurs a kilometer away, the light arrives almost immediately (1/300,000 of a second) but it takes sound nearly 3 seconds to arrive. If you prefer to think in terms of miles, it takes sound nearly 5 seconds to travel 1 mile. Next time you see lightning count the number of seconds before the thunder arrives, then divide this number by 5 to find out how far away the lightning is.
Outer ear
Main article: Outer ear
The folds of cartilage surrounding the ear canal are called the pinna. Sound waves are reflected and attenuated when they hit the pinna, and these changes provide additional information that will help the brain determine the direction from which the sounds came.
The sound waves enter the auditory canal, a deceptively simple tube. The ear canal amplifies sounds that are between 3 and 12 kHz. At the far end of the ear canal is the eardrum (or tympanic membrane), which marks the beginning of the middle ear.
[edit]Middle ear
Middle ear
Main article: Middle ear
Sound waves traveling through the ear canal will hit the tympanic membrane, or eardrum. This wave information travels across the air-filled middle ear cavity via a series of delicate bones: the malleus (hammer), incus (anvil) and stapes (stirrup). These ossicles act as a lever and a teletype, converting the lower-pressure eardrum sound vibrations into higher-pressure sound vibrations at another, smaller membrane called the oval (or elliptical) window. The malleus articulates with the tympanic membrane via the manubrium, where the stapes articulates with the oval window via its footplate. Higher pressure is necessary because the inner ear beyond the oval window contains liquid rather than air. The sound is not amplified uniformly across the ossicular chain. The stapedius reflex of the middle ear muscles helps protect the inner ear from damage. The middle ear still contains the sound information in wave form; it is converted to nerve impulses in the cochlea.
Inner ear
Cochlea
Diagrammatic longitudinal section of the cochlea. Scala media is labeled as ductus cochlearis at right.
Main article: Inner ear
The inner ear consists of the cochlea and several non-auditory structures. The cochlea has three fluid-filled sections, and supports a fluid wave driven by pressure across the basilar membrane separating two of the sections. Strikingly, one section, called the cochlear duct or scala media, contains endolymph, a fluid similar in composition to the intracellular fluid found inside cells. The organ of Corti is located in this duct on the basilar membrane, and transforms mechanical waves to electric signals in neurons. The other two sections are known as the scala tympani and the scala vestibuli; these are located within the bony labyrinth, which is filled with fluid called perilymph, similar in composition to cerebrospinal fluid. The chemical difference between the two fluids (endolymph & perilymph) is important for the function of the inner ear due to electrical potential differences between potassium and calcium ions.
Hair cell
Main article: Hair cell
Hair cells are columnar cells, each with a bundle of 100-200 specialized cilia at the top, for which they are named. There are two types of hair cells. Inner hair cells are the mechanoreceptors for hearing: they transduce the vibration of sound into electrical activity in nerve fibers, which is transmitted to the brain. Outer hair cells are a motor structure. Sound energy causes changes in the shape of these cells, which serves to amplify sound vibrations in a frequency specific manner. Lightly resting atop the longest cilia of the inner hair cells is the tectorial membrane, which moves back and forth with each cycle of sound, tilting the cilia, which is what elicits the hair cells' electrical responses.
Inner hair cells, like the photoreceptor cells of the eye, show a graded response, instead of the spikes typical of other neurons. These graded potentials are not bound by the “all or none” properties of an action potential.
At this point, one may ask how such a wiggle of a hair bundle triggers a difference in membrane potential. The current model is that cilia are attached to one another by “tip links,” structures which link the tips of one cilium to another. Stretching and compressing, the tip links may open an ion channel and produce the receptor potential in the hair cell. Recently it has been shown that cdh23 and pchh15 are the adhesion molecules associated with these tip links. It is thought that a calcium driven motor causes a shortening of these links to regenerate tensions. This regeneration of tension allows for apprehension of prolonged auditory stimulation.
Neurons
Main article: Hair cell neural connection
Afferent neurons innervate cochlear inner hair cells, at synapses where the neurotransmitter glutamate communicates signals from the hair cells to the dendrites of the primary auditory neurons.
There are far fewer inner hair cells in the cochlea than afferent nerve fibers - many auditory nerve fibers are innervated by each hair cell. The neural dendrites belong to neurons of the auditory nerve, which in turn joins the vestibular nerve to form the vestibulocochlear nerve, or cranial nerve number VIII.[1]
Efferent projections from the brain to the cochlea also play a role in the perception of sound, although this is not well understood. Efferent synapses occur on outer hair cells and on afferent (towards the brain) dendrites under inner hair cells
Cochlear nucleus
Main article: Cochlear nucleus
The cochlear nucleus is the first site of the neuronal processing of the newly converted “digital” data from the inner ear (see also binaural fusion). In mammals, this region is anatomically and physiologically split into two regions, the dorsal cochlear nucleus (DCN), and ventral cochlear nucleus (VCN).
[edit]Trapezoid body
Main article: Trapezoid body
The Trapezoid body is a bundle of decussating fibers in the ventral pons that carry information used for binaural computations in the brainstem.
[edit]Superior olivary complex
Main article: Superior olivary complex
The superior olivary complex is located in the pons, and receives projections predominantly from the ventral cochlear nucleus, although the posterior cochlear nucleus projects there as well, via the ventral acoustic stria. Within the superior olivary complex lies the lateral superior olive (LSO) and the medial superior olive (MSO). The former is important in detecting interaural level differences while the latter is important in distinguishing interaural time difference.
Lateral lemniscus in red, as it connects the cochlear nucleus, superior olivary nucleus and the inferior colliculus. Seen from behind.
[edit]Lateral lemniscus
Main article: Lateral lemniscus
The lateral lemniscus is a tract of axons in the brainstem that carries information about sound from the cochlear nucleus to various brainstem nuclei and ultimately the contralateral inferior colliculus of the midbrain.
[edit]Inferior colliculi
Main article: Inferior colliculus
The IC are located just below the visual processing centers known as the superior colliculi. The central nucleus of the IC is a nearly obligatory relay in the ascending auditory system, and most likely acts to integrate information (specifically regarding sound source localization from the superior olivary complex and dorsal cochlear nucleus) before sending it to the thalamus and cortex.
[edit]Medial geniculate nucleus
Main article: Medial geniculate nucleus
The medial geniculate nucleus is part of the thalamic relay system.
[edit]Primary auditory cortex
Main article: Primary auditory cortex
The primary auditory cortex is the first region of cerebral cortex to receive auditory input.
Perception of sound is associated with the left posterior superior temporal gyrus (STG). The superior temporal gyrus contains several important structures of the brain, including Brodmann areas 41 and 42, marking the location of the primary auditory cortex, the cortical region responsible for the sensation of basic characteristics of sound such as pitch and rhythm. We know from work in nonhuman primates that primary auditory cortex can probably itself be divided further into functionally differentiable subregions.
Primary auditory cortex is surrounded by secondary auditory cortex, and interconnects with it. These secondary areas interconnect with further processing areas in the superior temporal gyrus, in the dorsal bank of the superior temporal sulcus, and in the frontal lobe. In humans, connections of these regions with the middle temporal gyrus are probably important for speech perception. The frontotemporal system underlying auditory perception allows us to distinguish sounds as speech, music, or noise.
