Sound wave 'Receptors' - Ear, Microphone
The sound energy transmitted through medium, starting from the vibration of the origin (source), is transmitted to the receptors such as the human ears. As representative receptors, ear and microphone, it will be explained how human ear and microphones receive and recognize sound energy.
1. Human Ear
The human ear is divided into Outer Ear, Middle Ear, and the Inner Ear depending on its structure and role.
As you can see in the picture, the outer ear is composed of pinna, external audibility canal, and the diaphragm of the eardrum. Pinna effectively collects the reached sound and sends it to auditory canal, and the sound through the auditory canal hits the diaphragm of the eardrum and shakes the eardrum. The outer ear also plays a role of estimation of location (Localization) for the source of sound. The sound reaching both pinna is refracted at various angles depending on the degree of bending of the pinna, entering the auditory canal, and the total travel distance to the auditory canal varies depending on the angle of refraction. The degree of intensity of beating the diaphragm of the eardrum varies depending on the refracted angle and the distance of movement of the sound. Both eardrums recognize the nature of the sound source and information of localization based on the information from the sound pressure on the diaphragm.
The middle ear consists of the eardrum and the three ear bones (Stapes, Incres, Maleus), oval window connected to Cochlea of inner ear and the Eustachian tube. The Eustachian tube is connected to the outer ear, which is keeping the air pressure same both in inside and outside of the eardrum constantly. The other three middle ear organs (eardrum, three ear bones, oval windows) are effectively transmitting sound energy delivered to the eardrum to Cochlea filled with liquid material. The sound energy that is transmitted to the eardrum is the energy of the sound wave that moves from the air, and when the sound energy moves inside the liquid Cochlea tube, the impedance of the moleculesincreases (because the density of the liquid molecules is greater than the density of the air molecules), so the loss of the sound energy transmitted increases.In order to prevent the loss of sound energy inside the liquid in Cochlea tube, middle ear increases the pressure of sound energy transmitted to the eardrum and transmitted it into the Cochlear tube. The structure of each organs in the middle ear has evolved to fulfill this function. Firstly, the surface area of the eardrum under pressure from sound energy transmitted from the air is 27 times larger than the surface area of the Oval window connected to the entrance of the inner cochlea. Therefore, the pressure of sound energy per unit surface area in oval window is 27 times larger than that of the eardrum. Every time sound energy reaches the eardrum, the surface of the eardrum is pounding toward the middle ear (as in the woofer movement of the speaker). The three ear bones connected to the eardrum and the oval window use the principle of lever movement in order to effectively convey the pressure of sound energy transmitted to the eardrum onto the surface of the oval window. This structure of the middle ear ultimately increases the pressure of sound energy transmitted to the surface of the eardrum, allowing sound energy to be effectively transmitted to each organ of the inner ear while minimizing the loss of energy inside the liquid-filled Cochlea tube.
The sound waves entering into the Auditory canal are transmitted to the eardrum and moved to the ear bones, converting to mechanical form of sound energy through the vibration of the eardrum and the lever movement of the ear bones. This mechanical form of sound energy is moved into the inside of Cochlea of liquid state, and the fluid current generated by the opening and closing motion of the Oval window and the Round window stimulate the hair cells on the surface of the Cochlea generating electricity which transform the mechanical form of sound energy into the electrical energy form of sound energy. The sound energy powered by this electricity stimulates the auditory nerve fibers connected to the Cochlea and transmits information about the sound energy to the human brain. Eventually, the sound energy reached to the pinna of the outer ear reach the human brain through: the acoustic energy (energy of the sound waves moving in the air) —> the mechanical Energy —> the electric energy —> the neural energy.
Meanwhile, the sound energy entering the liquid inside of Cochlea is bumped against into the membrane of the Cochlea (AKA Reissner’s Membrane, Basilar Membrane) as it moves inside the Cochlea. The frequency of sound energy that the auditory nerve perceives depends on the location of the collision, and if the location is closer to the oval window and round window connected to the middle ear then the auditory nerve perceives higher frequency, and if the location is closer to the deeper inside of inner ear then the auditory nerve perceives lower frequency. The hair cells located on the surface between the membranes of the Cochlea (Reissner’s Membrane, Basilar Membrane) detect the vibration of the sound energy being struck, and convert sound energy into electrical energy to the auditory nerve fibers. Thus, the inner ear is analyzing sound energy and delivering it to the brain through the auditory nerve.
Generally, sound energy transmitted to a microphone stimulates the diaphragm with sound pressure, which is in the microphone connected to the coil of the electromagnet, form a magnetic field, and the magnetic field formed is converted to electrical energy. Thus, a transducer is a device that converts a form of energy (sound energy) into another form of energy (magnetic power, electric energy). The sound energy delivered to the converter, the microphone, is converted into electrical energy, moved to the speaker via cable, and then the speaker converts the electrical energy into the sound energy, and finally, the sound energy transmitted to the microphone can be heard through the speaker.
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