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National research Council (US) Committee on disability Determination for people with hear Impairments; Dobie RA, van Hemel S, editors. Listening Loss: identify Eligibility for Social security Benefits. Washington (DC): national Academies press (US); 2004.


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National research study Council (US) Committee on special needs Determination for individuals with listening Impairments; Dobie RA, van Hemel S, editors.

In this chapter we review basic information about sound and about how the human auditory mechanism performs the procedure called hearing. We define some an essential auditory functions that human beings perform in their day-to-day lives, and also some environmental variables that may complicate the listening task. We likewise discuss the types of hearing ns or disorder that deserve to occur and their causes.


INTRODUCTION come SOUND1

Hearing enables one come identify and recognize objects in the world based upon the sound lock produce, and hearing makes interaction using sound possible. Sound is obtained from objects the vibrate developing pressure variations in a sound-transmitting medium, such together air. A push wave is propagated external from the vibrating source. When the press wave encounters another object, the vibration deserve to be imparted to that object and the push wave will propagate in the tool of the object. The sound tide may additionally be reflected from the thing or it may diffract approximately the object. Thus, a sound wave propagating outside from a vibrating object deserve to reach the eardrum of a listener leading to the eardrum to vibrate and also initiate the process of hearing.

Sound waves deserve to be mathematically defined in two ways, that is, in 2 domains. In the time domain, sound is explained as a succession of pressure transforms (oscillations) that take place over time. In other words, the time-domain description of a sound tide specifies just how the sound press increases and also decreases end time. In the frequency domain, the spectrum specifies sound in terms of the tonal components that comprise the sound. A tonal sound has a time-domain summary in i m sorry sound pressure changes as a continual (sinusoidal) duty of time. If one knows the tonal components of sound as characterized in the frequency domain, one deserve to calculate the time-domain description of the sound. Utilizing the exact same analytic tools, the frequency domain representation of a sound can additionally be calculated indigenous the time-domain description. Thus, the time and frequency domain descriptions of sound room two different ways of measure the exact same thing (i.e., the time and also frequency domains are sensible equivalents). Thus, one can explain sound as temporal fluctuations in pressure, or one can define sounds in terms of the frequency materials that create the sound.

Largely since tonal (sinusoidal) sounds are the bases that the frequency domain description of sound, a an excellent deal that the examine of listening has encountered tonal sounds. However, day-to-day sounds are complicated sounds, i beg your pardon are made up of many tonal frequency components. A common facility sound provided to examine hearing is noise. Noise includes all possible frequency components, and the amplitude the the noise different randomly over time. A noise is claimed to be “white noise” if it consists of all frequency contents each in ~ the same mean sound level.

A sound waveform has actually three an easy physical attributes: frequency, amplitude, and temporal variation. Frequency refers to the number of times per second that the vibratory sample (in the time domain) oscillates. Amplitude describes sound pressure. There are many aspects to the temporal sport of sound, such together sound duration. Sound press is proportional come sound strongness (in systems of strength or energy), so sound magnitude deserve to be measure in systems of pressure, power, and also energy. The typical measure that sound level is the decibel (dB), in i beg your pardon the decibel is the logarithm the the ratio of two sound intensities or two sound pressures. Frequency is measure in units of hertz (Hz), cycles every second. Steps of time space expressed in various temporal systems or have the right to be analyzed into phase measured in angular degrees. Below are some interpretations of terms and also measures provided to explain sound.


Sound push (p): sound press is equal to the force (F) developed by the vibrating object split by the area (Ar) end which that force is gift applied: p = F/Ar.
DekaPascals or daPa; the Système worldwide unit that pressure. One daPa = 100 dynes every cm2, and also one environment = 10132.5 daPa.
Sound soot (I): sound strongness is a measure up of power. Sound intensity equates to sound push squared separated by the thickness (po) of the sound-transmitting medium (e.g., air) time the speed of sound (c): ns = p2/poc. Power is a measure up of the capability to perform work and is same to power times the term of the sound, or E = PT, wherein P is power and T is time (duration) in seconds.
Phase (angular degrees): one bike of a periodic adjust in sound pressure have the right to be to express in terms of completing the 360 levels of a circle. Thus, fifty percent a bicycle is 180 degrees, and also so on. Thus, time (t) within a cycle deserve to be express in terms of phase (θ, express in degrees), θ = 360o(t)(f), whereby f = frequency in Hz, and also t = time in seconds.
Tone (a basic sound): a ton is a sound who amplitude transforms as a sinusoidal function of time: Asin(2 πft + θ), wherein sin is the trigonometric sin function, θ = optimal amplitude, f = frequency in Hz, t = time in seconds, and θ = starting phase in degrees.
Spectrum: the summary of the frequency contents of sound; amplitude spectrum defines the amplitude of each frequency component; phase spectrum describes the phase of every frequency component.
Noise: a facility sound that consists of all frequency components, and whose instantaneous amplitude different randomly.

The hatchet “noise” can refer to any type of sound that may be undesirable or might interfere with the detection that a target or signal sound. In some contexts, a speech sound might be the signal or target sound, and another speech sound or a mixture of various other speech sounds might be presented as a “noise” come interfere v the auditory handling of the target decided sound. Often a mixture the speech sound is referred to as “speech babble.”


The hear System

The ear is a really efficient transducer (i.e., a maker that alters energy native one kind to another), changing sound press in the air into a neural-electrical signal the is interpreted by the brain as speech, music, noise, etc. The external ear, center ear, within ear, brainstem, and mind each have a specific role in this transformation process (see number 2-1).


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FIGURE 2-1

The anatomy that the hear system. Indigenous Yost (2000, p. 66). Reprinted with permission that author.


The exterior ear has the pinna, which helps record sound in the environment. The outside ear canal channels sound to the tympanic membrane (eardrum), which separates the external and also middle ear. The tympanic membrane and the three middle ear bones, or ossicles (malleus, incus, and stapes), aid in the move of sound push in air into the fluid- and tissue-filled inner ear. Once pressure is transferred from air to a denser medium, such as the inside ear environment, most of the push is reflect away. Thus, the within ear offers impedance come conducting sound push to the fluid and tissue of the inner ear. The move of press in this situation is referred to as admittance, while impedance is the border of the deliver of pressure. The hatchet “acoustic immittance” is provided to define the transfer process within the middle ear: words “immittance” combines the indigenous impedance and also admittance (im + mittance). Together a an outcome of this impedance, there is as much as a 35 dB ns in the infection of sound pressure to the within ear. The external ear, tympanic membrane, and also ossicles connect when a sound is existing to emphasis the sound pressure into the within ear so that many of the 35 dB impedance loss is overcome. Thus, the fluids and also tissues of the inside ear vibrate in solution to sound in a really efficient manner.

Sound tide are generally transmitted with the ossicular chain the the middle ear come the stapes footplate. The footplate rocks in the oval window of the within ear, setup the fluids the the inside ear in motion, v the parameters of that activity being dependency on the intensity, frequency, and temporal nature of the signal. The within ear consists of both the vestibular system (underlying the feeling of balance and also equilibrium) and also the cochlea (underlying the feeling of hearing). The cochlea has actually three separate liquid compartments; 2 contain perilymph (scala tympani and scala vestibuli), comparable to the body"s extracellular fluid, and the other, scala media, contains endolymph, i m sorry is comparable to intracellular fluids.

The scala media contains the sensorineural hair cells the are engendered by changes in fluid and tissue vibration. There room two species of hair cells: inner and outer. Within hair cells are the auditory biotransducers translating sound vibration right into neural discharges. The shearing (a form of bending) of the hairs (stereocilia) that the within hair cells caused by these vibrations induces a neural-electrical potential the activates a neural an answer in listening nerve fibers of the eighth cranial nerve the neurally attach the hair cells to the brainstem. The outer hair cells serve a various purpose. When their stereocilia room sheared, the dimension of the outer hair cells transforms due to a biomechanical alteration. The rapid change in outer hair cell dimension (especially that is length) transforms the biomechanical coupling in ~ the cochlea.

The frameworks of the cochlea vibrate in solution to sound through a details vibratory pattern. This vibratory pattern (the traveling wave) permits the inner hair cells and their connections to the listening nerve to send signals to the brainstem and brain about the sound"s vibration and its frequency content. The is, the travel wave motion of cochlear vibration helps type out the frequency content of any type of sound, so the information around the frequency contents of sound is coded in the neural responses being sent to the brainstem and brain.

The reality that the various frequencies of sound room coded by different auditory nerve yarn is referred to as the ar theory the frequency processing, and also the listening nerve is claimed to it is in “tonotopically” organized in that each nerve fiber carries details to the brainstem and brain about a narrow range of frequencies. In addition, the temporal pattern of neural responses of the listening nerve fibers responds come the temporal pattern of oscillations that the just arrived sound as lengthy as the temporal sport are much less than about 5000 Hz.

In general, the more intense the sound is, the greater the number of neural discharges that space being sent out by the auditory nerve come the brainstem and also brain. Thus, the cochlea sends neural details to the brainstem and brain via the listening nerve about the three physical nature of sound: frequency, temporal variation, and also level. The biomechanical an answer of the cochlea is an extremely sensitive come sound, is extremely frequency selective, and also behaves in a nonlinear manner. A good deal of this sensitivity, frequency selectivity, and also nonlinearity is a duty of the motility of the outer hair cells.

There space two significant consequences that the nonlinear role of the cochlea: (1) neural calculation is a compressive function of sound level. This way that, at short sound levels, over there is a one-to-one relationship in between increases in sound level and increases in neural output; however, at higher sound levels, the rate at i beg your pardon the neural output rises with increases in sound level is lower. (2) The cochlea and auditory nerve create distortion products. Because that instance, if the sound input has two frequencies, f1 and also f2, distortion assets at frequencies equal to 2f1, 2f2, f2-f1, and 2f1-f2 may be created by the nonlinear duty of the cochlea. The distortion product 2f1-f2 (the cubic-difference tone) might be especially strong and this cubic-difference distortion product is provided in several actions of listening function.

At 60 dB SPL the skeletal of the skull start to vibrate, bypassing the middle ear system. This direct vibration the the skull can reason the cochlea come vibrate and, thus, the hair cell to shear and to start the process of hearing. This is a an extremely inefficient means of hearing, in the this means of exciting the listening nervous mechanism represents at least a 60 dB hear loss.

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There are countless neural centers in the brainstem and in the brain that procedure the information provided by the listening nerve. The main centers in the auditory brainstem in order of your anatomical ar from the cochlea to the cortex are: cochlear nucleus, olivary complex, lateral lemniscus, worse colliculus, and medial geniculate. The outer, middle, and also inner ears in addition to the auditory nerve make up the peripheral hear system, and also the brainstem and brain constitute the central auditory nervous system. Together the peripheral and central nervous systems room responsible because that hearing and auditory perception.