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2 Basics of Sound, The Ear, and Hearing
Pages 42-68

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From page 42...
... When the pressure wave encounters another object, the vibration can be imparted to that object and the pressure wave will propagate in the medium of the object. The sound wave may also be reflected from the object or it may diffract around the object.
From page 43...
... Noise contains all possible frequency components, and the amplitude of the noise varies randomly over time. A noise is said to be "white noise" if it contains all frequency components each at the same average sound level.
From page 44...
... · White noise: a noise in which all of the frequency components have the same average level. The term "noise" can refer to any sound that may be unwanted or may interfere with the detection of a target or signal sound.
From page 45...
... The tympanic membrane and the three middle ear bones, or ossicles (malleus, incus, and stapes) , assist in the transfer of sound pressure in air into the fluid- and tissue-filled inner ear.
From page 46...
... of the inner hair cells caused by these vibrations induces a neural-electrical potential that activates a neural response in auditory nerve fibers of the eighth cranial nerve that neurally connect the hair cells to the brainstem. The outer hair cells serve a different purpose.
From page 47...
... The outer, middle, and inner ears along with the auditory nerve make up the peripheral auditory system, and the brainstem and brain constitute the central auditory nervous system. Together the peripheral and central nervous systems are responsible for hearing and auditory perception.
From page 48...
... Listeners can detect the presence of a sound; discriminate changes in frequency, level, and time; recognize different speech sounds; localize the source of a sound; and identify and recognize different sound sources. The auditory system must often accomplish these workplace tasks when there are many sources producing sound at about the same time, so that the sound from one source may interfere with the ability to "hear" the sound from another source.
From page 49...
... The audiogram is a plot of the thresholds of hearing referenced to the appropriate MAP or MAF thresholds shown in the figure. Thus, a person with no hearing loss at all will have a flat audiogram at zero dB HL (dB HL means decibels of hearing level, in which the reference decibel values are the appropriate MAP or MAF dB SPL values shown in Figure 2-2)
From page 50...
... Because people with hearing loss have elevated thresholds but have the same upper limit of audibility as people with normal hearing, the change in loudness grows more rapidly as a function of increasing sound level above threshold for a person with a hearing loss than for a person without a hearing loss. This rapid growth of loudness is referred to as loudness recruitment, and it is experienced by almost all people with sensorineural hearing loss.
From page 51...
... Maskers with frequencies higher than 1069 Hz or lower than 936 Hz will be less effective in masking the 1000 Hz signal. Thus, a white noise filtered so that the noise contains frequency components between 936 and 1069 Hz will be maximally effective in masking a 1000 Hz tonal signal.
From page 52...
... Masking means the detection threshold of a signal sound has been elevated by the presence of the masking sound. Loudness and pitch refer to subjective attributes of sound that are highly correlated with sound level and frequency, respectively.
From page 53...
... Thus, in the workplace, very small changes in sound level, frequency, and duration can be discriminated even when some masking sounds also exist. As long as the level of a sound does not vary too rapidly, listeners in the workplace should be able to determine that the sound is fluctuating in level (in loudness)
From page 54...
... One rule of thumb for listeners with normal hearing is that for a broadband masking stimulus such as a white noise, approximately 50 percent intelligibility occurs when only the speech and noise information is provided and the overall levels of the speech words or syllables and noise are about equal (i.e., when the S/N ratio is zero dB)
From page 55...
... However, the higher the fidelity of the reproduction system, the better speech recognition is likely to be when interfering sound sources are present. Hearing loss can lead to a significant loss of speech recognition even with high-quality amplification systems.
From page 56...
... The head-related transfer function (HRTF) describes the spectral changes that a sound undergoes between the sound source and the outer ear canal.
From page 57...
... Listeners are usually not confused about the nature of the actual sound source, including its location, in many reverberant spaces, presumably because the auditory system processes the first sound arriving at the ears and inhibits the information from later-arriving reflected sounds. Since the sound from the source will arrive at the listener before that from any longer-path reflection, auditory processing of the direct sound takes precedence over that of the reflected sound, usually allowing for accurate sound processing even in fairly reverberant environments.
From page 58...
... The ability to detect a signal source can be improved if potential masking sound sources are spatially separated from the signal sound source. Having a hearing loss can compromise a person's ability to locate sounds, and hearing aids may not assist him or her in locating sound sources.
From page 59...
... -- and can include fluid filling the middle ear space. Sensorineural hearing loss indicates a problem in the inner ear, auditory nerve, or higher auditory centers in the brainstem and temporal lobe.
From page 60...
... In the past, sensorineural hearing loss was referred to as "nerve deafness"; however, in most instances of sensorineural hearing loss, the auditory nerve is intact and an impairment in the hair cells within the inner ear results in the hearing loss. Loss of hair cells and the neurotrophic factors that they produce eventually lead to nerve cell loss.
From page 61...
... Infections (viral or bacterial) , disorders such as Meniere's disease and autoimmune inner ear disease, hereditary disorders, trauma, and ototoxic drugs are other causes of sensorineural hearing loss.
From page 62...
... Autosomal recessive disorders associated with sensorineural hearing loss are Pendred's syndrome, Usher syndrome, and Jervell and Lange-Nielsen syndrome. Sex-linked syndromes associated with sensorineural hearing loss are Norie's syndrome, otopalatodigital syndrome, Wildervaank's syndrome, and Alport's syndrome.
From page 63...
... components, and normal otoacoustic emissions. The otoacoustic emissions and CM findings in patients with auditory neuropathy usually indicate that the cochlear hair cells, at least the outer hair cells, are functioning normally while the abnormal auditory brainstem response is indicative of disease in the inner hair cells, auditory nerve, or brainstem.
From page 64...
... Aminoglycosides are directly toxic to outer hair cells, but they can also affect ganglion cells. Ototoxicity has been observed within the "safe" limits of nephrotoxicity.
From page 65...
... Subjective tinnitus almost always occurs when there is no acoustic stimulus at all, and only one person can hear the sound. People with subjective tinnitus typically describe their sensations as ringing, buzzing, humming, whistling, or hissing sounds.
From page 66...
... Patients who go to specialized tinnitus treatment clinics are very frequently found to meet formal psychiatric criteria for diagnosis of major depressive disorder, and about half of these have a history of major depression or anxiety disorder prior to the onset of tinnitus (Sullivan et al., 1988; Zoger, Svedlund, and Holgers, 2002)
From page 67...
... . Treatment of hyperacusis has usually consisted of desensitization to gradually increasing sound levels or treatment of underlying psychiatric disorders.
From page 68...
... 68 HEARING LOSS sources that constantly surround one. There are many causes of damage to the auditory system that result in a hearing loss that reduces one's ability to detect, discriminate, localize, or identify sound.


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