4.23 Block diagram of a dynamic loudness meter (DLM) [46].
One advantage of the DLM discussed and shown in Figure 4.23 is that, by suitable modification of the DLM loudness transformation (step 5), the loudness perception of both normal hearing and hearing impaired subjects can be simulated [22]. This fact is important since the aging population in industrialized countries has suffered varying degrees of hearing impairment that should be allowed for in sound quality simulations. Some fraction of the younger population may have suffered mild to moderate hearing loss because of exposure to intense noise during recreational or leisure time activities [22].
4.4 Hearing Loss and Diseases (Disorders)
Some are born with the severe handicap of deafness. Others suffer sudden hearing loss later in life or (much more commonly) gradually lose their hearing over a period of time. For all these persons, deafness is a crippling loss of one of the most important senses. Deafness can be very slight or complete. There are different legal and medical definitions of deafness. For our purpose we will define the onset of impaired hearing when a person's hearing level is 25 dB, averaged at 500, 1000, and 2000 Hz. We will define total deafness when a person's hearing level is 92 dB averaged at the same important speech frequencies [47] (see Section 5.7). Someone who is hard‐of‐hearing, then, may have a hearing level anywhere between these two extremes of 25 and 92 dB. The average hearing level of a nonimpaired person is 0 dB.
Deafness not only causes a lowering in the level at which sound is heard, but in many cases also causes a loss in hearing quality so that consonants cannot be distinguished and words are confused. In addition, some deaf people also suffer from tinnitus or a ringing in the ears which is disturbing and competes with other sounds.
Because of their affliction, deaf people tend to withdraw from society. This is sometimes reinforced by society itself. In ancient Rome, the deaf were classed with the mentally incompetent. In modern societies, education, advances in medicine, and use of hearing aids are now improving the plight of the deaf. Strict control of noise levels in industry will also reduce the number of people receiving needless hearing damage and consequent deafness. A review on noise‐induced hearing loss is available in the literature [48].
Hearing disorders (deafness) can be divided into two main types: conduction deafness and sensory‐neural deafness. Conduction deafness is related to disorders in the outer and middle ear, while sensory‐neural deafness occurs in the inner ear in the organ of Corti, the auditory nerve, or the auditory cortex in the brain.
4.4.1 Conduction Hearing Loss
This normally manifests itself as a fairly uniform decrease in hearing over most frequencies. Background noise usually causes people to speak louder and those with conduction deafness can then often hear. This type of deafness can normally be overcome by a hearing aid with sufficient amplification. There are several causes of conduction deafness, some of which may be easily corrected. For example the ear canal may become blocked with wax. This can now be cleaned out, medically restoring hearing.
Until 65 years ago, middle ear infections were the largest cause of hearing loss. Such infections, often occurring in children, could be caused by inflammations of the tonsils or adenoids. Unchecked, these inflammations could spread to the Eustachian tube which would swell and become blocked. Fluids would form in the middle ear cavity causing pressure on the eardrum and pain. Frequently infection could spread to the mastoid bone surrounding the middle ear cavity. Death often resulted. Mastoiditis was treated by surgically removing some of the affected mastoid bone. With the advent of penicillin and other antibiotics, there is no need for middle ear infections to progress so far and now they rapidly can be cleared up with drugs in almost every case.
Another common cause of hearing loss occurs when bony material forms around the footplate of the stapes preventing it moving and transmitting vibrations through the oval window into the cochlea. This condition known as Otosclerosis normally causes a severe loss in hearing at most frequencies of 50 dB or more. Otosclerosis can often be helped quite successfully with a hearing aid. By providing appreciable amplification of the sound pressure at the eardrum, sufficient vibration amplitude can be transmitted to the oval window despite the calcification.
However, calcification can now be successfully treated by surgical methods. Toward the end of the nineteenth century, two German surgeons, first Johann Kessel, and later Karl Passow, tried to remove the calcified stapes and make an opening to the inner ear [47]. Although some operations were briefly successful, this technique had to be abandoned because of the crudity of the surgical instruments available and the danger of admitting infection into the cochlea. However, in the U.S. in 1938, Julius Lempert pioneered the surgical technique known as fenestration. In this operation the incus and malleus and some of the middle ear bone are removed and the ear canal is thus extended and put into contact with one of the semicircular canals. The middle ear is thus bypassed completely. This operation is normally quite successful and restores hearing, although at a lower level.
Other surgical techniques have recently become preferred and fenestration is seldom used now. In 1952, Samuel Rosen accidentally freed the stapes during ear surgery and this technique has been refined into an operation known as stapes mobilization. It is difficult to accomplish, however, because just the right amount of force is needed.
Too much force can damage the inner ear. Also, because otosclerosis is a progressive disease, the condition of calcification can recur. A surgical technique used for many years is stapedectomy: the complete removal of the stapes and its replacement with a plastic part. The operation must be conducted using a microscope and can be accomplished in less than 30 minutes. After the stapes is chipped away it is replaced with an artificial one which is attached both to the incus and the oval window and almost perfect hearing is restored. Another microsurgical option is stapedotomy which is thought by many surgeons to be safer and less prone to complications. In this procedure, a skilled surgeon drills a tiny hole in the stapes using a laser, in which to secure a prosthetic.
In the middle of 1970s the first cochlear implants were performed. A cochlear implant is an electronic device used in individuals with a severe or profound hearing loss. Basically the implant consists of a microphone, speech processor, transmitter and receiver, and a stimulator which converts the signals into electrical impulses to be sent by electrodes to the cochlea. The surgeon makes an incision in the skin behind the ear were the implant is placed. Then the surgeon drills into the mastoid to reach the inner ear where the electrode array is inserted directly into the cochlea. Although it is still an expensive procedure, many deaf people have recovered the sense of hearing after this surgery.
4.4.2 Sensory‐Neural Hearing Loss
There are several causes of sensory‐neural hearing loss and all are associated with disorders of the inner ear, the auditory nerve fibers, the auditory cortex in the brain, or combinations of all three. Unlike conductive deafness, sensory‐neural deafness is often most severe at higher frequencies. Background noise can thus mask the consonants in speech and make it unintelligible. Although a hearing aid brings some relief, it is perhaps not so useful as with conductive deafness. Very profound sensory‐neural hearing loss may be restored only by using a cochlear implant.
Congenital deafness is often a cause of sensory‐neural deafness. Here genes producing imperfect hearing mechanisms are inherited. During the first three months of pregnancy, if a mother contracts certain viral diseases such as mumps, influenza, or German measles, these can impair the development of the hearing mechanism in the fetus. During birth itself, brain damage can affect the hearing mechanism. In early childhood, diseases such as meningitis can damage the inner ear or auditory nerve, producing deafness.
