Hearing
Our ears are amazing organs! They connect us to each other and to our environment. We usually take for granted how perfect our sense of hearing is until we start to lose it. If we do suffer hearing loss it usually is a very gradual, painless process that we are unaware of.
We most often divide the anatomy of our hearing organ into three segments; the outer ear, middle ear and inner ear. The outer ear consists of the pinna or auricle, ear canal and eardrum. The pinna helps gather and intensify sound, directing it down the ear canal. The ear canal, by the nature of its physical characteristics, enhances sounds that are critical to human speech communication. The traveling sound next hits the eardrum and the formerly acoustic sound waves are converted to mechanical sound waves, being intensified once again in the process.
The middle ear is the air filled space found behind the eardrum. Connected to the inside of the eardrum is the malleus, which in turn is connected to the incus, which in turn is connected to the stapes. These three bones are known collectively as the ossicles and are the smallest bones in our bodies. The sound wave, (now a vibration) is transmitted along this ossicular chain. It is also into the middle ear space that the Eustachian tube has an opening, connecting the middle ear space to the throat. The Eustachian tube acts to equalize the pressure in our middle ear to that of atmospheric pressure. When we “pop” our ears we are opening this tube and causing pressure equalization.
The stapes makes contact to the fluid filled cochlea of the inner ear via a thin membrane covered opening called the oval window. It is in the cochlea that the hair cells or nerves are found. As the vibrating stapes “pushes” into the cochlea, the fluid is disturbed causing a wave to be set up that travels through the cochlea. The hair cells are thus disturbed/bent (envision a kelp bed swaying as ocean waves pass through it) causing a signal to be sent along the auditory nerve up to the auditory centers of the brain. Different areas of the cochlea and the corresponding hairs cells located there are responsible for receiving specific sounds. Those hair cells responsible for hearing higher pitched tones are located at the beginning of the cochlea (near the oval window) while those responsible for lower pitches are located towards the end of the cochlea. We are still working to gain a full understanding of the biochemistry of the cochlea. We have both inner and outer hair cells and know that the outer hair cells work to amplify and fine tune incoming sound while inner hair cells send the information to our brain for interpretation. While much information is traveling up to the brain we also know that the brain is sending information back to the cochlea, telling it which sounds to amplify and pay attention to and which ones to ignore.
HEARING LOSS
Any thing that impedes this transmission of sound along our auditory pathway will cause hearing loss. Possible problems include, wax in the ear canal that blocks the incoming sound, a disconnected ossicular chain, noise damaged outer hair cells, to an injury resulting in damage to hearing center or auditory cortex of the brain.
Many hearing losses can be corrected through medical or surgical intervention. Some however are permanent in nature and need to be remedied with hearing instruments. To know how to help correct or at least minimize the impact of loss we need to know the degree and nature of the loss. All individuals will have an audiogram, which is a graphic representation of hearing. Our primary concern is to find out how the loss will impact daily communication ability. Therefore we need to know the severity (how loud does a sound have to be for the individual to hear it) the louder the sounds needs to be the greater the loss. We also want to know what areas of speech are being impacted if a person has a low pitched loss they will have more trouble hearing the vowel sounds of English while if the loss is in the higher pitches they will start to lose the consonants of English. When we do not hear all of the sounds of English at a comfortable volume, speech will become not only softer but distorted. When you hear someone say that people mumble or I can hear but not understand, they are describing hearing loss. We test using tone signals in order to break the important parts of speech into discreet segments. We also do many types of testing using speech, again to assess the individuals ability to discriminate and understand what is being said.
WHAT DOES THE DEGREE OF HEARING LOSS MEAN?
The degree of hearing loss varies from person to person. Between the two extremes of hearing well and hearing nothing, there are many degrees of impairment. The terms used to describe the degree of hearing loss are mild, moderate, severe and profound. Most hearing losses are mild to moderate.
- Mild hearing loss: unable to hear soft sounds, difficulty understanding speech clearly in noisy environments.
- Moderate hearing loss: unable to hear soft and moderately loud sounds, considerable difficulty understanding speech, particularly with background noise.
- Severe hearing loss: some loud sounds are audible but communication without a hearing instrument is impossible.
- Profound hearing loss: some extremely loud sounds are audible but communication without a hearing instrument is impossible.
Once we have documented the areas of loss we know what we want the hearing instrument to do help make speech audible and more intelligible. Figures indicate that one out of seven individuals does not have full hearing and one out of ten hears so poorly that a hearing instrument would help. Studies (Maastricht report on hearing impairment, 1999) also underline the fact that only a minority – less than 14% in the EU – of those for whom a hearing instrument would be beneficial actually use one.