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A hearing test—or audiogram—represents how well we hear sounds and speech testing will generate data on how well we “understand” speech. They are separate, but related portions of a complete hearing evaluation.
The audiogram consists of a graph of vertical and horizontal lines, and the vertical lines represent pitch or frequency, designated by “Hertz,” or cycles per second. Pitch progresses from low to high as you move left to right across the graph, and is represented by “Hz.”
The horizontal lines represent loudness or intensity, where soft sounds are at the top and sounds gets louder as you go down toward the bottom of the graph. Loudness is represented by decibels or “dB.” A symbol is placed on the graph when a person indicates that they have barely heard that specific pitch. This is called their threshold. A threshold is obtained for each pitch on the graph. Thresholds are obtained with two different types of sound generating head pieces.
Learn more about the aspects of hearing tests by clicking below:Air Conduction
Most people are familiar with headphones. They generate sound that is transmitted through the entire ear mechanism using sound waves (air). This is called air conduction. The symbol for right ear air conduction is a red circle, the symbol for left ear air conduction is a blue X. Air conduction tests the entire ear mechanism.
The other head piece used in a hearing test is a bone vibrator. This piece looks like a headband with a bone vibrator attached to one end. The bone vibrator is usually placed on the mastoid bone behind the pinna. It generates sound that is transmitted through the bones of our head, and therefore ultimately into our inner ear.
This transmission of sound directly into our inner ear allows us to bypass the outer and middle ear to obtain a threshold for bone conduction only. The symbol for right ear bone conduction is a red arrow pointing to the right; the symbol for left ear bone conduction is a blue arrow pointing to the left. The difference between the air conduction threshold and the bone conduction threshold can be used to diagnose ear conditions.
A normally functioning ear will have no difference between the air conduction threshold and the bone conduction threshold. If there is a difference, it indicates that there is a problem in the outer ear and/or middle structures.
A complete evaluation includes speech testing. This may be live or recorded in nature.
The first unit of speech testing is called a Speech Reception Threshold or SRT. With this type of testing the patient repeats two-syllable words from a comfortable loudness level to the very softest level they can hear. This test is not a test of understanding, but more of a “self-check” for the audiologist. The SRT should closely match the average threshold of the audiogram.
The second unit of speech testing is a word recognition test. Again, the patient repeats the words, but this time the list consists of short words, given at a comfortable loudness level. This results in a percentage score, based on the number of words correctly repeated. The higher the percentage, the better the patient's understanding ability. This understanding or “clarity” cannot be improved; it is due to damage to the inner ear.
The ear consists of three sections:
(Click the diagram at the right to see a larger version.)
The Outer Ear
The outer ear consists of the pinna (also known as the auricle) and the ear canal (external auditory meatus).
The pinna is shaped a bit like a funnel. Its function is to channel sound waves into the ear canal. Its shape and position on the head also results in a slight reduction of the sound waves coming from behind us, which assists us in sound localization (the ability to know where sounds are coming from).
The ear canal’s main function is to channel sound toward the eardrum.
The outer portion of the ear canal is housed in a cartilaginous framework, but as you progress toward the drum, that framework becomes osseous or bony
The ear canal produces bitter wax which helps deter insects and keep the skin moist.
Hair cells are also present to keep debris from the ear canal.
The middle ear is often improperly referred to as the inner ear. The middle ear consists of the tympanic membrane (or eardrum) and the ossicles (or bones.)
The eardrum consists of three layers. The first layer is the same skin that exists in the ear canal, the second layer is more fibrous in nature, and the third layer is the same as the inside of the middle ear cavity, mucous membrane. The eardrum’s function is to continue the transmission of sound waves that have been channeled into the ear canal.
The ear bones are all connected—not only to each other—but also to the eardrum. They are called the malleus (hammer), the incus (anvil), and the stapes (stirrup). They are very intricately connected to each other and any disruption of this bony chain can rarely be repaired without some loss of function, which results in some amount of hearing loss.
The ear bones not only continue the transmission of the sound waves to the inner ear, but along with the eardrum they increase the energy of sound waves to counteract the natural loss of energy as the sound waves move through the different parts of the ear. Their job is two-fold: transmission and amplification.
The stirrup bone is the last bone in the ossicular chain. It attaches, by means of a “gasket-like” seal, to the oval window of the cochlea, which is a part of the inner ear.
The inner ear houses the cochlea, which is used for hearing, and the vestibular system, which is used for balance. The cochlea is the “snail shaped,” bony portion which houses the sensory hair cells used for converting sound waves into nerve responses to the brain.
The vestibular system houses the three fluid-filled semi-circular canals which contribute to our balance function. The three canals are each positioned in a different plane of space, so that as we move, the fluid will shift and “level” itself. One canal is horizontally positioned, the other two are vertical, but at “right angles” to each other. These changes in fluid level and position cause hair cells located at the base of each canal to move. This hair cell “bending” results in the firing of the nerve cells attached to these hair cells and the transmission of that information to the brain. This information, along with information from our vision and “touch” results in our ability to maintain our balance.
The cochlea is a bony structure which houses an almost identical membranous structure inside it. Contained in this membranous tube is another membrane that runs from beginning to end. On this membrane is the organ of corti, where the sensory hair cells are. Different areas of this basilar membrane represent different areas of “pitch.” These hair cells can be thought of as the keys of a piano keyboard.
The cochlea receives the sound waves from the stirrup bone that vibrates in the oval window. The vibrations create a traveling fluid “wave,” that progresses from the beginning of the basilar membrane to its end. This traveling wave “bends” the appropriate hair cells, which will then fire the respective nerve.
Imagine a piano keyboard rolled up into a coil, and each key representing a hair cell. The action of a piano key striking its wire is similar to the hair cell bending and firing its nerve. This generates a specific pitch. Combining all of this “pitch” information allows us to interpret the sounds we hear.