Sound is produced in nature as a mechanical wave. The human ear converts that wave into an electrical signal that is transmitted to the brain. The sound waves enter our ear through the external auditory canal. This canal directs the waves to the eardrum (tympanic membrane). The eardrum vibrates when the sound waves hit it. That vibration is transmitted into three ear bones: the malleus (hammer), the incus (anvil), and the stapes (stirrup), which connect with each other, forming a chain.
These are the smallest bones in the human body. The vibration passes first to the malleus that is attached to the eardrum. The malleus passes the vibration to the incus, which in turn transmits it to the stapes. The stapes moves in and out like a piston.
There is a significant difference between the surface area of the eardrum and the surface area of the bottom of the stapes, called the footplate. This size mismatch amplifies the intensity of the wave that is passed to the inner ear. The footplate interfaces with the fluid that fills the hearing part of the inner ear—the cochlea. This in turn creates a fluid wave within the cochlea. The cochlea contains approximately 18,000 hair cells. As the fluid wave moves along within the cochlea it bends the hair cells.
When the hair cells are bent, they generate an electrical signal that is then transmitted to the brain. The brain interprets the signal as sound. These hair cells are spread out along the cochlea based on frequency. Sound waves of a given frequency travel up the cochlea a specific distance and stimulate the hair cells in that area. This creates a “tonotopic” distribution, and gives the cochlea its pitch-perceiving ability.
Hearing loss is categorized as conductive, sensorineural (nerve), or mixed (both conductive and sensorineural). When there is a problem that interferes with the sound wave being conducted through the eardrum or the middle ear bones (ossicles) it creates a conductive hearing loss. When there is a malfunction in the inner ear (cochlea) or the nerve pathway it causes a sensorineural (or nerve) hearing loss. Occasionally, both systems can have a problem at the same time, which causes a mixed hearing loss.
Sound level is measured in decibels (dB). The higher the decibel, the louder the sound. To check someone’s hearing we can measure how loud a sound has to be before the person acknowledges that a sound has been made. This is recorded over a range of frequencies—typically from 250 hertz to 8,000 hertz. The normal hearing is between 0 and 25 dB. Most conversational speech occurs at a level between 45 and 55 dB, and between the frequency range of 500 Hz (low-pitched) to 3,000 Hz (high-pitched).
Most people like to quantify how much hearing loss they have. The most commonly used classification breaks hearing loss into categories: mild, moderate, severe, and profound. Mild hearing loss is defined to occur when a person needs sound to be between 25 and 40 dB to be heard, moderate between 40 to 60 dB, severe between 60 to 80 dB, and profound when greater than 80 dB. Occasionally, hearing loss is given as a “percentage.” There are several different formulas that are used to calculate this percentage. Often the “percentage” is just an estimate given by the clinician.
How loud a sound must be before we are able to acknowledge it is only one part of hearing. The other part of the hearing process is how well we are able to understand the things we hear. This is called discrimination. Discrimination is tested by giving a list of words at a loudness above the person’s reception threshold (usually 40 dB louder).
For example, let’s say that a sound had to be 30 dB loud before a person acknowledged that a sound had been made. The examiner would then add 40 dB to that level, making the discrimination testing level 70 dB. They would then give a list of phonetically balanced words at 70 dB, and see how many the person being tested was able to repeat back correctly. Normally, a person will get 92% or more of the words correct. Some loss of discrimination often accompanies severe hearing loss. Loss of discrimination is difficult to rehabilitate or improve.
Tinnitus, or “ringing in the ears,” is present in 80-90% of those who have hearing loss. This article will not discuss tinnitus. However, realize that tinnitus is a big part of hearing loss.
