Every ear is different. Therefore, we need to understand a bit about how ears ultimately influence the performance of a hearing aid. You will encounter big ears, small ears, sensitive ears and even missing ears. Helen Keller once stated that “vision puts us in touch with things; hearing puts us in touch with people.” It is true that communication is crucial in developing and maintaining relationships; therefore our field is dedicated to improving communication skills through better hearing.
Ear anatomy consists of three different sections: The outer ear, the middle ear and the inner ear.
Ear Anatomy: The Outer EarThe most external part of the ear is easily seen on each side of the head. This is known as the outer ear. The outer ear anatomy consists of the Pinna and the external auditory meatus or external auditory canal (EAC). The function of the outer ear is to collect sound and channel it into the ear canal. The Pinna, with its unique “bowl-like” shape, helps to enhance sounds coming from the front of the listener. This is called the “Pinna Effect.” The EAC also assists in enhancing certain frequencies that are important for speech understanding. This is known as the Ear Canal Resonance and it is sometimes measured prior to a hearing aid fitting through Real-Ear Unaided Responses (REUR). Sound waves enter the ear canal and push, vibrate or move the eardrum. This movement on the eardrum causes the ossicles (three tiny bones that are attached to the eardrum) to move as well.
Ear Anatomy: The Middle Ear
The eardrum and ossicles make up the middle ear anatomy. The ossicles are made up of the Malleus, Incus and Stapes. The role of the ossicles is to help channel sound from air-filled middle ear space to the fluid-filled chamber of the inner ear. The ossicles, collectively, are small enough to fit on a dime! As a matter of fact, they are the smallest bones in your entire body. The ossicles move back and forth in response to sound pressure on the eardrum; the Stapes exerts force on the Oval Window, which leads into the inner ear.
Ear Anatomy: The Inner Ear
The inner ear anatomy is comprised of two chambers: a balance chamber, called the Vestibular System and a hearing chamber, called the Cochlea. For the purposes of this training manual, we will focus on the hearing aspects of the ear. Inside the cochlea, the sound waves are converted to neural signals by the “hair cells.” If the hair cells are destroyed or damaged (exposure to gun fire or other loud noises), this conversion from sound waves to neural signals does not happen effectively and the person develops a hearing loss. Once the cochlea receives the sound information, the sounds are divided by frequency and transferred by the auditory nerve to the portion of the brain responsible for hearing and understanding, known as the Auditory Cortex.
The Auditory Pathways
The auditory system pathways traveling from the cochlea to the Auditory Cortex continuously work together to determine the signal from the noise. Different levels of the auditory pathways are responsible for different aspects of “cleaning-up” the signal. For instance, the Superior Olivary Complex (SOC) is thought to be the place in the auditory pathways where information between the two ears (inter-aural) is combined. As information from each ear is received, there is typically a slight difference in sound intensity due to the time it takes for the information to arrive at the right ear versus the left ear. This called the Head-Shadow Effect.
The auditory system is so amazing that it can recognize these slight differences and determine what is useful information (the signal) and enhance it and what is distracting information (the noise) and reduce it. This is called Binaural Summation and Binaural Subtraction. Advanced Auditory Processing skills such as these rely heavily on both ears to deliver a full picture of the auditory surroundings from outside of the ear to the cochlea, to the auditory pathways and then to the Auditory Cortex.
Other important frequency information for localizing sounds (finding the sound source), is hindered if the two ears are not working together to deliver a synchronized picture of low and high frequencies to the Auditory Cortex. This is critical to understand as hearing loss enters the picture, since the inability to hear sounds immediately makes processing information impaired. You might say that anyone with a hearing loss also has an auditory processing disorder. The ultimate goal of hearing aid technology is to always mimic the normal ear.