Course:PHYS341/2022/Project25
Hearing Aids
The Ear
The ear is divided into 3 regions: the outer ear, middle ear, and inner ear. The outer ear consists of the pinna (external ear), meatus (ear canal), and tympanic membrane (eardrum). The purpose of the pinna is in the capture and localization of sound, the meatus recesses the delicate components of the ear into the head, and the tympanic membrane transmits sound pressure variations at the back of the ear canal, to the bones of the middle ear. The middle ear comprises the tympanic membrane and the ossicles which consist of the malleus, incus, and stapes. The ossicles work together to produce an efficient conversion of sound energy, from a small force on a big area to a large force on a small area. The malleus is pushed on one side by the eardrum and moves back and forth, pushing the incus on the other side. The incus which is pushed back and forth by the malleus, moves back and forth pushing the stapes. Finally, the stapes which is pushed back and forth by the incus moves back and forth, pushing the oval window, a soft spot on the cochlea. The cochlea is the sole component of the inner ear that is responsible for converting mechanical vibrations into a nervous impulse.
In essence, sound pressure on the eardrum pushes the ossicles onto the oval window. Then, the fluid in Scala Vestibuli flows part way along the cochlea which pushes in the Reissner’s Membrane and makes the fluid in the Scala Media flow. The fluid in Scala Media moves, stretching the basilar and tectorial membranes in different directions. The shearing motion between the membranes pushes back and forth the stereocilia on hair cells and the hair cells respond by sending an electrical signal to the nerve cells, which will communicate it down the auditory nerves to the brain.
Types of Hearing Loss
Hearing loss can occur when any part of the auditory system or ear is not functioning properly. There are 4 types of hearing loss: Sensorineural hearing loss, conductive hearing loss, mixed hearing loss, and auditory neuropathy spectrum disorder. The root cause of sensorineural hearing loss is in the inner ear which can be caused by illnesses, aging, inherited hearing loss, a blow to the head, a problem in the formation of the inner ear, listening to loud noises, and drugs that are toxic to hearing. Sensorineural hearing loss is the most common type of permanent hearing loss and hearing aids are a solution to hear better. Conductive hearing loss occurs when sound cannot go through the outer and middle ear due to some form of obstruction such as ear infections, earwax, deformation of the outer or middle ear, and poor function of the Eustachian tube. Medicine or surgery can often resolve conductive hearing loss as well as hearing aids. When both sensorineural and conductive hearing loss are present, it is known as mixed hearing loss. Auditory neuropathy spectrum disorder occurs when sound enters the ear normally, but due to damage of the inner ear or the auditory nerve, sound is not organized in a way that the brain can comprehend. Hearing aids can assist with all of these types of hearing loss.
Audiograms show the degree and configuration of hearing loss. The degree of hearing loss ranges from mild to profound and is measured in decibels (dB). Additionally, audiograms show how loud the sounds need to be at different frequencies for one to hear. The graph of one’s hearing on an audiogram will form a shape which is the configuration of hearing loss. These can be described as unilateral/bilateral, pre-lingual/post-lingual, symmetrical/asymmetrical, progressive/sudden, fluctuating/stable, and congenital/acquired onset.
Types of Hearing Aids
There are many different styles of hearing aids, each catering to the needs of a patient and the severity of one’s hearing loss. Common types of hearing aids include the behind-the-ear (BTE), receiver-in-canal (RIC), in-the-ear (ITE), in-the-canal (ITC), and completely-in-canal (CIC) hearing aids.
Behind-the ear (BTE) hearing aids are suitable for people of all ages with mild to profound hearing loss. It consists of a hard plastic case that is worn behind the ear and connects to a plastic earmold that fits inside the outer ear. Sound travels from the hearing aid through the earmold and into the ear. Receiver-in-canal (RIC) hearing aids are similar to BTE with the speaker in the ear canal but instead of an ear mold, there is a tiny wire that connects the piece behind the ear to the speaker. In-the-ear (ITS) hearing aids fit completely inside the outer ear and the case holding the electronic components is made of hard plastic. It is used for mild to severe hearing loss and is not typically worn by young children because as the ear grows, the casings need to be replaced. In-the-canal (ITC) hearing aids are designed to fit the shape and size of a person’s ear canal and are used for mild to moderately severe hearing loss. Completely-in-canal (CIC) hearing aids are almost hidden in the ear canal and like ITC, are suitable for mild to moderately severe hearing loss. Both canal hearing aids are very small which is not recommended for young children or for people with severe to profound hearing loss due their limited power and volume.
There are two types of electronic components in hearing aids: analog and digital. Analog aids convert sound waves into electrical signals that are amplified and can be used in all types of hearing aids. Digital aids convert sound waves into numerical codes before amplifying them. Digital circuitry allows for more flexibility in programming and adjusting to a user’s needs. Like analog aids, they can be used in all types of hearing aids.
How a Hearing Aid Works
There are 3 basic parts of a hearing aid: a microphone, amplifier, and speaker. Sound is received through the microphone, which converts the sound waves to electrical signals. The electrical signals are sent to the amplifier which increases its power and sends the signals to the ear through the speaker. Essentially, hearing aids magnify the sound vibrations entering the ear. There are multiple ways in which digital hearing aids process sounds. Microphones capture mechanical wave energy and convert it into electrical energy, and modern hearing aids have two microphones: the omnidirectional microphone and the directional microphone. The omnidirectional microphone captures sound from any direction while the directional microphone is more focused on capturing speech. Analog signals coming from the microphones are converted into a digital signal (A/D) in a process called analog to digital conversion and is subject to digital signal processing (DSP). The processed digital signal is converted back into an acoustic signal (D/A) which enters the ear canal through the receiver. The limited dynamic range of the analog to digital conversion process at 96 dB (common 16-bit A/D converter) may prevent sounds outside of that range from being amplified. Frequencies of sound are split up into a number of channels, where each channel spans a different frequency range and is analyzed and processed separately. Bands control the volume or gain at different frequencies. Channels and bands allow for raising the level of a specific frequency range, better separation of speech from background noise, feedback cancellation, noise reduction, and matching the volume and compression at different frequencies specific to the hearing aid user.
References
- “Conductive Hearing Loss.” American Speech-Language-Hearing Association, American Speech-Language-Hearing Association, https://www.asha.org/public/hearing/conductive-hearing-loss/.
- “Configuration of Hearing Loss.” American Speech-Language-Hearing Association, American Speech-Language-Hearing Association, https://www.asha.org/public/hearing/configuration-of-hearing-loss/.
- “Hearing Aids.” National Institute of Deafness and Other Communication Disorders, U.S. Department of Health and Human Services, Sept. 2013, https://www.nidcd.nih.gov/health/hearing-aids#:~:text=A%20hearing%20aid%20has%20three,the%20ear%20through%20a%20speaker.
- Moore, Guy D. "Physics of Music Lecture Notes." Physics 224: The Physics of Music, 14 July. 2006, McGill University, https://theorie.ikp.physik.tu-darmstadt.de/qcd/moore/ph224/lecnotes.pdf
- “Sensorineural Hearing Loss.” American Speech-Language-Hearing Association, American Speech-Language-Hearing Association, https://www.asha.org/public/hearing/sensorineural-hearing-loss/.
- Sieber, Tina. “Hearing Aids: How Do Hearing Aids Work?” SoundGuys, 3 Mar. 2022, https://www.soundguys.com/how-do-hearing-aids-work-57482/.