After this lesson, students should be able to:

• Explain the process of hearing and identify ways in which hearing can be lost.
• Explain how hearing aids are used, how they have been designed to address different hearing problems, and describe their different components.
• Identify the types of engineers who design these devices, be able to describe the difficulties with each design, and suggest possible solutions.

The Structure of the Ear (see attached Human Ear Anatomy Visual Aid)

The outer ear consists of the pinna, earlobe, ear canal and tympanic membrane (or eardrum). The pinna is the curved outer region of the visible part of the ear that collects sound waves and directs them into the ear canal. The ear canal acts much like a tube. The sound waves bounce back and forth along its length until they reach the eardrum and the middle ear.

The middle ear consists of three tiny bones collectively called the occicles; they are the malleus (or hammer), the incus (or anvil), and the stapes (or stirrup). The malleus acts like a mallet; it hits the incus, which vibrates in turn and causes the stapes to vibrate also. As the stapes vibrates, it tugs on the oval window and sends sound vibrations into the inner ear.

The inner ear consists of the cochlea and auditory nerves (or hair cells). The snail-shaped cochlea is filled with a fluid and the hair-like auditory nerves. As the vibrations pass into the cochlea, the fluid moves, which cause the auditory nerves to shift, sending an electrical signal to the brain. The brain detects these electrical signals as sound.

Ear anatomy. click for copyright

Sources of Hearing Loss

Conductive hearing loss is a mechanical problem in the outer or middle ear that blocks the conduction of sound. Causes of conductive hearing loss include: blockage of the ear canal, perforation of the tympanic membrane (eardrum), mechanical difficulties in the bones of the middle ear, or fluid accumulation in the middle ear. Conductive hearing loss is often reversible

Sensorineural hearing loss occurs when hair cells in the cochlea become unresponsive or damaged and no longer send impulses to the brain. Causes of sensorineural hearing loss include: aging, brain tumors, viral infection in the inner ear, genetics, loud noises, mumps or meningitis. Sensorineural hearing loss is permanent.

Engineers to the Rescue

Biomedical engineers study the function of the ear and develop systems that solve hearing problems, whether they are conductive or sensorineural.

Electrical engineers develop the tiny circuits that control the microphones and amplifiers that work together to convert sound waves to electrical impulses.

Material engineers develop the encasement that holds the hearing-aid components. A hearing aid must be made of a durable, flexible plastic and shaped to fit comfortably in the ear or ear canal.

Types of Hearing Aids

External hearing aids are devices intended for those with mild to moderate hearing loss, and are worn in the ear, in the canal, completely in the canal, or behind the ear.

• Digital and programmable hearing aids are instruments that contain a computer chip programmed by a computer. This technique offers a way to match a particular hearing loss with the amplification that the user needs most.
• Conventional hearing aids are devices that use conventional electronics to produce quality, amplified sound at all levels of intensity. Because these devices are not programmable, many users find that some sounds are amplified too much, while others are not amplified enough.

Internal hearing aids (implants)

Ear with cochlear implant. click for copyright

• Unlike hearing aids that amplify sound waves, the inner component of cochlear implants are surgically implanted into the inner ear to stimulate the hair cells in the cochlea with electrical impulses. The external components, which attach to the skull behind the ear, include a microphone to gather sound waves, a speech processor to select and arrange sounds picked up by the microphone, and a transmitter to convert the sound into electrical impulses. The internal components include connecting wires and an electrode array to collect the impulses from the transmitter, sending them to different parts of the auditory nerve. This device is intended for those with severe hearing loss or deafness.
• Ossicular implants are small pieces of metal, usually titanium, surgically implanted in the occicles to replace damaged bones in the middle ear and improve the conduction of sound. This method to improve hearing is still in the process of development.

The Newest Hearing Aid Being Developed by Engineers

Biomedical engineers have developed a new idea that could lead to a very small hearing-aid device. The concept includes a tiny nanoparticle and a transmitter. The magnetically responsive nanoparticle is implanted in the inner ear and the transmitter is worn outside the ear. The transmitter produces a magnetic field, causing the nanoparticle, and the tissue in which it is implanted, to move, which sends a signal to the brain.

Ask the class if anyone has a family member who uses a hearing aid. Invite this person into the classroom to discuss his/her hearing loss, how the problem came about, and the type of hearing aid s/he uses to improve his/her hearing. Have him/her describe the features of the device and if s/he has encountered any issues while using the hearing aid.

Introduce students to the decibel reading of various noises and why high-level readings damage hearing. Have students arrange sound types and decibel readings from the lowest to highest decibel levels. See the TeachEngineering Sound Lines activity.

Have students examine how we know sound exists by listening to and seeing sound waves, then describing sound in terms of its pitch, volume and frequency. This ties together how biomedical engineers study sound waves to help people who cannot hear or talk. See the TeachEngineering Seeing and Feeling Sound Vibration activity.

To further students' understanding of sound energy, have them identify different pitches and frequencies as they create high- and low-pitch sound waves with a vibrating ruler and a straw kazoo. See the TeachEngineering Pitch & Frequency activity.