Hearing aid
A hearing aid is a device designed to improve hearing by making sound audible to a person with hearing loss. Hearing aids are classified as medical devices in most countries, and regulated by the respective regulations. Small audio amplifiers such as personal sound amplification products or other plain sound reinforcing systems cannot be sold as "hearing aids".
Early devices, such as ear trumpets or ear horns, were passive amplification cones designed to gather sound energy and direct it into the ear canal.
Modern devices are computerised electroacoustic systems that transform environmental sound to make it audible, according to audiometrical and cognitive rules. Modern devices also utilize sophisticated digital signal processing, aiming to improve speech intelligibility and comfort for the user. Such signal processing includes feedback management, wide dynamic range compression, directionality, frequency lowering, and noise reduction.
Modern hearing aids require configuration to match the hearing loss, physical features, and lifestyle of the wearer. The hearing aid is fitted to the most recent audiogram and is programmed by frequency. This process, called "fitting", can be performed by the user in simple cases, by a Doctor of Audiology - also called an audiologist, or by a Hearing Instrument Specialist or audioprosthologist. The amount of benefit a hearing aid delivers depends in large part on the quality of its fitting. Almost all hearing aids in use in the United States are digital hearing aids, as analog aids are phased out. Devices similar to hearing aids include the osseointegrated auditory prosthesis and cochlear implant.
Uses
Hearing aids are used for a variety of pathologies including sensorineural hearing loss, conductive hearing loss, and single-sided deafness. Hearing aid candidacy was traditionally determined by a Doctor of Audiology, or a certified hearing specialist, who will also fit the device based on the nature and degree of the hearing loss being treated. The amount of benefit experienced by the user of the hearing aid is multi-factorial, depending on the type, severity, and etiology of the hearing loss, the technology and fitting of the device, and on the motivation, personality, lifestyle, and overall health of the user. Over-the-counter hearing aids, which address mild to moderate hearing loss, are designed to be adjusted by the user.Hearing aids are incapable of truly correcting a hearing loss; they are an aid to make sounds more audible. The most common form of hearing loss for which hearing aids are sought is sensorineural, resulting from damage to the hair cells and synapses of the cochlea and auditory nerve. Sensorineural hearing loss reduces the sensitivity to sound, which a hearing aid can partially accommodate by making sound louder. Other decrements in auditory perception caused by sensorineural hearing loss, such as abnormal spectral and temporal processing, and which may negatively affect speech perception, are more difficult to compensate for using digital signal processing and in some cases may be exacerbated by the use of amplification. Conductive hearing losses, which do not involve damage to the cochlea, tend to be better treated by hearing aids; the hearing aid is able to sufficiently amplify sound to account for the attenuation caused by the conductive component. Once the sound is able to reach the cochlea at normal or near-normal levels, the cochlea and auditory nerve are able to transmit signals to the brain normally.
Common issues with hearing aid fitting and use are the occlusion effect, loudness recruitment, and understanding speech in noise. Once a common problem, feedback is generally now well-controlled through the use of feedback management algorithms.
Candidacy and acquisition
There are lots of ways of evaluating how well a hearing aid compensates for hearing loss. One approach is audiometry which measures a subject's hearing levels in laboratory conditions. The threshold of audibility for various sounds and intensities is measured in a variety of conditions. Although audiometric tests may attempt to mimic real-world conditions, the patient's own every day experiences may differ. An alternative approach is self-report assessment, where the patient reports their experience with the hearing aid.Hearing aid outcome can be represented by three dimensions:
- hearing aid usage
- aided speech recognition
- benefit/satisfaction
Types
There are many types of hearing aids, which vary in size, power and circuitry.Among the different sizes and models are:
Body-worn
Body worn aids were the first portable electronic hearing aids, and were invented by Harvey Fletcher while working at Bell Laboratories. Body aids consist of a case and an earmold, attached by a wire. The case contains the electronic amplifier components, controls and battery, while the earmold typically contains a miniature loudspeaker. The case is typically about the size of a pack of playing cards and is carried in a pocket or on a belt.Without the size constraints of smaller hearing devices, body worn aid designs can provide large amplification and long battery life at a lower cost. Body aids are still used in emerging markets because of their relatively low cost.
Behind the ear
Behind the ear hearing aids are one of two major classes of hearing aids – behind the ear and in the ear. These two classes are distinguished by where the hearing aid is worn. BTE hearing aids consist of a case which hangs behind the pinna. The case is attached to an earmold or dome tip by a traditional tube, slim tube, or wire. The tube or wire courses from the superior-ventral portion of the pinna to the concha, where the ear mold or dome tip inserts into the external auditory canal. The case contains the electronics, controls, battery, and microphone.The loudspeaker, or receiver, may be housed in the case or in the earmold or dome tip. The RIC style of BTE hearing aid is often smaller than a traditional BTE and more commonly used in more active populations.BTEs are generally capable of providing more output and may therefore be indicated for more severe degrees of hearing loss. However, BTEs are very versatile and can be used for nearly any kind of hearing loss. BTEs come in a variety of sizes, ranging from a small, "mini BTE", to larger, ultra-power devices. Size typically depends on the output level needed, the location of the receiver, and the presence or absence of a telecoil. BTEs are durable, easy to repair, and often have controls and battery doors that are easier to manipulate. BTEs are also easily connected to assistive listening devices, such as FM systems and induction loops. BTEs are commonly worn by children who need a durable type of hearing aid.
In the ear
In the ear aids devices fit in the outer ear bowl. Being larger, these are easier to insert and can hold extra features. They are sometimes visible when standing face to face with someone. ITE hearing aids are custom made to fit each individual's ear. They can be used in mild to some severe hearing losses. Feedback, a squealing/whistling caused by sound leaking and being amplified again, may be a problem for severe hearing losses. Some modern circuits are able to provide feedback regulation or cancellation to assist with this.Venting may also cause feedback. A vent is a tube primarily placed to offer pressure equalization. However, different vent styles and sizes can be used to influence and prevent feedback.
Traditionally, ITEs have not been recommended for young children because their fit could not be as easily modified as the earmold for a BTE, and thus the aid had to be replaced frequently as the child grew. However, there are new ITEs made from a silicone type material that mitigates the need for costly replacements.
ITE hearing aids can be connected wirelessly to FM systems, for instance with a body-worn FM receiver with induction neck-loop which transmits the audio signal from the FM transmitter inductively to the telecoil inside the hearing instrument.
Mini in canal or completely in canal aids are generally not visible unless the viewer looks directly into the wearer's ear. These aids are intended for mild to moderately severe losses. CICs are usually not recommended for people with good low-frequency hearing, as the occlusion effect is much more noticeable. Completely-in-the-canal hearing aids fit tightly deep in the ear. It is barely visible. Being small, it will not have a directional microphone, and its small batteries will have a short life, and the batteries and controls may be difficult to manage. Its position in the ear prevents wind noise and makes it easier to use phones without feedback. In-the-canal hearing aids are placed deep in the ear canal. They are barely visible. Larger versions of these can have directional microphones. Being in the canal, they are less likely to cause a plugged feeling. These models are easier to manipulate than the smaller completely in-the-canal models but still have the drawbacks of being rather small.
In-the-ear hearing aids are typically more expensive than behind-the-ear counterparts of equal functionality, because they are custom fitted to the patient's ear.
In fitting, the audiologist takes a physical impression of the ear.
The mold is scanned by a specialized CAD system, resulting in a 3D model of the outer ear.
During modeling, the venting tube is inserted.
The digitally modeled shell is printed using a rapid prototyping technique such as stereolithography.
Finally, the aid is assembled and shipped to the audiologist after a quality check.