A hearing aid or deaf aid is a device designed to improve hearing. Hearing aids are classified as medical devices in most countries, and regulated by the respective regulations. Small audio amplifiers such as PSAPs or other plain sound reinforcing systems cannot be sold as "hearing aids".
Earlier 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 more intelligible or comfortable, according to audiometrical and cognitive rules. Such sound processing can be considerable, such as highlighting a spatial region, shifting frequencies, cancelling noise and wind, or highlighting voice.
Modern hearing aids require configuration to match the hearing loss, physical features, and lifestyle of the wearer. This process is called "fitting" and is performed by audiologists. The amount of benefit a hearing aid delivers depends in large part on the quality of its fitting. Devices similar to hearing aids include the bone anchored hearing aid, and cochlear implant.
When the primary auditory cortex does not receive regular stimulation, this part of the brain loses cells which process sound. Cell loss increases as the degree of hearing loss increases.
Damage to the hair cells of the inner ear results in sensorineural hearing loss, which affects the ability to discriminate between sounds. This often manifests as a decreased ability to understand speech, and simply amplifying speech (as a hearing aid does) is often insufficient to improve speech perception.
- Hearing aids are incapable of truly correcting a hearing loss; they are an aid to make sounds more accessible. Three primary issues minimize the effectiveness of hearing aids:
The occlusion effect is a common complaint, especially for new users. Though if the aids are worn regularly, most people will become acclimated after a few weeks. If the effect persists, an audiologist or Hearing Instrument Specialist can sometimes further tune the hearing aid(s)(.
The compression effect: The amplification needed to make quiet sounds audible, if applied to loud sounds would damage the inner ear (cochlea). Louder sounds are therefore reduced giving a smaller audible volume range and hence inherent distortion. Hearing protection is also provided by an overall cap to the sound pressure. Also of protective value is impulse noise suppression, available in some high-end aids.
The initial fitting appointment is rarely sufficient, and multiple follow-up visits are often necessary. Most audiologists or Hearing Instrument Specialists will recommend an up-to-date audiogram at the time of purchase and at subsequent fittings
There are several 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
The most reliable method for assessing the correct adjustment of a hearing aid is through real ear measurement. Real ear measurements (or probe microphone measurements) are an assessment of the characteristics of hearing aid amplification near the ear drum using a silicone probe tube microphone.