eMedicine Specialties > Otolaryngology and Facial Plastic Surgery > Audiology
Hearing Aids
Updated: Oct 20, 2008
Basic Components and Functions of Hearing Aids
The basic components of a contemporary hearing aid include a microphone, an amplifier, a receiver, and a power supply.
A microphone is a transducer that converts the sound signal into electrical energy. The amplifier is a transformer that increases the amplitude of the electrical signal that is sent to the receiver. The receiver then changes the modified electrical signal back into sound energy that is directed into the ear.
A variety of microphones, amplifiers, and receivers are used, depending on the type and degree of hearing loss. The American National Standard Specification of Hearing Aid Characteristics specifies the electroacoustic tests that a manufacturer must perform and publish for each hearing aid before the instrument is shipped.1 The standard states the tolerance allowed so that the audiologist can perform the same tests to verify the performance of an instrument against specifications.
Currently used hearing aid microphones are primarily electrical devices that have good linear behavior over a frequency range of 50-6000 Hz. This range can be modified to be more appropriate for specific hearing losses.
Directional microphones have been developed that can vary with both the amplitude and the direction of the sound source relative to the microphone. They can reduce the sounds coming from the back of a hearing aid wearer compared with the sounds coming from the front by as much as 15 dB. This change can greatly improve the signal-to-noise ratio of the listener and thus the understanding of speech in the presence of noise.
Hearing aid amplifiers are transformers primarily composed of transistors that are built into an integrated circuit. These transistors provide a current source and serve a variety of functions. In these transistors, the primary function of the amplifier is to increase the power of the electrical signal received from the microphone.
Typically, hearing aids have 2 or more stages of amplification. The first stage is the preamplifier, which is at the level of the microphone. The preamplifier helps to amplify the initial input signal. At this level, the gain is relatively low.
Most amplification is supplied by the power amplifier. These amplifiers are typed in a particular class. The most common are referred to as class A, class B, and class D. They are distinguished by their power consumption, gain, and output abilities.
Each amplifier can be modified to limit the maximum output of the hearing aid. For linear amplification, the amplifier may be limited by peak clipping. This occurs when the electrical signal exceeds the maximum output of some component of the hearing aid circuit. This type of limiting causes various forms of distortion that have been found to reduce the intelligibility and the subjective quality of speech.
A hearing aid that has some type of level-dependent signal processing is termed a nonlinear hearing aid. Most nonlinear hearing aids reduce gain as input or output levels increase.
Nonlinear hearing aids are designed to amplify a wide range of sounds so that they are audible to the hearing-impaired listener without becoming uncomfortably loud. These aids usually use some form of compression circuit that reduces the gain of the instrument when either the input to the device or the output of the device exceeds a predetermined level. This process results in a comfortable amplification for the wearer and prevents the hearing aid from saturating.
Compression hearing aids can provide amplification of the speech components that are essential for intelligibility and can reduce impulsive or high-level sounds that normally cause discomfort.
The hearing aid receiver is an output transducer and handles more power than a microphone. Receivers in hearing aids are very small because of cosmetic considerations.
In general, larger receivers can supply larger output signals. Therefore, the small receivers on hearing aids may be taxed to their output capabilities.
The receiver must also be chosen to match its amplifier. A mismatch in design produces limited output and increases distortion.
Because of the receiver's open position in the external ear canal, it is vulnerable to damage from debris in the ear canal and from the aid being dropped. Manufacturers state that approximately 40% of hearing aids returned for service have damage or blockage to the receiver.
Hearing Aid Sizes and Styles
The first wearable electronic hearing aid was the body hearing aid (see Image 1). This type of aid included a variably sized case that was worn on the body of the user and contained the microphone, amplifier, battery, on/off switch, and volume control. Leading from the case were the receiver cord and the receiver. Attached to the receiver was an ear mold that was fitted to the wearer's ear.
Because of the size of the aid and the placement of the microphone on the body rather than in the ear, very few body aids are currently dispensed.
The behind-the-ear hearing aid is worn behind the pinna (see Image 2). The body of the instrument contains the microphone, amplifier, receiver, on/off switch, and volume control. Leading from the receiver is the ear hook, which loops around the ear and carries the amplified sound to the tubing attached to the ear mold.
The behind-the-ear hearing aid was the most common aid dispensed from the early 1960s until the early 1980s. However, since 1983, the in-the-ear–type hearing aids have captured the largest part of the hearing aid market. In 1987, approximately 80% of hearing aids dispensed in the United States were in-the-ear instruments; most of the remaining hearing aids were behind-the-ear instruments.
A new thin tube variety of behind-the-ear hearing aid has taken up over 50% of the total market in 2007. These behind-the-ear aids are very small and are nearly invisible when fit behind the ear. The longer thin tubing and fitting software allow access to higher frequency amplification with increased bandwidth from 6000-8000 Hz. These open-fitting hearing aids provide excellent sound quality, better directional microphone placement, and less occlusion compared with many in-the-ear hearing aids.
A variety of these hearing aids fit the receiver of the aid in the ear canal of the wearer. This allows for a smoother frequency response and increased gain before feedback occurs. Another advance in hearing aid technology with behind-the-ear hearing aids is wireless transmission of signals form outside sources such as cell phones to the hearing aids and the use of this technology to have hearing aids in a bilateral fit communicate with each other.
The in-the-ear hearing aids can be broken down into full-shell, half-shell, canal, and completely in-the-canal instruments. Every in-the-ear instrument contains its microphone, amplifier, and receiver. The faceplate of the instrument includes the battery door, on/off switch, volume control (if available), and microphone opening. Most of the shells for each of these aids are made from ear mold impressions taken from the individuals in whom these aids are to be fitted.
The full-shell instrument fills the concha of the individual. It is the largest of the in-the-ear hearing aids (see Image 3). It typically can address more severe hearing losses with greater ease because of its ability to fill the canal and the concha of the external ear. It thus can reduce the chance of feedback from the hearing aid.
The half-shell is an instrument that fills only the concha cavum and the canal and is approximately half the size of a full-shell instrument. Because of its smaller size, it is cosmetically more appealing and could be appropriate for moderate-to-severe hearing losses.
The canal-sized in-the-ear aid primarily fits within the concha and in the outer half of the canal. The faceplate of this aid is accessible to the user to allow changing the volume control and turning the aid on and off. This aid provides some advantage in gain at higher frequencies because of its depth of insertion and the acoustic resonance in the unblocked concha.
The completely in-the-canal aid, or what may be termed a peritympanic hearing aid, is fitted deep into the ear canal and is the smallest of all hearing aids (see Image 4). It typically fits entirely within the ear canal, and the deepest portion of the aid is in close proximity to the tympanic membrane. The faceplate is usually not accessible to the user. The aid also needs a short cord or wire attached to the faceplate for the wearer to use while removing the aid.
These aids are regarded as the most cosmetically pleasing, and, because of the close proximity to the tympanic membrane, they can reduce or eliminate the occlusion effect. Additionally, patients with this type of aid can use the telephone like individuals without hearing aids.
Hearing Aid Batteries
The power supply to the hearing aid is derived from its battery. Hearing aid batteries used currently are of 2 main types, zinc-air and mercury, although most in use today are zinc-air cells.
The primary feature of the zinc-air cell is its longer shelf life compared with the mercury- or silver-based hearing aid batteries. Zinc-air cells are not activated until a tape seal is removed from the positive side of the battery. This side contains small holes through which air enters to initiate activation. In most situations, the zinc-air cells last longer than their mercury counterparts.
Hearing aid batteries have a relatively flat discharge rate, and the battery's capacity is rated in milliampere hours (mAh). If the current drain of a hearing aid is known, an estimate of the expected life of the battery can be calculated by dividing the battery's capacity by the current drain measured in milliamperes (mA).
Hearing Aid Candidacy
A variety of factors must be taken into account before an individual is considered to be a candidate for a hearing aid. These factors also depend on whether the individual is an adult or a child.
One major misconception concerning the candidacy of hearing aid use revolves around the type of hearing loss. In the past, some physicians were trained to believe that hearing aids were not helpful for sensorineural hearing losses. However, most patients fitted with hearing aids have sensorineural hearing loss, and those who are fitted properly have reported increased communication abilities from the use of hearing aids.
Audiologic test results are primarily used to determine the hearing aid candidacy for adults. Other factors, such as motivation, perceived handicap, and social needs, are also considered.
Audiologic results including pure-tone thresholds, speech reception thresholds, and speech discrimination scores in quiet and noise are used to define the type, degree, and configuration of hearing loss.
In addition, most comfortable loudness and uncomfortable loudness levels help in determining the patient's dynamic range. The dynamic range for speech can be defined as the difference in decibels between the speech reception threshold and the uncomfortable loudness level.
All the above tests are used to determine the frequency response curve, gain, and maximum output of the patient's hearing aid.
In addition to these tests, which are completed with the patient using earphones, sound-field versions of these tests can help determine the binaural hearing abilities of the patient and help with fitting verification of the hearing aid.
Motivational factors can be determined by interviewing the patient and obtaining information regarding the impact of the hearing loss on everyday life and the patient's perceived need for amplification. Patients who are highly motivated and perceive that they will hear better with hearing aids or that their understanding of speech will improve with hearing aids are most likely to adapt to and obtain maximum use from the aid.
A variety of hearing-handicap scales can be used to measure the self-perceived hearing handicap of a patient. Several self-report scales, including the Hearing Handicap Inventory for the Elderly (HHIE) and the Client Oriented Scale of Improvement (COSI), are relatively short and assess the areas of hearing handicap, including the social and emotional effects of hearing loss. These scales also can be used after fitting of the hearing aid to determine the benefit of its use.
Motivation and the amount of perceived handicap are major factors in determining the candidacy for hearing aid use in adults.
Any child with a verifiable hearing loss is a candidate for amplification. A combination of objective electrophysiologic tests and behavioral tests are usually needed to determine the degree, type, and configuration of the hearing loss when evaluating a young child.
Infants who are identified with sensorineural hearing loss can be fit with amplification when younger than 6 months. Because behavioral test results at this age are limited, electrophysiologic test results are primarily used to determine hearing aid candidacy.
With all very young children, the hearing aid evaluation and fitting process should be ongoing. Children need to be monitored on a regular basis to determine if the fit of the hearing aid is appropriate and if the aid is set for maximum aided results.
Hearing Aid Selection
In addition to the hearing evaluation and the patient profile obtained regarding the patient's hearing handicap and motivation, other testing may be completed in the sound field, especially speech reception in the presence of noise. These tests can help determine the type of hearing aid to be chosen. They also serve as nonaided pretests that will be compared with aided posttesting.
After reviewing the test results and determining the type and degree of loss to each ear, a decision must be made whether to recommend one hearing aid (monaural amplification) or 2 hearing aids (binaural amplification). For most binaural hearing losses, 2 hearing aids are recommended.
One of the primary reasons for this recommendation is the mounting clinical evidence that indicates that failure to fit hearing aids on both ears of patients with binaural hearing loss can result in temporary and perhaps permanent decrease in the auditory function in the unaided ear.
The deterioration over time of auditory perceptual function in the unaided hearing-impaired ear has been referred to as the "auditory deprivation" effect.
In addition to avoiding the possible deprivation effects, other advantages exist to binaural amplification, including better sound localization, improvement of speech reception in the presence of noise, improved speech clarity, and more natural and less stressful listening.
Besides determining the size of the hearing aid and whether to fit monaurally or binaurally, the determination of frequency response, gain, and overall output of the hearing aid must be decided. In many settings, Real Ear measurements are made to help select the proper characteristics of the hearing aid.
Specifications of hearing aids from manufacturers are produced using the ANSI S3.22 standards concerning the gain, output, and frequency response of the hearing aid. These measurements are made in a 2-cm3 coupler. This coupler is used to simulate the condition of the aid in an ear, but many differences exist between a metal 2-cm3 coupler and the volume and texture of an ear canal and eardrum, and many individual differences exist between ears. Because of these differences, a Real Ear probe-tube measurement is used to reveal the exact frequency response, gain, and maximum output of the hearing aid in the ear at the site of the eardrum.
Using the Real Ear equipment, the audiologist places a probe microphone into the ear canal and presents a known auditory signal to the patient. The information from the microphone when the stimulus is present yields a Real Ear unaided response (REUR). This response reveals the resonating characteristics of the ear canal without the aid in place and can assist in formulating the best 2-cm3 coupler response for a patient at the time a hearing aid is ordered.
A variety of prescriptive techniques for fitting hearing aids use information from Real Ear measures. These techniques include the half-gain rule and the prescription of gain and output (POGO).
One of the most popular prescriptive techniques is the procedure developed by the National Acoustics Laboratory (NAL) in Australia for selecting gain and frequency response of a hearing aid. The NAL algorithm is used to calculate the most appropriate Real Ear gain.
From the algorithm, a hearing aid is selected with the required frequency response and gain characteristics, and comparisons are made between the predicted gain and the Real Ear measurements obtained from the hearing-impaired client.
This can be obtained by measuring the real-ear aided response (REAR). The REAR is taken with the hearing aid and the probe microphone in the ear, and the aid's gain is turned to match the calculated Real Ear gain. The REAR is the gain in decibels relative to the stimulus level presented to the patient.
The real-ear insertion gain (REIG) is the difference between the REAR and the REUR and is used to verify that the predetermined target insertion gain has been achieved.
Within the past 10 years, hearing aids using digital signal processing (DSP) have been introduced into the market. These aids, when compared with standard analog hearing aids, allow for a more precise control over a broader range of parameters. In 2005, more than 90% of all hearing aids dispensed in the United States were digital.
Some analog hearing aids can be digitally programmed; the digital programmer can adjust the gain, frequency response, and output of the analog circuit. Some analog hearing aids also may have multiple channels (frequency bands) that can be digitally programmed.
The difference between a DSP hearing instrument and an analog aid is that the analog signals from the microphone are converted into a digital form by an analog-to-digital converter. Once in the digital form, the signals are manipulated by sophisticated processing algorithms and then converted back to analog form by digital-to-analog conversion.
The digitally controlled hearing aids usually use an external programming unit that the dispenser uses to adjust the gain, output, and frequency response of the unit. Many of these aids have multiple channels that allow the dispenser to program individual gain, output, and compression for each frequency channel.
Most of the digital hearing aids and some of the digitally programmed analog hearing aids use a common computer platform database called NOAH. This database can carry the audiometric information and office-based information on each patient. Software from each manufacturer can be installed on the platform. The aids are connected to a common interface called HI-PRO that allows the software from the manufacturer to interface with the hearing aid. The fitting paradigms vary with each manufacturer.
Ear mold impressions
For behind-the-ear fittings, ear mold impressions of the patient are taken, and these impressions are sent to a manufacturer who makes the ear mold that will be fit to the chosen behind-the-ear instrument. The manufacturer is instructed on the type of material to be used, the type of mold to be made, and any modifications, venting, and tubing that is to be included with the mold.
For in-the-ear instruments, ear mold impressions are sent to the hearing aid manufacturer, who makes the casing of the in-the-ear hearing aid from the impression.
Hearing aid manufacturers are also instructed by the dispenser regarding the type of in-the-ear aid that is to be made and the type of microphone, amplifying circuits, and receivers that are most appropriate for the patient.
Recently, hearing aid manufacturers have provided dispensers the technology that allows for ear mold impressions to be digitally scanned in the office and electronically sent to the manufacturer. This process allows for a more accurate ear mold and eliminates the shipping of ear mold impressions and order forms and reduces turnaround time by several days.
Hearing Aid Fitting and Orientation
After the hearing aid has been ordered and sent to the hearing aid dispenser, it is ready to be fitted to the patient. In this process, the hearing aid is inserted into the patient's ear, and the acoustic performance of the aid is evaluated. This can be accomplished by using Real Ear equipment or by sound field–aided test results.
With the Real Ear equipment, a Real Ear aided response can be obtained, and the insertion gain of the aid can be measured. This gain can be compared with the target gain generated by a particular prescriptive method chosen by the dispenser (eg, NAL), and the hearing-aid settings can be adjusted until a reasonable match is observed. In addition to the gain, similar adjustments are made to the total output of the aid to ensure that the aid does not exceed the patient's loudness discomfort levels.
After Real Ear measurements are taken, the patient may be placed in a sound booth where aided sound-field testing of the speech reception threshold and the speech discrimination in quiet and in noise can be made. The difference between the aided and the unaided measures (ie, functional gain) provides a general indication of the benefit provided by the hearing aid.
Once the hearing aid has been fitted and evaluated, the patient is given a general orientation concerning the hearing aid, including all the components of the device, how to insert and remove the aid, the care and maintenance of the aid, and how and when to change batteries.
During the orientation, the patient is counseled about the use of the aid in various settings, common problems faced by individuals using hearing aids in these settings, and strategies to maximize hearing aid benefit.
Questions that the patient may have concerning wearing and using the aid are answered. If the patient appears to understand how to insert and remove the hearing aid and understands how to turn the aid on and off and adjust the volume control, he or she is allowed to leave with the aid.
In most settings, the hearing aid is dispensed with a 30-day trial period, and the dispenser sets up 2-3 appointments with the patient during this time. During these follow-up visits, the patient's ear mold may need to be modified for a more comfortable fit or to reduce feedback problems.
Near the end of the trial period, the dispenser may retest the patient in the sound field to obtain aided sound-field measures. The dispenser also may obtain aided measures of the patient's self-assessed hearing handicap to assess the patient's subjective perception of the benefits of the hearing aid.
If the patient decides to purchase the hearing aid, then the warranty for the aid begins. Most hearing aids come with at least a 1-year warranty. Extended warranties (up to 3 y) are also available. These types of warranties are most appropriate for children or other individuals who may be at risk of damaging the hearing aid.
Summary
During the past decade, hearing aids have progressed from rather simple linear analog amplifiers to hearing aids with sophisticated digital programmable analog circuits and digitally programmable digital circuits. These digital devices can contain a variety of channels and programs to function in various listening situations.
Although the digital technology provides a more precise fit, complete counseling during the fitting and orientation sessions remains necessary to maximize the communication abilities of the patient who is hearing impaired.
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Keywords
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Keywords
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