A Contemporary Review of Hearing Aids

The Laryngoscope C 2009 The American Laryngological, V Rhinological and Otological Society, Inc. Contemporary Review A Contemporary Review of Heari...
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The Laryngoscope C 2009 The American Laryngological, V

Rhinological and Otological Society, Inc.

Contemporary Review

A Contemporary Review of Hearing Aids Catherine V. Palmer, PhD

A contemporary review of hearing aids is provided with a focus on current styles of amplification devices, the goals of a hearing aid fitting and the signal processing schemes that allow the clinician to meet these goals, and the need to couple outside devices to hearing aids. A variety of new features available on hearing aids that improve the listener’s experience with amplification are described. Some future challenges in hearing aid design and hearing evaluation are presented. Key Words: Amplification, hearing aids. Laryngoscope, 119:2195–2204, 2009

significant hearing loss, including sensorineural, conductive, or mixed hearing losses of any degree’’ (p. 2). Children are now being fit with hearing aids by the age of 6 months due to successful early identification programs and the new guidelines that suggest children should be screened for hearing loss by 1 month, diagnosed by 3 months, and fit within 1 month of diagnosis and no later than 6 months (1–3(þ1)–6 goal).3 The AAA Guideline for the Audiologic Management of Adult Hearing Impairment4 does not specifically outline candidacy but indicates that adult amplification recommendations will be based on the hearing evaluation and the patient’s reported difficulties. If an adult is having problems hearing, then they are a candidate for hearing assistance.

DEFINING HEARING AIDS For the purpose of this report, hearing aids are defined as external electronic devices used to help individuals with hearing loss. The components of traditional hearing aids include a microphone, an analog to digital converter, digital signal processor including an amplifier stage, a digital to analog converter, and a receiver (loudspeaker) that delivers an acoustic signal into the external auditory canal (directly or through tubing leading to the ear canal). This report will not include devices that are implanted (fully or partially implantable hearing aids, bone anchored hearing aids, or cochlear implants). These systems also are continually changing and would merit contemporary reviews of their own. Kirkwood1 reported that 97% of all hearing aids sold in 2008 used digital signal processing. The discussion in this review will focus only on the current digital hearing aids.

WHO IS A CANDIDATE FOR HEARING AIDS? The AAA Pediatric Amplification Guidelines2 indicate that ‘‘Amplification with hearing instruments should be considered for a child who demonstrates a From the Department of Otolaryngology and Department of Communication Science and Disorders, University of Pittsburgh, Pittsburgh, Pennsylvania, U.S.A. Editor’s Note: This Manuscript was accepted for publication July 21, 2009. Send correspondence to Catherine V. Palmer, PhD, Director, Audiology, 203 Lothrop Street, Eye and Ear Institute, 4th Floor, Pittsburgh, PA 15213. E-mail: [email protected] DOI: 10.1002/lary.20690

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CURRENT STYLES The choice of hearing aid style includes degree of hearing loss, site of lesion, ear shape and size, desired battery life, and patient preference. Figures 1 to 4 illustrate traditional, air-conduction hearing aid styles. Figure 1 shows a behind-the-ear hearing aid (BTE) attached to an earmold that couples the hearing aid to the ear canal. Figure 2 and 3 show an in-the-ear (ITE) and in-the-canal (ITC) style hearing aid, respectively. Figure 4 illustrates a completely-in-the-canal (CIC) hearing aid. The newest style consists of a mini-BTE coupled to the ear with slim tubing ending in a small dome (Fig. 5). It is common to refer to this style as an ‘‘open fit,’’ because the slim tube and small dome leave the ear canal open. The openness of the fit is only related to the final coupling in the ear canal, not to the size of the behind-the-ear hearing aid or the size of the tubing connecting the BTE to the earmold. The chart in Figure 6 shows the various combinations of the behind-the-ear hearing aids that are currently available. The numerous arrows illustrate that the size of the BTE, the size of the tubing, and the final coupling in the ear canal are all independent decisions. For example, the slim tube that is commonly associated with the open fit can actually couple a standard or mini-BTE to an occluding earmold if the hearing loss warrants this type of earmold (more severe hearing loss). This can provide the user with the cosmetic advantage of the slim tube but with the power Palmer: Hearing Aids


Fig. 1. Behind-the-ear style hearing aid with the hearing aid coupled to the ear with an earmold. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]

advantage of a full-size behind-the-ear hearing aid and traditional occluding earmold. The second outcome of this new style of hearing aids has been a distinction between having the receiver (loudspeaker) cased in the BTE or at the end of the sound channel (slim tubing). Again, this choice is not the same as whether the fitting is open or not, but rather where the receiver (loudspeaker) is housed. The

Fig. 2. In-the-ear style hearing aid. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]

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Fig. 3. In-the-canal style hearing aid. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]

receiver in the aid (Fig. 7, right side) is the traditional configuration and can be coupled to standard or slim tubing and to an open or closed earmold. The receiver in the canal (RIC, Fig. 7, left side), removes the receiver (loudspeaker) from the BTE case, and a wire running down the slim tube connects the BTE circuitry to the receiver (loudspeaker) that is now at the end of the slim tube (in the ear canal). This style allows for a smaller

Fig. 4. Completely-in-the-canal style hearing aid. [Color figure can be viewed in the online issue, which is available at www.interscience. wiley.com.]

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Fig. 5. Mini–behind-the-ear coupled to an open fitting via slim tubing. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]

BTE case while still having the amplification power of a more traditional BTE. It also allows room for a larger battery in a smaller case. The potential disadvantage is consistent with the disadvantages of the ITE, ITC, and CIC styles in that the receiver (loudspeaker) is now more exposed to moisture, heat, and cerumen in the ear canal. Once again, the RIC can be configured as an open or closed fitting. Cerumen guards have provided a way to protect receivers (loudspeakers) that are in the ear canal by providing an acoustically transparent barrier. The hearing aid user is able to change the cerumen guard with a tool, thereby reducing repeated trips to the clinic for hearing aid repairs. Figure 8 shows a white cerumen guard at the end of a RIC product. The tool needed to remove and replace the cerumen guard is shown as well. Individuals with normal hearing (or aidable impaired hearing) in one ear and an unaidable hearing

loss on the other side continue to present a challenge in terms of amplification solutions. The traditional contralateral routing of signal (CROS) provides a microphone on the unaidable side and then transfers the signal via wire or FM signal to a device on the normal hearing side that receives the signal from the contralateral side and delivers it to the normally hearing ear. This solution is appropriate for the individual who indicates that they are frustrated by not hearing an individual sitting on their unaidable side. This arrangement does not generally improve localization, and these individuals usually have continued difficulty understanding in noise. If the unaidable ear actually has no hearing (e.g., the acoustic nerve has been cut due to surgery), then a transcranial CROS may be appropriate. In this arrangement, a powerful hearing aid is fit in the unaidable ear, and the signal picked up by the microphone and amplified by the hearing aid provides enough power to overcome the interaural attenuation and crosses over to the normal hearing cochlea via bone conduction. The bilateral contralateral routing of signals (BiCROS) is a similar solution to the CROS, but the device that delivers the signal from the unaidable side also provides amplification because there is hearing loss in the good ear. The bone anchored hearing aid (not discussed in detail in this review) also is being used in cases of unilateral deafness working on the same premise as the transcranial CROS by providing a bone-conducted signal that will cross to the good cochlea via bone conduction.

GOALS OF A HEARING AID FITTING The basic goals of a hearing aid fitting are to provide audibility for the range of sounds encountered in daily life (range of intensity levels and frequencies), to allow the listener to hear in complex (noisy) situations commonly occurring in daily communication, and to do all this while being comfortable acoustically and physically. With the advent of a variety of consumer electronics made to couple to the ear, a newer challenge is making sure these devices couple to the hearing aid that is coupled to the ear.

Fig. 6. Chart of behind-the-ear (BTE), tubing, and earmold choices.

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frequency hearing. This signal processing technique takes the high-frequency sounds and transposes or compresses them in order to provide them in the lower-frequency range where the individual can perceive sound.12 Although not discussed in this paper, hybrid electroacoustic cochlear implants with traditional amplification in the low frequencies through a hearing aid and electrical stimulation in the higher frequencies through an implant also are being investigated to address this type of hearing loss.


Fig. 7. Receiver-in-the-canal hearing aid on the left side of the picture and receiver in-the-aid hearing aid on the right side of the picture. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]

AUDIBILITY Audibility is obtained for soft, moderate, and loud inputs without the need for constant volume wheel manipulation through the use of compression. Compression allows more gain to be applied to soft inputs, with less for moderate inputs, and even less for loud inputs. Currently, aspects of compression (e.g., compression threshold, compression ratio, attack time, and release time) can be controlled separately in over 20 frequency channels.5,6 Data suggest that no more than two to four channels are required for frequency shaping for intelligibility,7 but the large number of frequency-specific channels allows for other advanced signal processing, including automatic feedback management, which will be discussed in another section. The second aspect of audibility relates to bandwidth (frequency response). Stelmachowicz et al.8 and Pittman9 provide compelling data that children need audibility through 9,000 Hz to adequately hear the /s/ sound. The /s/ is significant in English as it represents plurals, possessives, and tense. More recently Ricketts et al.10 and Horwitz et al.11 have reported that adults have improved intelligibility with increased bandwidth past 4,000 Hz. Unfortunately, commercially available hearing aids do not provide usable gain past 4,000 to 5,000 Hz at this time. Constraints to bandwidth come from the sampling rate used by the digital chip and the resulting battery drain. Low-frequency audibility is equally important when considering the sound quality of music. The response of commercially available hearing aids rolls off below 500 Hz. Of course, in the new open fit style that is appropriate for individuals with good low-frequency hearing and mild to moderate high-frequency hearing loss, the individual will hear the low frequencies because the physical hearing aid is not blocking the sound. Frequency transposition or compression has been introduced recently by several manufacturers. This is a solution for individuals who do not have aidable highLaryngoscope 119: November 2009


The majority of adult patients come to the clinic complaining that they cannot hear well in noise. The ‘‘noise’’ the patient is referring to often refers to actual background noise, reverberation, distance from a person talking, and the need to attend to different talkers in a fast-paced conversation. The most important recommendation for the individual with hearing loss in both ears (whether symmetrical or asymmetrical) is to use bilateral amplification. The human brain is a far better signal detector than any hearing aid algorithm, and it is the input from both ears to the brainstem that allows the brain to detect the primary signal versus the noise. With this said, many individuals with sensorineural hearing loss will continue to have difficulty hearing in noise even when both ears are amplified due to frequency and temporal resolution problems that result in poor word recognition and are not solved through amplification.

Directional Microphones The use of directional microphone technology has been shown in both the laboratory and the field to provide significant benefit in listening in noise.13,14 An omnidirectional microphone has equal sensitivity in all directions (only impacted by head shadow and body baffle). A directional microphone generally maintains its sensitivity at the 0 azimuth (where the individual would be facing) and has reduced sensitivity in the back

Fig. 8. Receiver-in-the-canal hearing aid with a cerumen guard inserted with the tool used to remove and replace the cerumen guard. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]

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Fig. 9. Free-field omnidirectional microphone polar plot. This microphone is equally sensitive in all directions.

Fig. 11. Free-field cardioid microphone polar plot. This microphone is most sensitive to sounds in the front and to the sides of the listener.

and/or to the sides (depending on the particular polar plot). A range of polar plots are depicted in Figures 9 to 12. In the figures, the sensitivity of the particular microphone is illustrated by the dark line. To interpret the figures, pretend the listener’s head is in the center of the circle (looking down on the top of the head) with the listener’s nose pointing to 0 . The dark line illustrates the sensitivity of the microphone to sounds around the listener. If the listener has positioned him/herself so the unwanted noise is behind him/her and the desired signal is in front, then the directional microphone pattern illustrated in Figure 11 would be desirable.

Data indicate that most adults only need the directional setting in about 25% of their listening situations, but when they do need it, it is very valuable.14 Directional microphones actually were introduced in the late 1960s but disappeared rather quickly because they were only available on BTEs, and one had to choose to have a directional microphone or an omnidirectional microphone and could not switch between the two. In addition, the early 1970s saw the advent of custom products (all in the ear), and individuals were not interested in BTEs for the time being. In the mid-1990s directional

Fig. 10. Free-field bidirectional microphone polar plot. This microphone is most sensitive to sounds in front and in back of the listener.

Fig. 12. Free-field hypercardioid microphone polar plot. This microphone is most sensitive to sounds in front and at 90 to the listener as well as sound directly behind the listener.

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microphones were back and could be found on BTE, ITE, and some ITC hearing aids. In addition, the user could switch between the omnidirectional versus directional setting. As directional microphone fittings became commonplace, engineers used the ability of the new digital chips to increase the usability of the microphones by making autodirectional microphones. With the introduction of sound analysis in the hearing aid, an algorithm could be designed to detect what the hearing aid considered ‘‘speech’’ versus what the hearing aid considered noise. When noise was detected, the hearing aid would switch from an omnidirectional setting to a directional setting (one of the polar plots depicted in Figs. 10–12). Next, the auto adaptive directional microphone was introduced; now the hearing aid algorithm made the decision to switch from omnidirectional to directional, and further, several directional polar plots could be introduced depending on the analysis of the incoming signal. The current challenge is designing algorithms that switch the microphone sensitivity appropriately and determining the ideal transition times between polar patterns. Until recently, most audiologists did not recommend directional microphones for children because of the lack of data on whether the technology would be beneficial or detrimental in terms of reducing incidental hearing. Recent data from Ricketts et al.15 indicate that young children do face the signal of interest, and the impact on incidental hearing may not be substantial. The incoming signal analysis is the basis for all of the hearing aid decision making. Although the signal analysis process differs between commercially available hearing aids and their digital chips and algorithms, most are based on a similar decision matrix. The signal entering the hearing aid is evaluated in real time with the analysis focused on whether the signal is varying in intensity over time or steady state over time. The algorithm assumes that a varying signal is primarily speech, and a steady state signal is primarily noise. If the decision is that the signal is mostly noise, then the directional microphone is engaged, thereby improving the signal-to-noise ratio by decreasing the microphone sensitivity in the direction of the noise. Because no hearing aid algorithm can detect the users intent (e.g., what the listener wants to hear), the successful use of directional microphones requires environmental manipulation or awareness. The user must place him/herself so the signal of interest is in front and the noise is in back. In many cases, a room will be so reverberant that in actuality the noise will be all around. In this case, directional microphones will not be of benefit.

Noise Reduction The signal analysis described above is the basis for the implementation of noise reduction algorithms as well. When the algorithm detects noise (steady state signal), the gain is reduced. Therefore, the introduction of noise reduction does not increase the signal-to-noise ratio because the gain for all sounds is decreased. Studies focused on the implementation of noise reduction algorithms have reported that individuals may have an increase in comfort Laryngoscope 119: November 2009


when noise reduction is employed, but that intelligibility is not enhanced.16,17 These findings are consistent with the lack of change in signal-to-noise ratio, which is essential for an improvement in intelligibility.

Assistive Listening Devices For the patient with difficulty hearing in noise, the ideal solution is the use of an assistive listening device where the microphone can be placed in close proximity to the desired signal. The signal is picked up by the microphone of the assistive listening device and transmitted to an FM receiver that can be body worn or can be built into the hearing aids. The transmission generally will be wireless so the listener is not tethered to the person speaking. The signal is then processed through the hearing aids and delivered to the ear. This arrangement provides the desired signal without any of the interfering noise because of the microphone placement. Unfortunately, many hearing aid users do not feel comfortable or capable of placing the microphone in the desired location and operating the hearing aid in order to put it on the correct setting for the FM receiver input and, therefore, do not take advantage of these devices. The FM receiver portion of these devices has recently been integrated into the BTE hearing aid that simplifies the system significantly (rather than having a bodyworn FM receiver with needed coupling to the hearing aid). Even with this significant improvement in cosmetics and ease of use, the reality remains that the system is only useful if the user places the microphone by the signal of interest. Because classrooms are inherently noisy places, students with hearing aids will need the added advantage in signal-to-noise ratio that an assistive listening device provides. Individuals fitting hearing aids want to think ahead in the case of children and make sure that the hearing aid has the ability to couple to an assistive listening device. Ideally, the FM receivers will be built into the hearing aids, which is not only cosmetically appealing, but also creates less wear and tear than when the FM receivers have to be unplugged from the hearing aids and provides for less possibilities of mechanical failure than when a body-worn FM receiver with coupling to the hearing aids is used.

COMFORT Physical and acoustic comfort continue to be top priorities for hearing aid users and can be approached from several different aspects of the hearing aid fitting and hearing aid features.18 Two of the most common complaints from hearing aid users continue to be issues surrounding the occlusion effect and annoyance from feedback. Noise reduction is used for comfort, and appropriate output limiting continues to be essential for acoustic comfort.

Occlusion Effect and Feedback Management When an individual plugs his/her own ears and talks, the sound of one’s own voice is much louder. By Palmer: Hearing Aids

the very nature of the hearing aid’s placement, it plugs the user’s ears. Patients often describe this sound as ‘‘boomy’’ or ‘‘sounding like they are in a barrel.’’ One simple solution is to leave the ear as open as possible. In the past, this has been achieved through venting in the earmold of a BTE or custom shell of an ITE. The limitation has always been feedback. Feedback is a pure tone signal that is generated when amplified sound can leak out of the ear canal and reach the microphone, thereby being reamplified. This creates a feedback loop and an audible signal is generated. This is very annoying for the hearing aid user and anyone around them. The introduction of sophisticated digital signal processing algorithms has allowed the development of automatic feedback management systems. These systems are generally implemented in one of two ways. First, when a pure tone signal is detected by the signal detector in the digital chip, gain can be reduced in the corresponding frequency channel until the signal disappears. The algorithm will continue to reintroduce the sound while avoiding the gain that produced feedback. This system can reduce gain temporarily when an object gets close to the hearing aid (e.g., telephone) or can reduce the gain permanently if one channel is consistently creating feedback. This solution is dependent on multichannel instruments so the very least amount of the frequency range can be reduced. If a hearing aid only had two channels, then the entire high-frequency channel would have to be reduced in these example situations, which would diminish audibility and speech understanding. The second type of signal processing introduces an identical pure tone 180 out of phase in order to cancel the sound generation. The advent of this advanced feedback management has led to the proliferation of open fittings. Now that feedback is fairly well mitigated, the ear canal can remain much more open while the hearing aid is able to provide high-frequency gain for at least moderate hearing losses. The feedback management programs continue to improve, and more and more gain before feedback is being achieved. The open canal alleviates the sensation of occlusion. These two advances have likely been the most important changes in patient comfort in the past 30 years.

Output Limiting Complaints of hearing aids being too loud18 continue to be a primary reason for rejecting amplification. The compression output currently used to limit the maximum output of hearing aids is not new and has been used successfully for several decades. As is the case with audibility, the key to success is in the verification procedure. Mueller et al.19 found that the first fit for output limiting varied greatly between manufacturers, and the only way to set the output limiting correctly was to measure the individual’s frequency-specific loudness discomfort levels, and to present loud sounds in order to verify that loudness discomfort levels were not exceeded. For pediatric patients, loudness discomfort levels are estimated using data from Cornelisse et al.20 The Adult Amplification guidelines4 provide guidance related to the Laryngoscope 119: November 2009

measurement of loudness discomfort levels and the verification of the hearing aid settings.

COUPLING TO OTHER THINGS THAT WE PUT IN OUR EARS Consumer (e.g., MP3 players, mobile phones) and professional electronics (e.g., stethoscopes) continue to change, improve, and capture the attention of the public. When choosing the style and features of hearing aids, it has become essential to ask, ‘‘What else do you put in your ears?’’ In essence, the clinician needs to know what other signals/devices the hearing aid user will need to couple to in order to do his/her work or enjoy entertainment. When discovered ahead of time, there are a variety of decisions that can be made relative to the hearing aid style, technology, and features that will allow coupling to almost any device. These solutions add training time to the hearing aid delivery as the patient will need to be instructed on the coupling techniques. With a BTE hearing aid, a large vent (or open fitting) can be used if the listener needs to hear through a stethoscope. In this configuration, the listener may want to use an amplified stethoscope because the signal will be going directly into the ear without the amplification produced by the hearing aid. One essential item that must be compatible with the hearing aids is the telephone. Everyone uses a telephone for at least safety purposes, if not for social and work reasons. Hearing aids must now function with land line and mobile phones. The US Federal Communications Commission passed a rule that went into effect several years ago that requires that mobile phone manufacturers produce several models of phones that are hearing aid compatible (which is different from just having a volume control). Hearing aid compatible means that the telecoil circuit in the hearing aid can pick up the electromagnetic leak produced by the telephone. This strategy has been used over the years because the proximity of the telephone created feedback in the hearing aid. By activating the telecoil, the microphone is turned off and no feedback can be produced. With the advent of automatic feedback management, we may see less and less use of the telecoil for telephone communication, but the telecoil circuit will still be valuable as the coupling component for many assistive listening devices. Many patients now report using Bluetooth phones. Currently, Bluetooth receivers cannot be cased in hearing aids due to power restrictions. There are several hearing aids on the market that are sold with an accessory that hangs around the individual’s neck and receives the Bluetooth signal. This signal is then coupled to the hearing aid through proprietary transmissions. Therefore, there is an extra stage between the receipt of the Bluetooth signal and the person actually receiving the signal in their ear canals. This is an area that will likely see development in the future. No patient should leave the clinic without a viable telephone solution, one that does not include taking out the hearing aid. Palmer: Hearing Aids


DATALOGGING/DATALEARNING Datalogging is a function of the hearing aid that allows the clinician to see data related to the individual’s usage. The data may include overall hours of use and percent of use in different programs. This feature can be very useful in counseling patients (e.g., perhaps they have not become a full-time user) or may be helpful in fine tuning (e.g., the person spends the majority of the time in program #2, why not put this program first). This feature has a bit of a big brother feel to it if the clinician does not introduce it as a positive feature at the time of the fitting (e.g., I will be able to read the hearing aid when you come back and get a sense of how many hours you are using it and how you are using the memories. This information should help me fine tune the fitting further once you have used the hearing aid in your day-to-day environment). Datalearning is a more recent development that produces changes in the hearing aid fitting online as the person uses the hearing aid. So, if the individual consistently turns the volume down, the datalearning would eventually manipulate the start-up volume to a lower setting. The datalearning programs generally can be set to start to perform some time after the fitting. Careful consideration has to be given to the use of these programs and to the timing because there are data to indicate that hearing aid users need time to adjust to the new sounds they are hearing. An individual cannot adjust to what they do not experience, so if you have fit the hearing aid by verifying audibility for soft, moderate, and loud sounds, you will want the patient to experience these sounds in order to get used to them and make use of them. If a datalearning program were turned on from the beginning and the user found that they needed to turn the hearing aids down a bit as they started to use them, the actual settings would be changed before the user had a chance to adjust.21

NEW VALUE ADDED FEATURES Quite a few of the most recent advances in hearing aids have focused on features that enhance the experience of using amplification rather than on signal processing to improve communication. The most recent of these value added features are described below. It is likely that more of these features will be introduced and the new generation of baby boomer hearing aid users will appreciate them as they allow tailoring the fitting to particular lifestyles and preferences.

Hearing Aid Alerting Signals Traditionally, hearing aids produce a series of beeps when the listening programs are changed, when the volume control is manipulated, or when the battery is low. The newest generation of hearing aids produce speech to indicate what listening mode you are in (‘‘program two’’) or to let you know the battery is dying (‘‘change your battery’’). These hearing aids can be programmed to provide appointment reminders as well, and Laryngoscope 119: November 2009


different languages can be chosen based on the user’s preference.

Using Your Cell Phone as a Remote Control Hearing aid users, like any other individual in our modern world, are finding themselves dependent on a variety of devices that all have to be charged at night and have to be carried throughout the day. Remote controls are one more of these devices. Although rarely essential (the hearing aids can be controlled via push buttons on the aids themselves), remote controls can be very useful for certain individuals. The newest advance in remote controls is a hearing aid that can be controlled by the individual’s cell phone (e.g., for increasing and decreasing volume).

Hearing Aids Communicating With Each Other Approximately 2 years ago, ear-to-ear communication was introduced. This technology allows the two hearing aids to communicate with each other. This can be for ease of use (e.g., adjust one volume control and both hearing aids are adjusted) or it can be for signal processing purposes (e.g., the autoadaptive directional microphone in the left ear goes into a particular setting, and that same setting can be communicated to the opposite ear in order to guarantee similar signal processing in both ears). Potentially, beamforming, which produces a highly directional signal, can be achieved if the hearing aid signal processing in each hearing aid can coordinate its activities. There are additional issues of power consumption with beamforming as well.

Relaxation Sounds A recently released hearing aid comes with a program that actually produces low level sounds that are meant to be relaxing during a busy day of communicating. These also are being marketed to tinnitus sufferers who may want low levels of sound around them at all times.

Automatic Telecoil As discussed above, telephone use continues to be a challenge for some hearing aid users. The automatic telecoil uses the magnet that is contained in land line phones to switch the circuit in the hearing aid (shutting off the microphone and engaging the telecoil). When the phone is pulled away from the hearing aid, the hearing aid goes back to the normal listening program. This may help individuals who were reticent to try to push their program button to the correct setting while picking up the phone.

Selecting and Deselecting Features Some features are really designed for the clinician, which is true in the case of features that can be deselected. The clinician used to have to decide upfront whether the user would benefit from a volume control or directional microphone. Now these features can be included, but can be deactivated if they create confusion Palmer: Hearing Aids

or are not useful. In the case of the pediatric patient, one would expect the child to have the new set of hearing aids for 5 to 7 years, so there are features that may not be appropriate for a 6 year old but will be needed for an 11 year old.

FUTURE CHALLENGES Hearing aids are moving toward consumer electronics in looks and features as opposed to traditional medical devices. The challenge in function, however, is that hearing aids must meet the needs of individuals with wildly different hearing ability (both peripheral and central), whereas consumer electronics are designed for uniform hearing (assumed to be good). In addition, hearing aids must process constantly varying signals, whereas consumer electronics are designed generally to deal with one type of signal. No other audio system currently designed is expected to deal with such a wide variety of hearing and signal inputs. A great many challenges still lie ahead in terms of hearing aid design. It is rewarding that more and more collaboration between engineers, acousticians, auditory scientists, cognitive scientists, psychologists, audiologists, physicians, and end users is evident in the newest technologies being produced. A clear challenge that lies ahead includes the development of adequate rechargeable batteries for hearing aids. Battery insertion continues to be difficult for many users with dexterity challenges. Two manufacturers currently produce products with rechargeable batteries, but as of yet the charge time is often less than a full day of use, and this limits the use of one or two technologies for the individual, which may or may not match their needs. Moisture continues to be a challenge for the hearing aid user. The ear canal is a damp, warm place. In essence, we are placing unsealed electronics in this space, and the result is damage to the circuit over time. Several companies are now advertising water resistant cases, but the challenge remains that there are openings into the system by the inherent nature of the device (microphone input and battery door opening). The Lyric hearing aid (InSound Medical, Inc., Newark, CA) has been introduced to try to solve battery insertion, hearing aid insertion/removal, and moisture issues experienced by hearing aid users. This device is inserted deeply into the canal and is left in place for approximately 3 to 4 months. When the hearing aid stops functioning, it is removed and replaced with a new device. Users can shower with the devices inserted because they are water resistant. The company indicates that swimming and diving are not recommended. Users no longer have to deal with battery changes or insertion/ removal of the hearing aid. The Lyric is an occluding device in that it fills the ear canal. The device will be appropriate for a particular range of hearing loss, canal size, and ear health (e.g., no draining ears). The device is being introduced across the United States slowly at this time. Currently, the cost over time appears to be Laryngoscope 119: November 2009

greater than replacing a standard hearing aid every five years. The development of more function-specific hearing evaluations seems to be needed in light of the more focused technology that is being developed. The traditional hearing evaluation produces a very gross measure of function, whereas amplification technology continues to become more and more specialized. The future may see tests that will evaluate frequency and temporal resolution, and perhaps cognitive function, if there are hearing aid parameters that could be adjusted based on these results. New outcome measures may be warranted as the differences in signal processing become more subtle. One could imagine a clinical evaluation of effort (or resource allocation), as opposed to a gross measure of percent correct, in order to test whether a noise reduction or directional microphone system was having a significant impact on the individual’s quality of life.

CONCLUSION Matching technology to the person (hearing loss and communication needs/demands) is the critical role of the clinician. This is achieved through appropriate diagnostic evaluation and patient interview to determine individual communication needs and challenges and environmental constraints. Although many consumers of hearing health care continue to focus on the hearing aid as the key to mitigating their hearing problem, the reality is that it is the clinician who matches the correct technology to their needs and then programs the technology to produce audibility, comfort, and various other signal processing strategies that the individual requires. In addition, there is evidence that rehabilitation is critical in the patient’s success22,23 and should be part of a comprehensive hearing health care plan. The patient must take an active role in becoming a successful user of amplification.

BIBLIOGRAPHY 1. Kirkwood D. Hearing Journal report. Hear J 2009;62:7. 2. American Academy of Audiology: Pediatric Amplification Protocol. Reston, VA: American Academy of Audiology, 2003. Available at: www.audiology.org/resources/documentlibrary/documents/pedamp.pdf. Accessed May 1, 2009. 3. American Academy of Pediatrics, Joint Committee on Infant Hearing. Year 2007 position statement: principles and guidelines for early hearing detection and intervention programs. Pediatr 2007;20:898–921. 4. Guideline for the Audiologic Management of Adult Hearing Impairment. Reston, VA: American Academy of Audiology, 2007. Available at: www.audiology.org/resources/ documentlibrary/documents/haguidelines.pdf. Accessed May 1, 2009. 5. Shi LF, Doherty K. Subjective and objective effects of fast and slow compression on the perception of reverberant speech in listeners with hearing loss. J Speech Lang Hear Res 2008;51:1328–1340. 6. Warner Henning R, Bentler R. The effects of hearing aid compression parameters on the short-term dynamic range of continuous speech. J Speech Lang Hear Res 2008;51:471–484.

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